Complete guide to OBD2 diagnostic codes. Learn what P0300, P0171, P0420, and other fault codes mean, how to diagnose them, repair costs, and when it's safe to drive.
P0135 is the upstream companion to P0141. The heater in your pre-catalyst oxygen sensor has failed. Affects fuel economy more than P0141 because this sensor directly controls the fuel mixture.
P0300 means random misfires across multiple cylinders. P0301 through P0304 point to a specific cylinder. Here is how to tell which part failed, what it costs to fix, and whether you can keep driving.
P0340 means the ECU is not receiving a valid signal from the camshaft position sensor. Can cause no-start, rough running, or stalling. Usually a sensor or wiring issue with a straightforward fix.
P0446 means the vent valve in your fuel vapour system is not working correctly. Your car drives fine, but the code will not clear on its own and you will fail an emissions test. Simple diagnosis, cheap fix.
P0507 means your engine idles too fast. Instead of 700-800 RPM, it sits at 1,000-1,500 RPM or higher. Usually a vacuum leak, dirty throttle body, or stuck idle air control valve. Easy to diagnose.
P0016 means the crankshaft and camshaft are out of sync. Usually a timing chain stretch, VVT solenoid, or oil issue. This can be expensive if ignored. Here is how to diagnose it and what to expect.
P0101 means your mass air flow sensor is sending readings outside the expected range. Usually fixable by cleaning, sometimes needs replacement. Full diagnosis and cost breakdown.
P0128 means your engine isn't reaching operating temperature fast enough. Usually a stuck-open thermostat. Here's how to confirm it, what it costs, and why you shouldn't ignore it through winter.
P0141 means the heater in your downstream oxygen sensor has failed. Your car runs fine, but emissions increase and fuel economy drops. A straightforward fix on most vehicles.
P0172 means your engine is running too rich, too much fuel, not enough air. Common causes, how to diagnose it yourself with live data, and what repairs actually cost.
P0174 is the Bank 2 version of P0171. Too much air or not enough fuel on the opposite side of your engine. Causes, live data diagnosis, and repair costs for V6 and V8 engines.
P0401 means your EGR valve is not flowing enough exhaust gas back into the intake. Common on high-mileage vehicles, especially diesels. Carbon buildup is the usual culprit.
P0430 is the Bank 2 version of P0420. Your catalytic converter on the opposite side of the engine isn't working efficiently. Causes, diagnosis with O2 sensor data, and what replacement actually costs.
BMW fault codes explained by engine family. VANOS, HPFP, turbo, oil leak, and DME codes for N52, N54, N55 with EU repair costs and diagnostic tool options.
P0300 random misfire explained. Flashing check engine light? Stop driving. Causes, diagnosis order, and repair costs in euros.
EVAP leak causing a check engine light? P0442, P0455, P0456 explained. Start with the gas cap. Causes, diagnosis, and repair costs in euros.
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P0171 explained: what system too lean means, the most common causes in order, how to diagnose with fuel trims, repair costs (€50 to €800), and when it's safe to drive.
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