P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
Drive with caution — fix soon.
What P0420 means
P0420 means the computer compared the two oxygen sensors around your catalytic converter and decided the converter isn't reducing pollution efficiently enough. It doesn't always mean the converter is dead; a faulty sensor, an exhaust leak, or a separate engine problem can fool the system. A proper diagnosis checks the sensors and exhaust before condemning the expensive converter.
Common symptoms
- Check engine light is on, often steady
- Slightly reduced fuel economy
- Failed emissions or smog test
- Faint rotten-egg (sulfur) smell from the exhaust
- No obvious change in how the car drives
Common causes
- Aging or contaminated catalytic converter
- Faulty downstream oxygen sensor (Bank 1, Sensor 2)
- Faulty upstream oxygen sensor skewing readings
- Exhaust leak before or near the converter
- Engine misfire or oil/coolant contaminating the converter
- Rich or lean fuel mixture from another fault
Typical repair cost
$200 – $2200
Varies by vehicle and root cause.
Frequently asked questions
Can I drive with a P0420 code?
Yes, short-term driving is generally safe and the car runs normally. You'll fail emissions and could lose a little fuel economy, so get it checked within a few weeks before it worsens.
How much does it cost to fix P0420?
It ranges widely: an oxygen sensor or exhaust leak repair can be $200-$500, while replacing the catalytic converter often runs $900-$2,200 depending on the vehicle and whether you use OEM parts.
Is P0420 serious?
It's moderate. It won't strand you, but it signals reduced emissions performance and can lead to a costly converter replacement if an underlying issue like a misfire is ignored.
Does P0420 always mean I need a new catalytic converter?
No. Many P0420 cases are caused by a bad oxygen sensor, an exhaust leak, or another engine fault. A technician should verify the converter is actually failing before replacing it.