Camshaft and Crankshaft Position Sensors: Symptoms and Test

Your car cuts out at a junction. No warning, no splutter, just a dead engine in the middle of pulling away. You coast to the kerb and turn the key, and it fires straight back up and runs perfectly. A week later it does it again at a roundabout. Nothing is consistent. Nothing shows on the dash long enough to read, and the garage cannot reproduce it. This stranded-then-fine pattern is the calling card of a failing camshaft or crankshaft position sensor.

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What do the camshaft and crankshaft position sensors do?

These two sensors give the engine control unit (ECU) its sense of timing. Without them the ECU is firing blind.

The crankshaft position sensor (CKP) is the master clock. It watches a toothed reluctor wheel on the crankshaft and tells the ECU exactly where the pistons are and how fast the engine is turning. The ECU uses that signal to set ignition timing and to trigger the fuel injectors at the right crank angle. No crank signal means the ECU does not know when to fire, so on most cars the engine simply will not run.

The camshaft position sensor (CMP) tracks the camshaft, which turns at half crankshaft speed. Its job is to tell the ECU which cylinder is on its firing stroke, so the injectors can fire sequentially, one cylinder at a time, in the correct order. On many engines the camshaft signal is also what the ECU uses to control variable valve timing.

Why one is more critical than the other:

• Crankshaft sensor: the ECU cannot calculate ignition or injection timing without it. A dead CKP almost always means a no-start.
• Camshaft sensor: if it drops out while the engine is already running, the ECU can usually fall back to a default firing pattern (batched or estimated sequential injection) and keep the engine alive, just rougher.

That difference in how the engine reacts is the single most useful diagnostic clue you have, and we come back to it below.

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What are the symptoms of a bad crankshaft position sensor?

A crankshaft sensor fault tends to be dramatic rather than subtle, because the ECU loses its master timing reference. Watch for these signs:

No-start or hard start: the engine cranks normally but will not fire, because the ECU never sees the trigger to inject fuel and spark. Stalling while driving: the engine cuts out at speed or at idle, sometimes repeatedly. The heat-soak pattern: the engine quits once it is fully warmed up, then refuses to restart until it has cooled for 20 to 30 minutes. This is the classic signature of a crankshaft sensor breaking down as it heats up. Hesitation and rough running: intermittent signal dropouts cause stumbles and surging before the sensor fails completely. Check engine light: typically P0335 (crankshaft position sensor "A" circuit) or the related no-signal code P0336, which names the same circuit but is not always given its own lookup page.

If the car will not start at all, or only restarts after a long cooldown, the crankshaft sensor jumps to the top of the suspect list.

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What are the symptoms of a bad camshaft position sensor?

A camshaft sensor fault is usually less catastrophic but far more annoying, because the engine keeps limping along and the fault is intermittent. A rough idle is one common sign, though it has many other causes too. Look for these:

Stall, then immediate restart: the engine drops out for a moment, fuel cuts, then it fires straight back up and runs fine for hours. This stall-and-recover behaviour is the camshaft sensor's signature. Hard starting: longer cranking than usual, because the ECU has to work out cylinder identification before it can run cleanly. Rough idle and hesitation: the firing sequence drifts off when the cam signal is noisy. Loss of power and worse fuel economy: the fallback injection mode is less efficient than proper sequential firing. Check engine light: commonly P0340 (camshaft circuit), P0341 (range/performance), P0342 (circuit low), or P0343 (circuit high). Bank 2 codes such as P0344 cover the second cylinder head where fitted.

The tell is the recovery. A camshaft fault strands you for seconds, not for half an hour.

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How do I know if it is the camshaft or crankshaft sensor?

This is the question most owners actually want answered, because the two share so many symptoms. Three checks separate them without guessing.

1. Restart behaviour. This is the fastest filter. If the engine quits and will not restart at all, or only restarts after a 20 to 30 minute cooldown, lean crankshaft. If it stalls but fires back up within a few seconds and keeps running, lean camshaft. 2. Does the engine still run with the fault active? A car that drives in to the garage with a stored code, still running, points more toward the camshaft, because the crankshaft sensor failing usually leaves you stranded. 3. The stored code. This is the decisive one. P0335 and P0336 name the crankshaft circuit. P0340 through P0344 name the camshaft circuit. The code does not lie about which circuit the ECU flagged, even if the same wiring or connector problem can ultimately be the cause.

There is a fourth code family that complicates the picture. Understand it before you buy a sensor.

Reading the stored code and its freeze frame data is the difference between buying the right 40 euro sensor and throwing parts at the car. Plug a 15 euro Bluetooth OBD2 adapter into the port and Skanyx shows you the exact code plus the mileage and engine conditions it set at, then gives you plain-language guidance on whether you are looking at a sensor circuit or a timing problem. You fix the cause instead of the symptom.

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What about the cam-crank correlation codes (P0016 to P0019)?

If your scan returns P0016, P0017, P0018, or P0019, slow down before you order a sensor. These are correlation codes, and they mean something different.

A correlation code does not say "the sensor circuit is broken." It says the ECU compared the camshaft signal against the crankshaft signal and found them out of sync. The two are no longer lining up where the engine's timing map says they should.

That can be caused by a faulty sensor, but it is just as often caused by:

A stretched timing chain that has let the camshaft fall a few degrees behind the crankshaft, very common on certain BMW, VW, and PSA engines at higher mileage. A failing variable valve timing (VVT) solenoid or actuator that is not holding cam phasing where it should. Low or dirty oil starving the VVT system, which lives on oil pressure. A slipped or mis-installed timing chain or belt after a previous repair.

