15 Common Car Problems: Symptoms, Diagnosis, and Repair Costs

Most car problems announce themselves before they leave you stranded. The trick is learning to listen, and knowing what to do when your car starts talking. After years of working through diagnostic codes, pulling apart engines, and helping people figure out what's wrong with their vehicles, I've noticed the same handful of issues come up again and again. This guide covers those problems in plain language -- what you'll notice, what's probably causing it, and whether it's something you can tackle yourself or need to hand off to a professional.

Quick-Reference Summary

Engine and Performance Issues

1. The Check Engine Light

Let's start with the one everyone dreads. That little orange light triggers more anxiety than almost anything else on your dashboard, but here's the honest truth: most of the time, it's not catastrophic.

The check engine light covers an enormous range of conditions, from a loose gas cap (seriously, tighten it and see if the light goes away after a couple of drive cycles) to a failing catalytic converter. The light itself tells you almost nothing -- you need to pull the diagnostic trouble code to know what the car's computer is actually complaining about. A basic OBD2 scan takes about 30 seconds and gives you a specific code like P0420 or P0171 that points you toward the actual problem.

Here's what I've seen most often: oxygen sensor failures (€150-300 to replace), catalytic converter issues (€800-2,500 and climbing, especially with precious metal prices), worn spark plugs (€200-500 for a full set with labour), and yes, the infamous loose gas cap. If the light is solid, you've generally got time to investigate. If it's flashing, that means active misfires -- pull over when it's safe and don't keep driving, because you're risking serious catalytic converter damage with every kilometre.

2. Misfires and Rough Running

Your engine should idle smoothly enough that you barely notice it running. When it starts shaking at red lights, surging up and down in RPM, or hesitating when you step on the gas, something in the combustion process has gone wrong.

The diagnostic code will tell you a lot here. A P0300 means random misfires across multiple cylinders, which usually points to something systemic like fuel quality, a vacuum leak, or worn plugs. Codes P0301 through P0308 are cylinder-specific, and that's actually good news because it narrows the search. If cylinder 3 is misfiring, you can swap the ignition coil from cylinder 3 to cylinder 1. If the misfire follows the coil, you've found your problem -- about a €100-200 part that most people can replace with basic hand tools in under 20 minutes.

Spark plugs are the other usual suspect. Modern iridium plugs last 80,000-120,000 km, but I've seen plenty that were well past their prime. If you haven't replaced them in that range and you're getting misfires, start there. It's a moderate DIY job -- straightforward on most four-cylinders, trickier on V6s where the rear bank is buried under the intake manifold.

When misfires come with fuel trim codes (P0171 or P0172), you're looking at air-fuel mixture problems. A vacuum leak, dirty mass airflow sensor, or failing fuel injector can all cause this. Fuel injectors are where I'd draw the DIY line for most people -- they require more disassembly and proper torque specs.

3. Rough Idle Specifically

Sometimes the engine runs fine while driving but shakes and stumbles at idle. This is almost always one of three things: a dirty throttle body, a vacuum leak, or a failing idle air control valve (on older cars).

Throttle body cleaning is one of those satisfying DIY jobs. A can of throttle body cleaner costs about €8, the job takes 15 minutes, and the difference is often dramatic. Carbon builds up on the throttle plate and bore over time, and on some cars -- I'm looking at you, Volkswagen -- it happens faster than others. Clean it every 50,000 km and you'll avoid a lot of idle problems.

Vacuum leaks are trickier to find. The classic trick is to spray carburetor cleaner around the intake manifold gaskets and vacuum hoses while the engine idles. If the RPM changes, you've found your leak. Be careful though -- this spray is flammable near a running engine, so use caution and keep a fire extinguisher handy. A professional smoke test is safer and more thorough for hard-to-find leaks. Replacement hoses are cheap; a cracked intake manifold gasket is a bigger job but still manageable for a competent DIYer. Budget €100-300 depending on what's leaking and where.

4. Poor Fuel Economy

If you're filling up more often and your driving habits haven't changed, something is off. The frustrating part is that reduced fuel economy often has no other obvious symptoms.

Start with the basics: check your tyre pressure. Low tyres are the most common and most overlooked cause of poor mileage. Every 7 kPa (1 PSI) below spec costs you about 0.2% in fuel efficiency, and I've seen people driving around 40-50 kPa low on all four tyres without realising it. That adds up fast.

After tyres, a dirty air filter is the next cheapest fix. Pull it out and hold it up to a light. If you can't see light through it, it's past due. A new one costs €15-40 depending on the vehicle.

