Catalyst System Efficiency Below Threshold (Bank 1)
What does P0420 mean?
The P0420 code means the vehicle's onboard diagnostic system has detected: Catalyst System Efficiency Below Threshold (Bank 1). This is a moderate severity code.
Common Symptoms
- Check Engine Light on (steady)
- Slight reduction in fuel economy
- Sulfur or rotten-egg smell from exhaust in some cases
- May fail emissions testing
Probable Causes (Ranked by Likelihood)
- Worn or failed catalytic converter Very High
- Faulty downstream (post-cat) oxygen sensor High
- Engine oil consumption fouling the cat Moderate
- Coolant leak into combustion chamber Moderate
- Rich-running condition burning the catalyst Moderate
- Exhaust leaks before or after the cat Low
Step-by-Step Diagnostic Procedure
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Check freeze frame data -- if the code set under steady highway conditions, the cat is more likely the culprit. Under mixed driving, suspect a sensor.
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Compare upstream (pre-cat) and downstream (post-cat) O2 sensor waveforms with a scan tool. Downstream should show a slow, lazy signal (0.1-0.9V with few switches). If it mirrors upstream switching, the cat is not storing oxygen properly.
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Inspect for exhaust leaks upstream of the downstream sensor -- a leak can pull in fresh air and falsely indicate a bad cat.
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Check for oil fouling: blue smoke, excessive oil consumption, or a milky oily residue inside the tailpipe.
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Perform a temperature test: the cat outlet temperature should be 10-100 degrees F higher than the inlet. No rise indicates a dead cat.
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If the downstream O2 sensor is original and over 100,000 miles old, replace it before replacing the cat -- a sluggish sensor mimics a failed catalyst.
Frequently Asked Questions
Will P0420 cause my car to fail emissions?
Yes. In most OBD-II emissions states, any stored fault code with the MIL illuminated results in an automatic failure, regardless of tailpipe emissions results.
Can a bad O2 sensor cause P0420?
Yes -- a slow or lazy downstream oxygen sensor mimics the behavior of a failed catalytic converter. Always test or replace the downstream O2 sensor before purchasing a catalytic converter.
Does using premium gasoline fix P0420?
No. Fuel grade does not affect catalytic converter efficiency. Additives marketed as 'cat cleaners' have no proven effect on a mechanically failed converter.