Decode Your Car Diagnostic Code
Free step-by-step OBD-II diagnostic trouble code guides for Powertrain, Body, Chassis & Network systems
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P0402
The engine control module (ECM) has detected that more exhaust gas is being recirculated than the system expects. This usually means the EGR valve is stuck open, a vacuum leak, or a faulty flow sensor, causing too much exhaust to re‑enter the intake.
P0411
The engine control module (ECM) has detected that the secondary‑air injection (SAI) system is delivering too little or too much air compared with the PCM’s command. The pump, valve, wiring, or sensor data are usually at fault.
P0451
The engine control module has detected that the EVAP‑system pressure sensor is reporting values outside its calibrated range. The cause is usually a faulty sensor, damaged wiring, or an over‑pressurized EVAP system.
P0420
The P0420 code means your catalytic converter is not cleaning exhaust gases as efficiently as it should. The downstream oxygen sensor detects that the catalyst is underperforming compared to the upstream sensor readings.