Beyond the freebies, a failing oxygen sensor is the most common culprit. The car's computer relies on O2 sensor data to fine-tune the air-fuel mixture. When a sensor gets lazy, the mixture runs rich, and you burn more fuel. Replacing an O2 sensor costs €150-300 and is a moderate wrench-turning job -- the hardest part is usually getting the old one unstuck from the exhaust manifold.

5. Exhaust Smoke

The colour of your exhaust smoke tells a specific story, and none of the stories are ones you want to ignore.

White smoke (thick, not the thin wisps on a cold morning) means coolant is getting into the combustion chamber. That's typically a head gasket failure, and it's one of the more expensive repairs you'll face: €1,000-2,500 on most cars. However, I've seen people panic over normal condensation steam on cold mornings. The distinction is that a head gasket leak produces persistent, sweet-smelling white clouds regardless of engine temperature. Blue smoke means the engine is burning oil. Worn valve stem seals, piston rings, or turbo seals are the usual causes. If it only happens on startup and then clears, valve stem seals are the likely culprit (€500-1,200). If it's constant, you're probably looking at piston rings, which is a much bigger job (€1,000-2,000+). Black smoke on petrol engines indicates too much fuel in the combustion mixture. On modern fuel-injected cars, this usually points to a stuck fuel injector, faulty fuel pressure regulator, or sensor problems. This is typically the most affordable of the three to fix (€200-600) and often just requires a sensor replacement or injector cleaning. On diesel engines, the diagnostic path is different -- black smoke typically points to a clogged DPF, faulty injectors, a failing turbocharger, or EGR problems. See the DPF section below for more on diesel-specific issues.

All three of these generally need professional diagnosis and repair. The smoke colour gets you into the right neighbourhood, but pinpointing the exact failure requires proper diagnostics.

Brakes and Stopping

6. Brake Noise and Performance

Brakes are straightforward but safety-critical, so I'll be direct: if your brakes feel wrong, don't postpone the inspection.

Squealing when you press the brake pedal usually means the wear indicators are making contact -- those little metal tabs are designed to scream at you when the pads are getting thin. This is your friendly warning to schedule a pad replacement within the next few weeks. Brake pads run €100-300 per axle depending on the vehicle, and replacing them yourself is one of the more accessible DIY jobs. You need a jack, jack stands, basic sockets, and a C-clamp to push the caliper piston back. Grinding means you've gone past the pads and you're now metal-on-metal. At this point, you're damaging the rotors, and what could have been a €150 pad job becomes a €300-500 pad-and-rotor job. I've seen people drive on grinding brakes for weeks because "it still stops." It does stop, right up until the moment the pad backing plate separates from the caliper and it doesn't.

A soft or spongy brake pedal that goes further to the floor than usual is a different conversation entirely. That's either air in the brake lines (needs a bleed, €50-100), low brake fluid (check for leaks immediately), or a failing master cylinder (€300-500). A brake pedal that slowly sinks to the floor while you hold pressure at a red light is a classic master cylinder symptom.

Pulling to one side when braking typically indicates a stuck caliper or uneven pad wear. It can also be a brake hose that's swollen internally, acting as a one-way valve that doesn't let the caliper release properly. Calipers run €100-250 each, and a stuck caliper will eat through pads on that side much faster than normal.

If your ABS light is on, the antilock system is disabled but your standard brakes still work. An OBD2 scan can pull ABS-specific codes to identify which wheel speed sensor or module is having problems. ABS sensors are about €150-300 per corner to replace.

Electrical and Starting Problems

7. Battery and Starting Failures

"It won't start" is the most common complaint, and the diagnostic process is actually pretty logical once you know what to listen for. For a deeper dive, see our complete starting problems guide.

Rapid clicking when you turn the key: the battery has enough charge to engage the starter solenoid but not enough to turn the engine. This is usually a dead or dying battery. A healthy battery should read 12.6V at rest. Anything below 12.2V is struggling, and below 12.0V is effectively dead. Battery replacement costs €100-200 and is a 15-minute job with a wrench. A single heavy click: the starter motor solenoid is engaging but the motor isn't turning. This could be a dead battery (test voltage first) or a failing starter motor. Try tapping the starter with a wrench while someone turns the key -- if it starts, the starter motor is on its way out. Starter replacement runs €300-600 at a shop, or about half that in parts if you do it yourself. The difficulty depends heavily on the car. Some starters are right on top, easily accessible. Others are buried behind the intake manifold on transverse-mounted engines and take hours to reach. Complete silence, no clicks, no lights: check your battery terminals. Corrosion on the battery terminals is incredibly common and can completely block the electrical connection. Clean them with a wire brush and a baking soda/water solution. If the terminals are clean and the battery tests fine, you're looking at a starter relay or ignition switch issue. Diesel-specific note: if your diesel engine cranks but won't fire, especially in cold weather, check the glow plugs. They preheat the combustion chamber to help diesel fuel ignite. A failed glow plug or two might not matter in summer, but in winter it can prevent starting entirely. Glow plug replacement runs €100-300 for a set. In extreme cold, diesel fuel itself can gel or wax -- if you suspect this, don't keep cranking, as you risk damaging the fuel system.

