P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)
🟠 Moderate severity · drivable
You can usually keep driving short-term, but a failing catalytic converter can eventually hurt performance and clog, so get it diagnosed soon.
What P0420 means
The downstream oxygen sensor on bank 1 is telling the engine computer that the catalytic converter isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; if they read too similarly, the converter is underperforming and it stores P0420.
Symptoms
- Check engine light is on
- Reduced fuel economy
- Possible rotten-egg smell from the exhaust
- Often no noticeable driving symptoms early on
Common causes
- most common: Worn or failing catalytic converter
- common: Faulty downstream oxygen sensor (bank 1, sensor 2)
- common: Exhaust leak before or near the downstream sensor
- less common: Engine misfire or running rich that damaged the converter
- less common: Coolant or oil contamination of the converter
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Inspect for exhaust leaks and repair them before replacing anything | Yes | $50–200 |
| Test the downstream oxygen sensor with a scan tool and replace it if faulty | Yes | $60–200 |
| Replace the catalytic converter (bank 1) | Mechanic | $600–2500 |
Frequently asked questions
Can I drive with a P0420 code?
Yes, usually for a short time. The engine will still run, but continuing to drive a failing converter can hurt fuel economy and eventually cause it to clog, so have it diagnosed soon.
Will a P0420 clear itself?
Sometimes the light goes off on its own if the converter was only marginally underperforming, but it usually comes back. The underlying problem won't fix itself.
How much does it cost to fix a P0420?
Most repairs run $600–$2,500 depending on the cause. A bad oxygen sensor is the cheap fix ($60–$200); a catalytic converter replacement is the expensive one.
Related codes
P0430
P0430 Code: Catalyst System Efficiency Below Threshold (Bank 2)
P0136P0136 Code: O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
P0421P0421 Code: Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.