What does code P0420 mean on a Mitsubishi and is it safe to drive?
P0420: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means on your Mitsubishi, what usually causes it, what repairs typically cost, and how safe it is to keep driving.
What this code means
The downstream oxygen sensor detects that the catalytic converter is not storing or converting exhaust properly. This is one of the most common check engine codes.
Common causes for your Mitsubishi
- Failed or aging catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak before or after the catalyst
- Engine misfire or rich/lean condition damaging the catalyst
Symptoms
- Check Engine Light on
- Often no drivability issues
- Possible failed emissions test
- Rotten egg smell in some cases
Typical fixes
- Replace catalytic converter
- Replace oxygen sensor(s)
- Repair exhaust leaks
- Fix underlying fuel/ignition issues first
Repair cost range
For a Mitsubishi, repairs for P0420 often fall between $400 and $2400, depending on the cause and whether you use OEM or aftermarket parts. Labor rates vary by location.
Known issues on Mitsubishi
Mitsubishi P0420 on the 2.4L 4B12 MIVEC engine is relatively uncommon before 120k miles. When it occurs, first check the B1S2 (downstream) O2 sensor heat shield—it vibrates against the sensor wiring and chafes the connector. On Eclipse Cross 1.5T, the turbo downpipe carries the close-coupled catalyst; boost leaks from intercooler piping cause lean misfires that stress the converter. Outlander PHEV has a complex exhaust system that is structurally different—P0420 on the PHEV indicates a distinct fault in the ICE exhaust path during EV-to-ICE transitions.
Most commonly affected models