P0420: Catalyst System Efficiency Below Threshold (Bank 1)

What does P0420 mean?

The catalytic converter on bank 1 is not cleaning the exhaust as well as it should. The computer compares the O2 sensors before and after the converter; when the downstream one starts to mirror the upstream one, the converter is no longer storing oxygen effectively.

Generic OBD-II definition: Catalyst System Efficiency Below Threshold (Bank 1). Confirm applicability in your vehicle service information.

Common causes

• Aging or failed catalytic converter
• Faulty downstream or upstream O2 sensor
• Exhaust leak near the sensors
• Engine misfires or rich running that damaged the converter

Symptoms

• Often none besides the check engine light
• Possible rotten-egg smell
• Will likely fail an emissions inspection

Can I keep driving with P0420?

Severity: Medium Drive gently and get it diagnosed soon.

If the check engine light is flashing or the car runs badly, stop driving and have it checked.

Typical fixes

• Fix any misfire or fuel trim codes first
• Check for exhaust leaks
• Compare upstream and downstream O2 sensor activity
• Replace the catalytic converter if confirmed failed (use an emissions-compliant part for your state)