Diesel Trouble Codes

P0420 — Catalyst System Efficiency Below Threshold (6.7 Cummins (SCR / DEF))

Diagnostic reference for 6.7L Cummins SCR/DEF 2013–present. P0420 is a generic OBD-II catalyst efficiency code that is not used on this platform; the platform…

Updated September 2, 2026

What it means

On 6.7L Cummins SCR/DEF trucks, P0420 is a generic OBD-II catalyst efficiency code that is not used on this platform; the platform monitors DPF/SCR with manufacturer-specific codes, not P0420.1

Most likely causes

  1. Code mis-match / non-platform code — P0420 is a generic OBD-II catalyst efficiency code that is not used on 6.7L Cummins SCR/DEF trucks; the platform monitors DPF/SCR with manufacturer-specific codes, not P0420. If P0420 is present, verify tool calibration, aftermarket tuning, or cross-platform code reporting.1
  2. Diagnostic tool / software reporting error — Scan tool may report a generic P0420 from a non-Cummins library or from an aftermarket calibration. Confirm code with Cummins Insite or OEM scan tool and check for manufacturer-specific aftertreatment codes such as P2002, P20EE, P20E8.1
  3. Aftermarket emissions modification or sensor spoofing — Non-OEM tuning or emulator devices can generate generic catalyst efficiency codes that do not correspond to factory monitoring on this platform.1

How to diagnose

  1. Verify code with OEM/Cummins diagnostic tool; confirm code is present in ECM and not a generic translation.
  2. Check for manufacturer-specific aftertreatment codes: DPF efficiency P2002, SCR NOx efficiency P20EE, DEF pressure P20E8, etc.
  3. Review freeze frame and recent drive history for aftertreatment events; inspect for recent tuning, emulator installation, or scan tool mismatch.
  4. If no Cummins-specific aftertreatment codes are present and vehicle operates normally, treat P0420 as a non-platform reporting artifact and document tool/software discrepancy.

Parts and tools you may need

Rough cost and difficulty

No platform-specific repair is indicated for P0420 on 6.7L Cummins SCR/DEF; cost is diagnostic time to verify code source and correct tooling.

When to see a pro

If OEM scan tool confirms a genuine aftertreatment fault, have a qualified diesel technician inspect DPF/SCR system using manufacturer diagnostics. Do not attempt high-pressure fuel or aftertreatment service without proper training and equipment. If limp mode or derate occurs, tow to dealer.

Sources

  1. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎ ↩︎ ↩︎ ↩︎