P0420
Catalyst System Efficiency Below Threshold (Bank 1)
medium
i Description
The catalytic converter on Bank 1 is not working efficiently enough to reduce exhaust pollutants. The ECU compares upstream and downstream O2 sensor signals — if they look too similar, the catalyst is not doing its job.
? Common Causes
- Worn-out catalytic converter
- Engine running rich or lean (damaging catalyst over time)
- Contaminated catalyst (oil, coolant, or fuel additive)
- Faulty downstream O2 sensor
- Exhaust leak near O2 sensors
! Symptoms
- Check engine light
- Failed emissions test
- Reduced fuel economy
- Slight sulphur smell
- Usually no noticeable driveability change
# Diagnostic Steps
- Check for other DTCs that might be the root cause (misfires, fuel trim issues)
- Compare upstream and downstream O2 sensor waveforms
- Check for exhaust leaks
- Test downstream O2 sensor response
- If catalyst is confirmed failed, replace it
⚙ Typical Fix
Replace the catalytic converter. Fix any underlying issues (misfires, fuel trim) that caused premature failure.
Parts Needed
Catalytic converter, Gaskets
Estimated Cost
$500-$2500
↔ Related Codes
Q Frequently Asked Questions
Yes, the car will drive normally. The catalyst is not performing optimally for emissions but it will not damage the engine. You will fail an emissions test however.
Catalytic converters are expensive: $500-$2500 including labour. Aftermarket cats are cheaper ($300-$800) but may not meet all emissions standards. Always fix the root cause first.
Yes — always check the downstream O2 sensor before replacing the catalyst. A sluggish sensor can falsely trigger P0420. An O2 sensor is $100-$200 versus $500+ for a cat.
☞ Learn More
- What is OBD-II? — Complete guide to your car's diagnostic system
- How to Read DTC Codes — Step-by-step with a Bluetooth scanner
- OBD-II Scanner Buying Guide — Find the best scanner for your car