8. Broader Electrical Gremlins

Flickering lights, intermittently dead electronics, and fuses that blow repeatedly all point to electrical system problems. These can be genuinely maddening to diagnose because the symptoms are often inconsistent.

Start with the alternator. It should be putting out 13.5-14.5V while the engine is running. If it's below 13V, the alternator isn't charging properly and your battery is slowly draining while you drive. Alternator replacement runs €300-800 depending on the vehicle.

Bad ground connections are the hidden villain of automotive electrical problems. A corroded ground wire can cause all sorts of bizarre symptoms -- dim lights on one side, intermittent gauge readings, random warning lights. Checking and cleaning ground points is free, and it resolves more electrical problems than people expect.

For parasitic drains -- where your battery dies overnight even though everything appears to be off -- the diagnostic process involves pulling fuses one at a time with an ammeter connected to the battery. When the draw drops, you've found the circuit that's draining you. Common culprits include aftermarket stereo installations, trunk or glove box lights that stay on, and failing door lock actuators.

Cooling and Temperature

9. Overheating

This is the one where time really matters. An overheating engine can go from "needs attention" to "needs a new engine" in minutes if you keep driving.

If your temperature gauge climbs into the red, pull over and turn the engine off. Don't open the radiator cap while it's hot -- the system is pressurised and will spray boiling coolant.

Once things cool down, check the coolant level in the overflow reservoir. Low coolant is the most common cause of overheating and often means there's a leak somewhere. Top it up with the correct coolant type (don't mix colours/types) and look for drips under the car.

A failed thermostat is the second most common cause. It's a €15-30 part that controls coolant flow through the radiator. When it sticks closed, coolant can't circulate and the engine cooks. Replacing a thermostat is a moderate DIY job (€100-200 at a shop) and is usually accessible near the top of the engine where the upper radiator hose connects.

Water pump failures announce themselves with a coolant leak from the pump weep hole, a grinding/whining noise from the front of the engine, or overheating without an obvious leak. This one typically costs €300-600 to have done, and on interference engines, a water pump failure can lead to catastrophic timing belt/chain problems. On many cars the water pump is driven by the timing belt, so it makes sense to replace both at the same time. On many European engines with timing chains (BMW, Mercedes, some VAG), the water pump is separate and often electrically driven, so this bundling advice applies specifically to timing belt engines.

The worst-case scenario for overheating is a blown head gasket. Look for milky residue under the oil cap, coolant loss with no visible external leak, white exhaust smoke, or bubbles in the coolant reservoir when the engine is running. Head gasket repair runs €1,000-2,500, and on some vehicles it simply isn't economical -- at that point you're comparing repair cost to the car's value.

Transmission and Drivetrain

10. Transmission Troubles

Transmission problems range from a €50 fluid top-up to a €4,000 rebuild, so accurate diagnosis matters a lot here.

Slipping -- where the engine revs but the car doesn't accelerate proportionally -- is the most common symptom. In an automatic, this feels like the transmission is momentarily in neutral before it catches. In a manual, a slipping clutch is the equivalent: the engine revs up but the car doesn't accelerate in proportion, especially under load going uphill.

Before you assume the worst, check the transmission fluid. Many modern automatic transmissions, particularly European models with ZF, DSG, or Mercedes gearboxes, are sealed units with no dipstick. Fluid level checks on these require specific procedures and often dealer tools. On older automatics with a dipstick, check with the engine running and warm -- the fluid should be red or light brown. If it's dark brown or smells burnt, it's overdue for a change. Low fluid can cause slipping all by itself, and a fluid change (€100-200) is infinitely cheaper than a rebuild.

Delayed or harsh shifting often comes down to solenoid problems in automatics. The transmission control module uses solenoids to direct fluid flow and control shifts. A failing solenoid causes rough, delayed, or erratic shifts. Solenoid replacement runs €300-800 and requires dropping the valve body -- this is mechanic territory for most people.

