OBD-II trouble codes explained
Each page explains one diagnostic trouble code: what the engine computer saw, the usual causes, how urgent it is and what to ask the shop.
50 pages in this section
Emissions
- P0400 code: what an EGR flow fault means for your carThe P0400 code means exhaust gas recirculation flow is not working as expected. Learn the causes, driving risks and checks to request before buying parts.
- P0401 code meaning: insufficient EGR flow in city drivingP0401 means insufficient EGR flow. Learn why city trips cause carbon build-up, when you can drive, and what to ask before cleaning or replacing parts.
- P0420 code causes and how to confirm a catalyst faultP0420 can mean a worn catalyst, an exhaust leak or a sensor fault. Learn what to check first, when to drive and what to ask before paying for parts.
- P0430 code: Checking an imported used car before you buyA P0430 code flags low catalyst efficiency on bank 2. Learn what to inspect on an imported used car and how VIN history adds context before buying.
- P0440 code meaning: what your car’s EVAP warning meansP0440 means an EVAP system malfunction, often with little change in driving. Learn the causes, warning signs and checks to request from a shop.
- P0442 code causes and how a shop finds the leakP0442 usually means a small fuel vapour leak. Learn how a shop checks the cap, hoses and valves with a smoke test before you pay for replacement parts.
- P0455 code after refuelling: what you should checkP0455 means a large leak in the fuel vapour system. Check the fuel cap after refuelling, learn when a fuel smell is urgent, and see what to ask a shop.
- P0456 code meaning: Finding an intermittent EVAP leakP0456 means a very small fuel vapour leak. Learn why it returns after clearing, when you can drive, and what to ask before paying for diagnosis.
- P0457 code causes and the fuel cap check you can tryP0457 code causes often start with a loose fuel cap. Learn how to refit it, why the light stays on, and when a shop should check for an EVAP leak.
Fuel and air
- P0087 code: Low fuel rail pressure in your dieselP0087 means fuel rail pressure is too low. Learn how diesel quality, filter checks and proper diagnosis can help you avoid unnecessary pump replacement.
- P0101 code meaning: should you clean, test or replace the MAF?P0101 means airflow readings do not match engine expectations. Learn when MAF cleaning helps, how leaks cause it, and what to test before replacing parts.
- P0102 code causes and the wiring-first checks you needP0102 usually points to a low MAF circuit signal. Learn which connectors and wiring to check first, when to drive, and what to ask your shop.
- P0113 code: Why the intake air reading looks too coldThe P0113 code usually points to an unplugged IAT sensor or an open circuit. Learn what to check, how it affects starts and when to visit a shop.
- P0117 code meaning: why the computer thinks your engine is hotP0117 means the coolant temperature signal is too low. Learn why the engine reads hot, when to stop driving, and how a shop checks the cause.
- P0122 code causes when your accelerator stops respondingP0122 code causes include sensor, connector and wiring faults. Learn why limp mode happens, how to stop safely and what a shop should test first.
- P0128 code: why your engine stays cool in winterP0128 usually means the engine warms too slowly. Learn why winter heat suffers, when you can drive and what to ask a shop before replacing parts.
- P0131 code meaning: Is it the sensor or the engine?P0131 means a low upstream oxygen sensor signal. Learn how leaks or wiring can mimic a bad sensor, when to drive, and what to check before replacing it.
- P0133 code causes: why an ageing oxygen sensor responds slowlyP0133 code causes include sensor ageing, oil contamination and exhaust leaks. Learn what slow response means, when to drive and what a shop should test.
- P0135 code: What it means on cold morningsP0135 means an upstream oxygen sensor heater circuit fault. Learn why cold starts matter, what to check first and when replacement is justified.
- P0141 code meaning: what the downstream heater fault meansP0141 means a downstream oxygen sensor heater circuit fault. Learn why your car may drive normally, what to check, and why inspection readiness matters.
- P0171 code causes and how a shop finds the faultP0171 usually points to unmetered air or inadequate fuel delivery. Learn the causes, driving risks and checks to request before paying for parts.
- P0172 code: what a rich mixture means for your carP0172 means the engine is running too rich on Bank 1. Learn what fuel smell and black smoke mean, when to stop driving and which checks come first.
- P0174 code meaning: What one lean bank or both banks tell youP0174 means Bank 2 is running lean. Learn how P0171 changes the diagnosis, when you can drive, and what to ask your shop before replacing parts.
- P0175 code causes: Finding a rich mixture on bank 2P0175 means bank 2 needs excessive fuel correction. Learn bank-specific causes, LPG/CNG tuning issues, driving advice and what to ask your repair shop.