The classic tell for a stretched chain is a metallic rattle in the first few seconds after a cold start, often alongside the code. If you see P0016 together with P0017 (or P0018 with P0019) on the same bank, a stretched chain becomes a much stronger suspect than a sensor. The full diagnostic split lives in the P0016 camshaft position correlation guide, but the headline is this: a correlation code is a timing problem to investigate, not automatically a sensor to swap.

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How do you verify this with OBD2?

This is where being honest about tooling matters, because the gap between "any OBD2 app" and "a brand-specific tool" is wide on this topic.

What Skanyx and any generic ELM327 adapter give you:

The stored fault codes (P0335 and P0336 for crank, P0340 to P0344 for cam, P0016 to P0019 for correlation). The freeze frame data captured at the moment each code set.

• The live engine RPM signal on a standard PID.

The codes alone usually answer the cam-versus-crank question, and the freeze frame tells you whether the fault set hot or cold, at idle or under load, which narrows a heat-soak crank sensor versus a cold-start chain rattle. On an intermittent fault, watching live RPM while the engine stalls is genuinely diagnostic: a crank signal dropout shows the RPM reading collapsing to zero the instant the engine cuts, because RPM is derived from that very sensor. That is enough to confirm an intermittent crankshaft sensor on the spot, with no special equipment.

What you need a brand-specific tool or a scope for is the deeper timing data: the actual camshaft-to-crankshaft phase angle in degrees, the commanded versus actual VVT position, plus the raw sensor waveform shape. The phase angle and VVT data live on Mode $22 manufacturer-extended PIDs that generic OBD2 does not expose, so you read them with the matching specialist tool (Carly or ISTA on a BMW, OBDeleven PRO or VCDS on a VW, Audi, Skoda, or SEAT, XENTRY on a Mercedes), or with an oscilloscope on the sensor signal wire. A workshop will pull those numbers on a 30 to 50 euro diagnostic scan.

Practical workflow: read the generic scan first, because it is free beyond the adapter and answers most cases. If you have a plain P0335 or P0340 with the matching symptom pattern, replace the sensor. If you have a correlation code, or the symptoms point at timing, that is when the phase-angle data and the cold-start rattle test earn their keep, and where the brand-specific tool or a workshop pays for itself.

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How do you test the sensor yourself?

If the code and symptoms point at the sensor rather than the chain, a basic test takes minutes. You need a multimeter and, ideally, an assistant.

Step 1: Inspect the connector and wiring

Most "sensor" faults are actually connector faults. Unplug the sensor, check for corrosion, bent pins, oil contamination, and chafed wires near the connector. A green crusty pin or an oil-soaked plug is your answer before you even reach for the meter.

Step 2: Check resistance (for two-wire inductive sensors)

Many crankshaft sensors are passive inductive types with two wires. With the connector unplugged, measure resistance across the two pins with a multimeter set to ohms. Compare against the workshop spec for your engine, typically in the few-hundred to roughly 1,000 ohm range. An open circuit (infinite reading) or a dead short confirms a failed sensor. Three-wire Hall-effect sensors (most camshaft sensors and many newer crank sensors) need a powered test instead, so move to the live check.

Step 3: Watch the live signal while it fails

This is the test that catches intermittent faults a static resistance check misses. With the scan tool reading live RPM, have an assistant crank or run the engine while you gently wiggle the sensor connector and harness. If the RPM reading flickers or drops out as you move the wiring, or the engine stumbles in time with your hand, you have found an intermittent open. Do the same during the heat-soak window if the fault is temperature-related: catch it misbehaving while hot.

Step 4: Confirm with the freeze frame

Before you condemn anything, open the freeze frame on the stored code. Did it set at operating temperature (heat-soak crank sensor) or on a cold start (possible chain or cam issue)? Did it set at idle or under load? Matching the freeze frame conditions to your symptom story stops you replacing the wrong part.

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How much does it cost to replace a camshaft or crankshaft position sensor?

For a straight sensor swap, this is one of the cheaper diagnostic repairs. The complication is access: a crankshaft sensor buried low on the block behind the starter or the crank pulley takes far longer to reach than a camshaft sensor sitting at the top of the head.

Genuine Bosch, Hella, or Continental sensors dominate the European market and are worth the small premium over no-name parts on a timing-critical component. The number to watch is the bottom row: if the real fault is a stretched timing chain flagged by a correlation code, you are in a different cost universe, which is exactly why reading the code correctly before buying parts matters so much. A car that genuinely cranks but will not start needs the wider no-start diagnostic checklist run first, because a fuel-supply or spark fault, or a tripped immobiliser, can mimic a sensor.

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Frequently asked questions

Will a bad sensor always trigger a check engine light?

Usually, but not always immediately. A hard-failed sensor sets a code within a drive cycle or two. An intermittent fault can stall the car repeatedly while only setting a pending code, or none at all if it recovers before the ECU confirms it. This is why reading OBD2 live data, specifically watching the RPM signal during a stall, beats waiting for a stored code on intermittent faults.

Can a bad sensor damage the engine?

A failed position sensor itself does not damage the engine, it just stops it running. The risk is indirect: a long-running misfire from a marginal sensor can overheat the catalytic converter, and ignoring a cam-crank correlation code that turns out to be a stretched timing chain absolutely can lead to valve-to-piston contact and major engine damage on an interference engine. The sensor is cheap; the thing it might be warning you about is not.

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If the car strands you then restarts fine, plug in an adapter and read the code before you order any parts. A plain P0335 or P0340 with the matching restart pattern is a sensor you can often fit yourself for under 80 euros. A P0016 through P0019 correlation code is a different conversation, get the cam-crank phase angle and the cold-start rattle checked before you spend a cent on a sensor that was never the problem.