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Check Engine Light: Top 10 Codes & What They Actually Mean
These are common generic OBD-II codes. Exact repair varies by vehicle. We confirm with live data, smoke/pressure tests, and OEM procedures.
| Code | Plain-English Meaning | Common Causes / Next Steps |
|---|---|---|
| P0300 | Random misfire | Ignition (plugs/coils), fuel, vacuum leaks. If flashing, stop driving. |
| P0420 | Catalyst efficiency low (Bank 1) | Exhaust leaks, O2 sensors, aging catalyst. Verify fuel trims & misfire history. |
| P0171 | System too lean (Bank 1) | Vacuum/intake leaks, MAF contamination, low fuel pressure. Smoke test, MAF data. |
| P0442 | EVAP small leak | Loose gas cap, hose leaks, purge/vent valves. EVAP smoke test. |
| P0128 | Coolant temp below thermostat regulating temp | Stuck-open thermostat, low coolant, sensor. Check warm-up profile. |
| P0456 | EVAP very small leak | Cap/hoses/charcoal canister. Needs smoke test for pinpointing. |
| P0133 | O2 sensor slow response | Aging sensor, exhaust leaks upstream, wiring. Verify with live graphs. |
| P0101 | MAF range/performance | Dirty MAF, intake leaks, wrong air filter. Clean/verify grams/sec vs. RPM. |
| P0113 | IAT sensor high input | Unplugged sensor, wiring fault. Inspect connector/voltage. |
| P0507 | Idle control RPM higher than expected | Vacuum leak, dirty throttle body, PCV issues. Smoke test & throttle service. |
