P0420

PowertrainCatalysthigh severity

Catalyst System Efficiency Below Threshold Bank 1

What P0420 Means

The Bank 1 catalytic converter is not performing efficiently. The downstream O2 sensor is mirroring the upstream O2 sensor, indicating the catalyst is not storing and releasing oxygen properly.

Symptoms

Check Engine Light
Failed emissions test
Possible sulfur smell
Reduced fuel economy

Common Causes

Worn catalytic converter (most common)
Exhaust leak before downstream O2 sensor
Faulty downstream O2 sensor
Engine misfire damaging catalyst
Oil burning contaminating catalyst
Using leaded fuel or certain fuel additives

Diagnostic Steps

Compare upstream and downstream O2 sensor waveforms (downstream should be flat/steady)
Check for exhaust leaks
Test downstream O2 sensor response
Check for engine misfire codes
Inspect for oil consumption
Perform catalyst efficiency test with scan tool

Where to start: If catalyst is truly failed, replacement is needed. First rule out exhaust leaks and bad downstream O2 sensor. Aftermarket catalysts may not last as long as OEM.

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