What does code P0420 mean on a Nissan and is it safe to drive?
P0420: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means on your Nissan, 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 Nissan
- 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 Nissan, 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 Nissan
Nissan Altima 2.5L QR25DE is notorious for early catalyst failure—many cats fail before 100k miles. This is widely acknowledged; OEM replacement is the reliable fix. On Maxima and Pathfinder with VQ35DE, P0420 and P0430 often appear together (both bank cats failing simultaneously). Sentra CVT seal leaks can allow transmission fluid into the exhaust indirectly via engine contamination, accelerating cat degradation. On newer Rogues, confirm the rear O2 sensor connector hasn't corroded at the undercarriage clip point.
Most commonly affected models