P0420
Get Serviced SoonFix: Medium — get serviced in 1–2 weeksCatalyst System Efficiency Below Threshold (Bank 1) — The catalytic converter is not cleaning exhaust gases effectively.
Dashboard light triggered
Check Engine LightWhat to do
First rule out upstream causes (O2 sensors, misfires). A failed cat often means something else caused it — fix that first. May fail emissions.
Common causes
- Worn-out catalytic converter
- Faulty downstream O2 sensor (often mistaken for bad cat)
- Exhaust leak before the cat
- Unfixed misfire or rich condition that damaged the cat
Estimated repair cost
$300–$2,500
Exact cost depends on the cause, your vehicle, and labor rates in your area.
What to tell your mechanic
Use this script so you sound informed and get a clear diagnosis before agreeing to repairs:
“Code P0420, catalyst efficiency. Can you verify the O2 sensors are working before we assume the cat is bad? I've heard that's a common misdiagnosis.”
About the Check Engine Light
Code P0420 triggers the Check Engine Light. Indicates an issue with the engine or emissions system. Could range from a loose gas cap to a serious engine misfire.
Read full Check Engine Light guideFrequently asked questions
- What does OBD code P0420 mean?
- Catalyst System Efficiency Below Threshold (Bank 1) — The catalytic converter is not cleaning exhaust gases effectively.
- What dashboard light does P0420 trigger?
- P0420 triggers the Check Engine Light.
- How serious is P0420?
- Severity: yellow. Fix priority: Medium — get serviced in 1–2 weeks.
- How much does it cost to fix P0420?
- Repair costs typically range from $300–$2,500. The exact cost depends on the root cause and your vehicle.
Related OBD codes (same dashboard light)
These codes also trigger the Check Engine Light: