Catalytic converter
The catalyst and its efficiency. The most deceptive conclusion in the whole reference: the unit does not measure the state of the catalyst directly — it compares two oxygen sensors and draws a conclusion. So the culprit can be a sensor, an exhaust leak, or even the driving pattern.
4 codes in this section, 4 with a guide
What it usually turns out to be
- P0420 turns out to be the rear sensor or an exhaust leak more often than a dead catalyst.
- Short trips kill a catalyst faster than mileage: it never reaches working temperature.
- A warm-up code (P0421, P0431) is about the first minutes after starting, not about a worn-out core.
P0420Catalyst system efficiency below threshold (bank 1)guideP0421Warm-up catalyst efficiency below limit (bank 1)guideP0430Catalyst efficiency below limit (bank 2)guideP0431Warm-up catalyst efficiency below limit (bank 2)guide
Read about this group
Other sections
Fuel and air metering, general 33Intake measuring sensors 17Oxygen sensors 19Throttle and accelerator pedal 15Mixture: lean and rich 11Fuel and air metering: injectors 25Ignition system and misfires 29Exhaust gas recirculation (EGR) 7Secondary air injection 3Canister and purge valves 9Fuel vapour leaks 7Fuel level in the tank 4Diesel particulate filter 6Idle and auxiliary systems 22Control module and its circuits 21Transmission 17Communication between modules 20Brakes, ABS and stability control 17Body, airbags and lighting 12