P0420
Catalyst System Efficiency Below Threshold (Bank 1)
Your car's computer thinks the catalytic converter on the engine's first bank isn't cleaning exhaust gases as well as it should, which means it's not working efficiently.
The powertrain control module (PCM) has detected that the catalytic converter system for Bank 1 is not operating at the required efficiency.
Common causes
- high
Failing Catalytic Converter
This is the most common cause, especially on older vehicles or those with high mileage, indicating the catalyst material is degraded.
Typical part cost: $500-2500+
- medium
Oxygen Sensor (O2 Sensor) Malfunction
A faulty upstream or downstream O2 sensor can send incorrect readings, leading the PCM to falsely believe the converter is inefficient.
Typical part cost: $100-300
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First things to check
- 1Scan for other related codes (misfires, O2 sensor codes).
- 2Visually inspect exhaust system for leaks, damage, or loose connections.
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Related codes
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Frequently asked questions
Can I just clear the P0420 code?
Clearing the code will turn off the Check Engine Light temporarily, but if the underlying problem isn't fixed, the code will return. It's a diagnostic indicator, not the problem itself.
Is a P0420 code always a bad catalytic converter?
No, while it's a common cause, it's not always the case. Faulty oxygen sensors, exhaust leaks, or even engine performance issues can trigger this code. Proper diagnosis is crucial before replacing the converter.
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