Manual transmission issues are more mechanical. Grinding when shifting usually means synchros are wearing out. Difficulty getting into a specific gear points to a worn shift fork or synchro for that gear. A clutch that grabs only at the very top of pedal travel is nearly worn out. Clutch replacement on a front-wheel-drive car runs €500-1,500 because the transmission has to come out entirely. If your vehicle has a dual-mass flywheel (standard on most modern European cars), replacing it at the same time adds €300-600, but it's strongly recommended since you're already in there.

The key takeaway with transmissions: address problems early. A transmission that's slipping occasionally today will fail completely next month, and the repair bill grows with every kilometre you drive on a damaged unit.

Steering and Suspension

11. Steering Issues

If the steering feels heavy, pulls to one side, or the wheel shakes at certain speeds, there are a few distinct possibilities.

A vibration in the steering wheel at highway speeds (usually 90-120 km/h) is almost always a wheel balance issue. Getting your wheels balanced costs €40-80 for all four and fixes this about 90% of the time. If balancing doesn't fix it, warped brake rotors are the next suspect -- you'll notice the vibration is worse when braking.

Pulling to one side during normal driving (not braking) is typically an alignment issue. Hitting a pothole or kerb can knock the alignment out. A four-wheel alignment runs €50-150 and should be done any time you replace tyres or suspension components.

On cars with hydraulic power steering, a whining noise that gets louder when you turn the wheel means the power steering pump is either low on fluid or failing. Check the reservoir first -- a simple top-up might solve it. If the pump itself is going, expect to pay €300-600 for replacement.

Electric power steering (most cars made after 2010-2012) doesn't use fluid, so the failure mode is different. When the electric motor or sensor fails, you'll get a warning light and the steering will feel noticeably heavier. This is generally a dealer or specialist repair.

12. Suspension Wear

Suspension components wear gradually, so many people don't realise how degraded their ride has become until they drive a new car and feel the difference.

The bounce test is simple: push down firmly on each corner of the car and release. If it bounces more than once before settling, the shocks or struts on that corner are worn. Replacing shocks/struts costs €300-800 per axle including alignment afterward, and it makes a dramatic difference in ride quality and handling. This is usually a shop job because the struts are under heavy spring compression and require a spring compressor to disassemble safely.

Clunking noises over bumps, especially from the front end, typically indicate worn sway bar links (€50-150 per pair, moderate DIY job) or worn ball joints (€150-300 per side, generally a mechanic job because pressing them in/out requires special tools).

Uneven tyre wear tells its own story. Wear on the inner or outer edge indicates an alignment problem. Cupping or scalloping means the shocks/struts are worn and the tyre is bouncing rather than maintaining consistent road contact.

Climate Control

13. Air Conditioning Failures

AC problems usually boil down to one of two things: the system is low on refrigerant, or a component has failed mechanically.

If the AC blows air but it's not cold, low refrigerant is the most likely cause. AC systems are sealed, so if the refrigerant is low, there's a leak somewhere. Recharging the system costs €100-200, but if there's a leak, it'll just go low again. Finding and fixing the leak is the real repair. Common leak points are the compressor shaft seal, condenser (the radiator-like thing in front of the regular radiator), and the service port O-rings.

If the AC compressor clutch isn't engaging at all (you should hear a click and see the clutch plate engage when you turn the AC on), the system might be too low on refrigerant for the pressure switch to allow it to run, or the compressor itself has failed. Compressor replacement is one of those expensive jobs: €500-1,500 including the evacuation and recharge of the system. This isn't DIY territory -- it requires specialised equipment to handle the refrigerant legally and safely.

A blower motor that doesn't blow at all or only works on certain fan speeds is a separate issue from the AC itself. On older vehicles, the blower motor resistor controls fan speeds, and when it fails, you typically lose the lower speeds while the highest setting still works (because high bypasses the resistor). The resistor is usually a €20-50 part located behind the glove box, and replacing it is an easy DIY job. On many modern European vehicles, the blower speed is controlled by an electronic module (€80-200) rather than a simple resistor, so expect a higher parts cost if that's your situation.

Emissions

14. Catalytic Converter

The catalytic converter sits in the exhaust and converts harmful emissions into less harmful ones. When it fails, you'll get a P0420 code (or P0430 for the bank 2 converter on V6/V8 engines), reduced power, potentially a sulphur/rotten egg smell, and you won't pass emissions inspection.

