An OBD2 trouble code (DTC — Diagnostic Trouble Code) is your car’s way of flagging a problem it detected. The format is always a letter (P/B/C/U) plus four digits. Here are the 15 codes mechanics see most often and what to do about each.
Look up any code instantly with the OBD2 Code Lookup tool. To read codes while you drive — before they trigger a warning light — see the OBD2 Pro HUD.
Powertrain (P) codes — the most common
- P0300 — Random/multiple cylinder misfire. Severity: high. Causes: worn spark plugs, faulty ignition coil, bad fuel injector. Fix plugs and coils first.
- P0301–P0308 — Cylinder N misfire. Same as P0300 but identifies the specific cylinder. Easier to diagnose: swap the coil on that cylinder first.
- P0420 — Catalyst system efficiency below threshold (Bank 1). Severity: medium. Causes: aging catalytic converter, O2 sensor fault, exhaust leak. Often lasts months before causing drivability issues.
- P0430 — Catalyst system efficiency below threshold (Bank 2). Same as P0420 but for the second bank on V6/V8 engines.
- P0171 — System too lean (Bank 1). Severity: medium. Causes: vacuum leak, dirty MAF sensor, weak fuel pump. Fix vacuum leaks first — they’re free.
- P0174 — System too lean (Bank 2). As above on Bank 2. Often occurs alongside P0171 on V-engines.
- P0101 — MAF sensor range/performance. Severity: medium. Causes: dirty or failed mass airflow sensor. Clean the sensor with MAF cleaner spray before replacing.
- P0128 — Coolant temperature below thermostat regulating temperature. Severity: low/medium. Cause: failed thermostat stuck open. Engine runs rich until it warms up; fuel economy drops.
- P0401 — EGR flow insufficient. Severity: low-medium. Cause: clogged EGR valve or passages. Often cleanable without replacement.
- P0455 — EVAP system large leak detected. Severity: low. Very common cause: loose or cracked fuel cap. Tighten or replace the fuel cap first.
- P0442 — EVAP system small leak detected. As above. Check fuel cap, then EVAP hoses and purge valve.
- P0507 — Idle control system RPM high. Severity: low. Causes: vacuum leak, dirty throttle body. Clean the throttle body with aerosol cleaner.
- P0340 — Camshaft position sensor circuit malfunction. Severity: high. Can cause no-start. Replace the sensor; check the reluctor ring.
- P0011 — Camshaft position ‘A’ timing — over-advanced (Bank 1). Common on VVT engines with low or dirty oil. Change oil first.
- P0562 — System voltage low. Severity: medium. Causes: weak battery, bad alternator, loose connections. Test the battery and charging system.
Other code families
- B (Body) — airbag, climate, power windows. Usually shop-only.
- C (Chassis) — ABS, traction control, suspension.
- U (Network) — communication faults between modules. Often secondary codes; fix the primary first.
Read codes early with an OBD2 HUD
Most fault codes are stored in the ECU for days or weeks before triggering the check-engine light. An OBD2 HUD shows live coolant temperature, voltage, and alerts you to pending codes while you drive — letting you catch small problems before they become expensive ones.
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Frequently asked questions
What is the most common OBD2 code?
P0420 (catalyst efficiency below threshold) and P0300 (random misfire) are consistently the two most common codes worldwide. P0455 (loose fuel cap / EVAP leak) is very common and often the cheapest fix — check your fuel cap first.
Can I drive with an OBD2 trouble code?
It depends on the code. P0455 (loose fuel cap) is safe to drive with temporarily. P0300 (misfire) or P0340 (camshaft sensor) should be addressed immediately as they can damage the engine or catalytic converter. When in doubt, look up the specific code.
How do I clear OBD2 codes without a scanner?
You can disconnect the car battery for 15–30 minutes to clear codes, but this also resets the emissions readiness monitors, which may cause a fail at your next smog check. A proper OBD2 scanner lets you clear codes while preserving monitor data.