P0420 — Catalyst Efficiency Below Threshold
What it means: Catalyst Efficiency Below Threshold.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Check ash load
Don't risk it — look up listed providers near you and give them this code when you call.
Listed providers nearest Macon, GA
Closest first. Tell them P0420 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Macon, GA · (478) 538-3245
- 0.0 mi away — based in Macon, GA · (478) 216-8330
- 0.0 mi away — based in Macon, GA · (706) 536-3376
Seen on: Heavy Truck/Diesel
What P0420 means
Catalyst Efficiency Below Threshold. Common causes: Aged or contaminated DPF/DOC.
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.
Related codes
Codes that commonly appear alongside P0420, or share the same subsystem.
- SCR Operator Inducement — Severe Derate Active (SPN 5246 FMI 0)
- Aftertreatment 1 Diesel Particulate Filter Differential Pressure - Data Valid But Above Normal Operating Range, Most Severe Level (DPF severely restricted / plugged) (SPN 3251 FMI 0)
- Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect) (SPN 4364 FMI 1)
- DPF Differential Pressure — Data Erratic (SPN 3251 FMI 2)
- DEF Dosing Unit — Not Responding Properly (SPN 3361 FMI 7)
- SCR Conversion Efficiency — Below Threshold (SPN 4364 FMI 18)
- DEF Quality — Out of Spec (SPN 4094 FMI 31)
- EGR Temperature — Too High (SPN 412 FMI 0)
Same symptom: Regen problems
Other codes that block a regen or come back as soot load climbing.
- DPF Soot Accumulation Restriction (P2460) · Aftertreatment & Emissions
- DPF inlet temp sensor — voltage high (SPN 3241 FMI 3) · Aftertreatment & Emissions
- DPF outlet temp erratic (SPN 3242 FMI 2) · Aftertreatment & Emissions
- DPF outlet temp sensor — voltage high (SPN 3246 FMI 3) · Aftertreatment & Emissions
- DPF differential pressure — voltage high (SPN 3251 FMI 3) · Aftertreatment & Emissions
- DPF differential pressure — voltage low (SPN 3251 FMI 4) · Aftertreatment & Emissions
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.