P0421 - OBD-II Trouble Code
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Our emissions expert has put together the following information about the P0421 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.
OBD II Fault Code
- OBD II P0421
Fault Code Definition
Code P0421 is triggered when the Bank 1 Warm Up Catalytic Converter's emissions reducing efficiency has fallen below the minimum allowable threshold.
- Check Engine Light will illuminate
- In many cases, no abnormal symptoms may be noticed
- In some cases, there may be a noticeable lack of engine power
Common Problems That Trigger the P0421 Code
- Defective Warm Up Catalytic Converter
- Engine misfires have damaged the Catalytic Converter
- Oxygen Sensor(s) are replaced when the real problem is a damaged Warm Up Catalyst
- Warm Up Catalyst is replaced when the real problem is internal engine damage, which is producing elevated emissions levels
- Warm Up Catalyst is replaced when a misfiring engine is the real problem