What does code P0420 mean on a GMC and is it safe to drive?
P0420: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means on your GMC, 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 GMC
- 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 GMC, 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 GMC
GMC Sierra 5.3L P0420 almost always follows AFM (Active Fuel Management) lifter failure—the same issue as Chevrolet Silverado. A misfiring AFM cylinder sends unburned fuel into the exhaust, quickly poisoning the downstream O2 sensor and catalyst. Fix the AFM issue first; some owners permanently disable AFM via Range Device or cam swap. Terrain 2.4L shares the same oil consumption issue as Equinox—address piston ring wear before replacing the converter. Sierra 6.2L owners occasionally see P0420 from aftermarket exhausts using non-compliant O2 sensor configurations.
Most commonly affected models