What are the common causes of P0421?

A worn or contaminated catalytic converter on bank 1. A lazy upstream oxygen sensor on bank 1. An exhaust leak ahead of the bank 1 converter. A misfire or rich condition damaging the converter. Oil or coolant burning through the bank 1 converter.

Compare upstream and downstream O2 sensor switching on bank 1. Check for exhaust leaks before and after the converter. Check misfire counters and fuel trims. Inspect the converter for contamination or damage. Replace the bank 1 converter if efficiency stays low. Clear the code and retest.

P0421

Catalyst 1 Efficiency Below Threshold Bank 1

System: Auxiliary emission controls (P code)
Lamp: Check Engine Light
Category: Powertrain - Auxiliary Emissions Controls

What it means

P0421 means Catalyst 1 Efficiency Below Threshold Bank 1.

Check these first

A worn or contaminated catalytic converter on bank 1
A lazy upstream oxygen sensor on bank 1
An exhaust leak ahead of the bank 1 converter
A misfire or rich condition damaging the converter
Oil or coolant burning through the bank 1 converter

What to do next

Compare upstream and downstream O2 sensor switching on bank 1
Check for exhaust leaks before and after the converter
Check misfire counters and fuel trims
Inspect the converter for contamination or damage
Replace the bank 1 converter if efficiency stays low
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.