Here's what a lot of people don't realise: the converter itself rarely fails on its own. Something upstream almost always causes the failure. Engine misfires send unburned fuel into the converter, overheating it. A rich-running condition (bad O2 sensor, leaking injector) does the same thing. Oil burning coats the catalyst material and reduces its efficiency.

This is why it's important to diagnose properly before just throwing a new converter on the car. If the O2 sensors are giving lazy readings, replacing the sensors (€150-300) might clear the P0420 code entirely. If the underlying engine problem isn't fixed first, the new converter will fail just like the old one did.

Converter replacement itself costs €800-2,500 depending on the vehicle. European vehicles with integrated manifold cats can exceed €3,000. Some direct-fit converters are available in the aftermarket for less, but many regions require specific certifications for replacement catalysts. This is strictly a shop job, both for the welding/fabrication involved and for the legal requirements around emissions equipment.

15. DPF Problems (Diesel Vehicles)

If you drive a diesel, the diesel particulate filter is one of the most common issues you'll encounter in Europe. The DPF traps soot particles from the exhaust and periodically burns them off in a process called regeneration. When regeneration can't complete -- usually because you're doing too many short trips without sustained highway driving -- the filter clogs and problems begin.

Symptoms include a noticeable loss of power, increased fuel consumption, and a DPF warning light on the dashboard. Some cars go into a reduced-power "limp mode" when the DPF reaches a critical blockage level.

The first line of defence is a forced regeneration, which a mechanic can trigger with diagnostic equipment. This costs €150-300 and often resolves the issue. For more stubborn blockages, professional DPF cleaning (either chemical or thermal) runs €200-500. If the filter is too far gone, replacement costs €1,000-3,000 depending on the vehicle.

The best prevention is straightforward: take your diesel on a motorway run of at least 20-30 minutes once every week or two, maintaining steady speeds above 60 km/h. This allows the DPF to regenerate naturally. If your daily driving is almost entirely urban stop-and-go, a diesel might not be the best fit for your needs.

The Diagnostic Approach That Actually Works

Across all of these problems, there's a common thread: proper diagnosis saves money. I've seen too many people replace parts based on guesses -- swapping O2 sensors when the real problem was a vacuum leak, or buying a new starter when corroded battery terminals were the issue.

The smart approach is always the same: note your symptoms carefully (when they happen, how often, what conditions trigger them), pull the diagnostic codes, and then research what those specific codes mean for your specific vehicle. Some codes are generic across all cars; others have manufacturer-specific quirks. A P0171 on a Ford means the same thing as on a Toyota (system too lean), but the common causes differ by platform.

An OBD2 scanner paired with Skanyx can read codes, show live sensor data, and explain what's happening in the context of your specific vehicle -- so you're diagnosing rather than guessing. That's the difference between a targeted repair and an expensive parts-swapping adventure. Try Skanyx free at skanyx.com/download

Frequently Asked Questions

What's the single most important thing I can check myself?

Fluid levels. Engine oil, coolant, transmission fluid, brake fluid, and power steering fluid (if applicable). Most catastrophic failures start with something running low. It takes five minutes once a month, costs nothing, and catches problems while they're still cheap to fix.

My check engine light came on but the car seems fine. Can I keep driving?

If the light is solid (not flashing), you're generally safe to drive for a while, but get it scanned soon. The problem might be minor, but it could also be masking something that'll get worse. If the light is flashing, stop driving as soon as you safely can -- that indicates active misfires that can damage your catalytic converter within minutes.

How do I know if I should DIY a repair or take it to a mechanic?

Honest self-assessment is key. If you have basic hand tools, can follow instructions, and have a place to work, things like brake pads, spark plugs, air filters, and battery replacement are manageable. Anything involving the transmission, timing belt/chain, internal engine work, or AC refrigerant is usually better left to a professional. Also consider the stakes: messing up an air filter change is a minor inconvenience, but messing up a brake job is a safety hazard.

How often should I scan my car for codes?

Every few months is reasonable, or any time something feels off. Pending codes can appear before the check engine light triggers, so scanning proactively can catch problems in their early stages when they're cheapest to fix.

Skanyx's Health Monitor tracks your vehicle's condition over time, catching trends like rising fuel trim values or declining battery voltage before they become expensive problems. Download Skanyx free at skanyx.com/download

Is it worth fixing an older high-mileage car, or should I just replace it?

As a general rule, if the repair costs less than three months of car payments on a replacement, fix it. A known car with a known repair history is often a better bet than an unknown used car with unknown problems. But once you're stacking major repairs on top of each other -- head gasket plus transmission plus suspension all in the same year -- it's time for a realistic conversation about the car's remaining life.