The most common OBD2 codes account for the majority of check engine lights in the US: a loose gas cap, a lean condition, a misfire or a tired catalytic converter. Knowing what each code really means saves money — some are a five-minute fix, others need a shop. Below are the codes you are most likely to see, what usually causes them and what to check first. You can read them yourself with any ELM327 adapter and our free online OBD2 scanner.
How to read an OBD2 code
P 0 4 2 0
│ │ │ └┴─ specific fault number
│ │ └──── subsystem (4 = emissions control)
│ └────── 0 = generic SAE code, 1 = manufacturer-specific
└──────── P = powertrain, B = body, C = chassis, U = networkThe most common OBD2 codes
| Code | Meaning | Common causes |
|---|---|---|
| P0420 | Catalyst efficiency below threshold, bank 1 | aging catalytic converter, faulty downstream O2 sensor, exhaust leak |
| P0300 | Random / multiple cylinder misfire | worn spark plugs, ignition coils, vacuum leak, low fuel pressure |
| P0301–P0308 | Misfire in cylinder 1–8 | coil or plug on that cylinder, injector |
| P0171 / P0174 | System too lean, bank 1 / bank 2 | vacuum leak, dirty MAF sensor, weak fuel pump |
| P0442 | EVAP small leak | loose or worn gas cap, cracked EVAP hose |
| P0455 | EVAP large leak | gas cap missing or loose, purge valve |
| P0128 | Coolant below thermostat regulating temperature | thermostat stuck open, coolant temp sensor |
| P0401 | EGR flow insufficient | carbon-clogged EGR valve or passages |
| P0135 | O2 sensor heater circuit, bank 1 sensor 1 | failed upstream O2 sensor heater, fuse |
| P0101 | MAF sensor range/performance | dirty MAF, air leak after the sensor |
| P0507 | Idle speed higher than expected | vacuum leak, dirty throttle body |
| P0700 | Transmission control system fault | read the transmission module for the detailed code |
What to check first
P0420 — catalytic converter
Before replacing an expensive converter, look at the live data. If the downstream O2 sensor switches as fast as the upstream one, the converter is not storing oxygen. If it is steady, the sensor or an exhaust leak may be the real problem. Misfires and lean conditions damage the converter, so fix those codes first.
P0300 — random misfire
Check spark plugs and coils first, then look for vacuum leaks. Live data and freeze frame show the RPM and load when the misfire happened: misfires only at idle often mean a vacuum leak; misfires under load point to ignition.
P0171 — system too lean
Look at fuel trims in live data. If long-term trim is high at idle but normal at 2500 RPM, the cause is almost always a vacuum leak. If it is high everywhere, suspect the MAF sensor or fuel delivery.
P0442 / P0455 — EVAP leak
Tighten or replace the gas cap, clear the code, and drive for a few days. If it returns, have the EVAP system smoke-tested.
Use freeze frame and readiness
When a code is stored, the ECU saves a freeze frame: RPM, load, coolant temperature, fuel trims and speed at that moment. It is the best clue to when the fault happens. Readiness monitors tell you whether the car will pass an emissions test; they reset when you clear codes. Our ELM327 online guide shows how to read both.
Free online OBD2 scanner
Plug in an ELM327 adapter, open the page in Chrome or Edge and read codes, freeze frame and live data. No app, no account.
Read your codes nowFAQ
Can I drive with the check engine light on?
If it is steady, usually yes for a short time, but get it checked. If it is flashing, a severe misfire is happening — reduce load and stop as soon as it is safe, because the catalytic converter can overheat.
Does a loose gas cap really turn on the light?
Yes. P0442 and P0455 are among the most frequent codes, and a cap that does not seal is the number-one cause.
What does “pending code” mean?
The ECU saw the fault once but has not confirmed it on a second drive cycle. Pending codes do not always light the MIL, but they show what may come next.
Sources: OBD-II PIDs (Wikipedia).
