Diesel Trouble Codes

P0420 — Catalyst System Efficiency Below Threshold (6.7 Power Stroke)

Diagnostic reference for 6.7 Powerstroke P0420.

Updated September 1, 2026

What it means

Catalyst efficiency, judged by downstream sensors, is below threshold.1

Most likely causes

  1. Aged or contaminated diesel oxidation catalyst — Typical on this platform: aged or contaminated diesel oxidation catalyst, or exhaust-sensor drift.1
  2. Exhaust-sensor drift — Typical on this platform: aged or contaminated diesel oxidation catalyst, or exhaust-sensor drift.1

How to diagnose

  1. Verify code P0420 is present and check freeze-frame data for engine load, rpm, coolant temp at set.
  2. Inspect aftertreatment and oxidation catalyst for external damage, leaks, or contamination.
  3. Compare upstream and downstream exhaust sensor readings during steady cruise to assess catalyst efficiency.
  4. Check for related aftertreatment codes such as P2002 DPF efficiency below threshold. [1,2]
  5. Review recent regeneration history and driving conditions for interrupted regens that can affect catalyst performance.

Parts and tools you may need

Rough cost and difficulty

Cost varies by root cause; catalyst replacement is typically higher than sensor service.

When to see a pro

If catalyst efficiency remains below threshold after sensor verification, or if exhaust restrictions are suspected, have the aftertreatment inspected by a qualified diesel technician.

Sources

  1. Curated DTC seed — 6.7 Powerstroke (curated) ↩︎ ↩︎ ↩︎