P0421 OBD-II: Warm Up Catalyst Efficiency Below Threshold
P0421 Severity
Should you fix P0421 yourself?
Identifying an OBD code is just the start of a complete diagnostic routine. Most trouble codes are caused by a sensor reporting a value that is out of range. Just replacing a sensor may not fix underlying issues. Be sure to understand the code, the part or system in your car that the code refers to, and the risks of delaying professional repair.
Proper diagnosis will save you time and money on unnecessary repairs.
Urgent codes indicate immediate action is necessary. These are urgent and significant malfunctions that can cause severe damage to the vehicle or harm the driver and passengers.
What causes OBD-II fault code P0421?
The Catalytic Converter takes the exhaust gases left over by the combustion process and converts those gases to harmless emissions. The engine control module (PCM) compares the front and rear oxygen sensors to each other to determine catalytic converter efficiency. If the input from the rear oxygen sensor starts to match the front oxygen sensor, the PCM will set code P0421.Catalytic converters are covered under federally mandated 8yr/80,000 mile warranty by the vehicle's manufacturer. Catalytic converters usually fail as a result of something else (engine misfire, internal oil leak, or internal coolant leak) not being repaired. The original cause of the failure is usually not covered under warranty.
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