P0421 – Warm Up Catalyst Efficiency Below Threshold (Bank 1)

Meaning:

When the downstream O2 sensor on bank 1 has detected that the converter is not working properly, the P0421 happens
The P0421 is triggered when the downstream O2 sensor on bank 1 has detected that the converter is not working properly

Downstream O2 sensor on bank 1 has detected that the converter is not working according to specification.

Main Symptoms:

Possible Causes:

Diagnostic Steps:

  • Measure voltage of O2 sensor (both) and ensure they match the specs. If not replace as necessary
  • Visually inspect catalytic converters and check for red fumes (when car is running)
  • Smell fumes and try to find traces of excessive fuel
  • If the latter two are present replace catalytic converter

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