- P0299 code: Why your car loses power on mountain climbsThe P0299 code means turbo or supercharger underboost. Learn why power drops on climbs, what to check first, and when to book a diagnosis in Georgia.
Hybrid system
- P0A7F code: An early warning about hybrid battery healthThe P0A7F code signals hybrid battery deterioration. Learn how urgent it is, what cooling checks matter, and what to check before buying a used hybrid.
- P0A80 code meaning for Prius and Lexus hybrid ownersP0A80 means your hybrid battery has deteriorated. Learn what a shop should test, when to stop driving, and how repair and replacement options compare.
Ignition and misfires
- P0300: random or multiple cylinder misfire detectedP0300 means random/multiple cylinder misfire detected. Why several cylinders misfire, what to do if the light flashes, and what a shop checks.
- P0301 code causes and how a shop tests cylinder 1P0301 means cylinder 1 is misfiring. Learn the common causes, how a coil or plug swap test works, and when you should stop driving and ask a shop.
- P0302 code: What to check when the misfire stays on cylinder 2P0302 code means cylinder 2 is misfiring. Learn what injector, compression and leak-down tests reveal when swapping the plug and coil does not help.
- P0303 code meaning when your engine misfires in damp weatherP0303 means cylinder 3 is misfiring. Learn why damp mornings or a car wash can trigger it, when to stop driving, and what your shop should check.
- P0304 code causes when your engine misfires under loadP0304 code causes include weak coils, worn plugs and injector faults. Learn why climbs trigger misfires, when to stop and what to ask your shop.
- P0305 code: what a cylinder 5 misfire means for youThe P0305 code means cylinder 5 is misfiring. Learn when to stop driving, how shops diagnose it, and why engine access can drive repair costs.
- P0306 code meaning when your car misfires on gas or petrolP0306 means cylinder 6 is misfiring. Learn why it can happen on LPG, CNG or petrol, when to stop driving, and what your shop should test before repairs.
- P0351 code causes and how to check the coil circuitP0351 usually points to an ignition coil circuit fault. Learn likely causes, matching misfires, safe next steps and what to ask before replacing parts.
Network and communication
Sensors and valve timing
- P0011 code: Why you should check the oil firstP0011 means camshaft timing is too advanced or not responding correctly. Learn why checking the oil comes first, when to stop, and what to ask a shop.
- P0014 code meaning when your engine rattles at cold startP0014 means camshaft timing is too far advanced or not responding correctly. Learn what a cold-start rattle means and what to ask your repair shop.
- P0016 code causes and what the timing warning meansP0016 can signal timing chain, belt, sensor or VVT faults. Learn when to stop driving, what shops should check and what to ask before buying a used car.
- P0325 code causes: telling real knock from a circuit faultP0325 usually points to a knock sensor circuit fault, not bad fuel alone. Learn the causes, driving risks and checks to request before buying parts.
- P0335 code: Why your car stalls hot and starts after coolingP0335 points to a crankshaft position signal fault. Learn why your car stalls hot, when to stop driving, and what to ask a shop before replacing parts.
- P0340 code meaning when your engine takes longer to startP0340 means a camshaft sensor circuit fault that can cause long cranking. Learn what to check, when to drive and how a shop rules out timing problems.
Speed and idle
- P0500 code: Why your speedometer jumps and shifting changesP0500 means a vehicle speed signal fault. Learn why your speedometer jumps, shifting changes, and what a shop should check before replacing parts.
- P0505 code meaning after a battery change or throttle cleaningP0505 means an idle control malfunction. Learn why idle can change after battery work or throttle cleaning, when to drive, and what to ask your shop.
- P0507 code causes and checks for high or surging idleP0507 means your engine is idling higher than expected. Learn likely causes, safe driving advice and the checks to request before paying for parts.
- P0562 code: Is it the battery or the alternator?The P0562 code means low system voltage. Learn how battery, alternator and connection tests find the cause, and what to ask your shop before repairs.
Transmission
- P0700 code meaning when your reader shows nothing elseP0700 points to a fault stored in the transmission module. Learn what scan you need, when to stop driving, and what to ask before paying for parts.
- P0715 code causes: why your transmission shifts harshlyP0715 code causes include wiring faults, a failed speed sensor or transmission problems. Learn what harsh shifts mean and what to ask your shop.
- P0730 code: what slipping gears mean for your transmissionThe P0730 code means incorrect gear ratio. Learn when to stop driving, why fluid checks come first, and what affects transmission repair costs.
- P0741 code meaning: why highway driving can create extra heatP0741 means the torque converter clutch is not locking as expected. Learn why fuel use and heat rise, when to stop, and what to ask your shop.