Applicable Systems
OBD-II
Symptoms
Catalytic converter efficiency below threshold; post-cat O2 sensor switching too similar to pre-cat; emissions test failure.
Possible Causes
- Catalytic converter degraded (substrate meltdown)
- Pre-cat O2 sensor lazy (false reading)
- Exhaust leak before pre-cat sensor
Troubleshooting Steps
- Step 1: Compare pre-cat and post-cat O2 sensor switching
Post-cat should be steady, not switching like pre-cat - Step 2: Check for exhaust leaks before catalytic converter
No leaks found - Step 3: Verify pre-cat O2 sensor is not lazy
Pre-cat sensor switches normally - Step 4: If cat efficiency confirmed low, replace catalytic converter
Post-cat O2 steady after new cat - Step 5: Clear code and verify with drive cycle
No P0420 return after drive cycle
How to Reset
Clear with OBD-II scanner; requires complete drive cycle (incl. highway driving) to verify.
Common Replacement Parts
- Catalytic Converter Bank 1 (OEM spec)
Frequently Asked Questions
- Q: Can bad O2 sensor cause false P0420?
Yes. A lazy pre-cat O2 sensor can cause P0420 because post-cat sensor looks similar to slow pre-cat. Always verify O2 sensors before replacing catalytic converter.
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Source: SAE J2012 standard DTC list + OBD-II portal
Category: OBD-II
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