P0420 — Catalyst Efficiency Below Threshold
Seen on: Heavy Truck/Diesel
Catalyst Efficiency Below Threshold. Common causes: Aged or contaminated DPF/DOC.
Look up listed providers near you and give them this code when you call.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Aged or contaminated DPF/DOC
- Oil burning
- fuel contamination
First moves (roadside)
- Check ash load
- inspect for oil in exhaust
- perform regen
FAQ
Catalyst Efficiency Below Threshold. Common causes: Aged or contaminated DPF/DOC.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Aged or contaminated DPF/DOC. Other possibilities include Oil burning; fuel contamination.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack and more). Not a substitute for a scan-tool diagnosis on your specific truck.