Owner-help topic
OBD codes and emissions: 12 decision guides.
Read common diagnostic trouble codes as test directions instead of automatic instructions to replace the named component. The OBD codes and emissions desk holds 12 of the site's 126 ownership guides: 10 rated soon, 1 rated routine, 1 rated depends. A reader in a hurry can jump straight to the escalation rule. One with more time can work through the evidence first, because each guide follows the same five-section sequence: what to check, the safe action sequence, what not to do, the escalation rule, and sourced references. 14 related owner-help systems covering other symptom groups are linked below for the adjacent conditions this one is most often confused with, and each guide's own source and reviewed date sit beside its escalation rule.
See the complete ownership guides desk
| Guide | Urgency | First check |
|---|---|---|
| P0300 random or multiple-cylinder misfire code | Depends | Whether the engine light flashes under load |
| P0171 or P0174 system-too-lean codes | Soon | One bank versus both banks |
| P0420 catalyst-efficiency code | Soon | Misfire or fuel-trim codes that occurred first |
| P0455 large EVAP leak code | Soon | Cap seal and correct cap type |
| P0456 very small EVAP leak code | Soon | Cap gasket for cuts or hardening |
| P0128 coolant temperature below regulating range | Soon | Cold coolant level and leak history |
| P0299 turbo or supercharger underboost code | Soon | Oil level, smoke and turbo noise |
| P0101 mass-airflow performance code | Soon | Air filter and intake duct sealing |
| P0011 camshaft timing over-advanced code | Soon | Oil level, grade, condition and pressure history |
| P0401 insufficient EGR flow code | Soon | EGR command versus measured response |
| P0507 idle speed higher than expected | Soon | Idle speed cold and fully warm |
| Emissions readiness monitors show Not Ready | Routine | Which specific monitors remain incomplete |
P0300 random or multiple-cylinder misfire code
P0300 means the computer detected crankshaft-speed changes consistent with misfire across more than one cylinder; it does not identify one failed part. Ignition, fuel delivery, unmetered air, compression and contaminated fuel all remain possible.
P0171 or P0174 system-too-lean codes
A lean code means fuel correction reached a calibrated limit; it does not automatically mean an oxygen sensor is bad. Air leaks, low fuel delivery, biased airflow measurement, exhaust leaks and sensor errors must be separated with fuel-trim behavior.
P0420 catalyst-efficiency code
P0420 means catalyst monitoring fell below its calibrated threshold. An aged converter is possible, but exhaust leaks, misfire, fuel-control faults, oil burning and biased sensors can damage or imitate a converter problem.
P0455 large EVAP leak code
P0455 indicates the evaporative-emissions system could not hold the expected pressure or vacuum. A loose cap can cause it, but disconnected hoses, a stuck vent valve, damaged filler sealing and canister faults also qualify.
P0456 very small EVAP leak code
P0456 is a small sealing fault that may require a controlled smoke or pressure-decay test. The fuel cap is one possibility, but tiny hose cracks, valve leakage and filler-neck sealing are common diagnostic paths.
P0128 coolant temperature below regulating range
P0128 usually means the engine did not warm as quickly or as fully as expected. A thermostat stuck open is common, but low coolant, a biased temperature sensor and cooling-fan control can change the warm-up curve.
P0299 turbo or supercharger underboost code
P0299 means measured boost did not reach the commanded threshold. Charge-air leaks, control-solenoid or actuator faults, exhaust restrictions, sensor bias and turbocharger damage must be separated with commanded-versus-actual data.
P0101 mass-airflow performance code
P0101 means airflow measurement does not agree with the model’s expected range; it does not prove the sensor itself failed. Intake leaks, contamination, restricted air, incorrect parts and engine mechanical faults can distort the calculation.
P0011 camshaft timing over-advanced code
P0011 means commanded and measured cam timing did not agree in the stated bank. Oil level or viscosity, sludge, a control solenoid, wiring, phaser wear and base mechanical timing all require evaluation.
P0401 insufficient EGR flow code
P0401 means the expected exhaust-gas-recirculation effect was not detected. Carbon blockage, a valve or cooler fault, pressure-sensor passages and control circuits can all produce the same result.
P0507 idle speed higher than expected
P0507 means actual idle stayed above the commanded range. Unmetered air, a sticking throttle, incorrect relearn, purge flow, PCV faults and cable interference are more useful starting points than an idle-speed screw.
Emissions readiness monitors show Not Ready
Not Ready means the onboard test has not completed since codes were cleared, power was lost or enabling conditions were last met. It does not by itself prove a defect, though a monitor that never completes may have one.
Browse adjacent systems