Quick answer: Code P0420 means “Catalyst System Efficiency Below Threshold (Bank 1)”: the engine computer thinks the catalytic converter isn’t cleaning the exhaust well enough. The fix can be as cheap as $150–$400 for an oxygen sensor or exhaust leak repair, or $600–$2,500+ if the catalytic converter needs replacing.
What P0420 means
Your car has oxygen (O2) sensors before and after the catalytic converter. A working converter smooths out the oxygen readings, so the downstream sensor signal stays steady. When the downstream sensor starts to look like the upstream one, the computer sets P0420 (or P0430 for Bank 2 on V6 and V8 engines).
Common causes and repair costs
Typical independent shop prices, October 2026.
| Cause | How common | Typical repair cost |
|---|---|---|
| Worn or failing catalytic converter | Very common on older, high-mileage cars | $600 – $2,500+ |
| Faulty downstream (rear) O2 sensor | Common | $150 – $400 |
| Faulty upstream O2 / air-fuel sensor | Common | $200 – $500 |
| Exhaust leak before or near the sensors | Common | $100 – $500 |
| Engine misfires (spark plugs, coils) | Common root cause; see P0300 | $150 – $600 |
| Engine burning oil or coolant | Less common, serious | $1,000+ (engine repair) |
| Outdated engine software | Occasional | $0 – $150 (dealer update) |
See full converter prices in Catalytic Converter Replacement Cost.
Symptoms that come with P0420
- Check engine light (often the only symptom).
- Failed emissions or smog test.
- Rotten egg smell from the exhaust.
- Reduced performance or fuel economy if the converter is clogged.
Is it safe to drive with P0420?
Usually yes for the short term, if the car runs normally and the check engine light isn’t flashing. A flashing light means a severe misfire that can overheat and destroy the converter: stop driving and get it checked. Long term, P0420 will fail an emissions test and can get worse.
What to check before replacing the converter
- Scan for other codes: misfire (P0300–P0308) or fuel trim codes (P0171, P0174) point to the real cause.
- Check for exhaust leaks near the manifold and sensors.
- Look at the O2 sensor data: a lazy rear sensor can mimic a bad converter.
- Check for software updates or technical service bulletins for your model.
- Check your emissions warranty: federal law covers catalytic converters for 8 years or 80,000 miles on most cars.
Can you clear P0420 and pass inspection?
Clearing the code turns the light off, but the readiness monitors reset and the car usually can’t pass an emissions test until it has been driven enough to complete them, and the code often returns. Additives and “O2 sensor spacers” rarely fix the real problem and may be illegal in some states.
Is it worth fixing?
Yes, on most cars. If the car is older and needs a converter plus other expensive repairs, compare with the Repair or Replace Calculator.
Frequently asked questions
Can a bad O2 sensor cause P0420?
Yes. A slow or failing downstream sensor is a common cause and much cheaper to fix than the converter.
Will a new catalytic converter fix P0420?
Only if the converter is the problem and the root cause (misfires, oil burning, leaks) has been fixed.