Catalyst System Efficiency Below Threshold (Bank 1)
The catalytic converter on Bank 1 is not efficiently converting exhaust gases as expected. The oxygen sensors detect that the converter's catalyst material has degraded or failed, reducing its ability to reduce harmful emissions.
What You Might Notice
- Check Engine Light (MIL) illuminated
- Reduced fuel economy
- Rotten egg smell from exhaust
- Exhaust restriction causing reduced power
- Rough idle or hesitation during acceleration
Most Common Causes
- 1
Failed catalytic converter
The internal catalyst material has broken down or the honeycomb structure has collapsed, preventing proper chemical conversion of exhaust gases. This is the most common cause of P0420.
- 2
Faulty oxygen sensor (downstream)
The post-catalytic converter oxygen sensor may be reading incorrectly, causing the PCM to think the converter is not working when it actually is functioning properly.
- 3
Engine running too rich
Excessive fuel in the exhaust due to a failed fuel injector, bad air filter, or faulty mass airflow sensor can overheat and damage the catalytic converter.
- 4
Exhaust leaks before converter
Cracks or loose connections in the exhaust manifold or pipes before the catalytic converter allow unburned gases to bypass the converter, affecting efficiency readings.
How to Diagnose It
- 1
Check Failed catalytic converter — The internal catalyst material has broken down or the honeycomb structure has collapsed, preventing proper chemical conversion of exhaust gases. This is the most common cause of P0420.
- 2
Check Faulty oxygen sensor (downstream) — The post-catalytic converter oxygen sensor may be reading incorrectly, causing the PCM to think the converter is not working when it actually is functioning properly.
- 3
Check Engine running too rich — Excessive fuel in the exhaust due to a failed fuel injector, bad air filter, or faulty mass airflow sensor can overheat and damage the catalytic converter.
- 4
Check Exhaust leaks before converter — Cracks or loose connections in the exhaust manifold or pipes before the catalytic converter allow unburned gases to bypass the converter, affecting efficiency readings.
How to Fix It
- 1
Replace catalytic converter
Remove the failed catalytic converter and install a new OEM or quality aftermarket replacement. This addresses the root cause if the converter substrate has deteriorated.
- 2
Replace downstream oxygen sensor
Test and replace the post-catalytic converter oxygen sensor (O2 sensor) if it is providing faulty readings to the PCM. This sensor is critical for monitoring converter efficiency.
- 3
Inspect and repair exhaust leaks
Check the exhaust manifold, pipes, and connections for cracks or loose fasteners. Seal or repair any leaks found before the catalytic converter to prevent unburned gases from bypassing the converter.
- 4
Service fuel system and air intake
Replace air filter, inspect fuel injectors for leaking, and test the mass airflow sensor to ensure the engine is not running excessively rich and damaging the converter.
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