Diesel Trouble Codes

P20EE — SCR NOx Catalyst Efficiency Below Threshold (Bank 1) (L5P Duramax)

Diagnostic reference for L5P Duramax 2017–present 6.6L. P20EE indicates the SCR NOx conversion efficiency monitor is below threshold. Diagnose by confirming…

Updated September 2, 2026

What it means

On L5P Duramax, P20EE is an SCR NOx catalyst efficiency monitor. The ECM compares upstream and downstream NOx sensor signals and reductant dosing to judge SCR conversion; efficiency below threshold sets the code. The platform uses DOC/DPF/SCR aftertreatment; DEF/SCR codes apply to this platform.12

Most likely causes

  1. NOx sensor drift/fault or wiring issue causing incorrect efficiency calculation — Erratic or biased NOx sensor output can make the ECM calculate low conversion efficiency even with a functional catalyst. NHTSA reports describe P20EE with SCR NOx sensor 2 behavior that stops intermittently and returns after clearing.3
  2. Reduced SCR conversion due to reductant system performance issues — Insufficient or incorrect DEF dosing, pressure, or quality reduces NOx reduction, triggering efficiency codes. P204F is a DEF/SCR reductant system performance code that applies to L5P, and reductant pressure faults are monitored on this platform.24
  3. DEF delivery/quality problems affecting dosing accuracy — Low reductant pressure, clogged dosing unit, or degraded DEF can lower actual NOx conversion. P20E8 is reductant pressure too low on L5P.4
  4. Aftertreatment thermal or flow conditions outside expected range — SCR efficiency is temperature and flow dependent; conditions that keep the catalyst below light-off or alter exhaust flow can set efficiency below threshold. Platform has DOC/DPF/SCR aftertreatment monitored by DPF/SCR-specific codes.1
  5. Wiring harness/connector degradation in DEF/SCR circuits — DEF system electrical faults, including heater shorts and harness melt, can cause derate/limp and related SCR faults on Duramax trucks.567

How to diagnose

  1. Verify freeze frame and code history; confirm P20EE is present with related SCR/DEF codes such as P204F, P20E8, P203F, P2BAD, P2201.
  2. Inspect NOx sensor data: compare upstream vs downstream NOx readings at steady cruise and during a controlled drive; look for sensor rationality, response, and intermittent dropouts. NHTSA complaint notes sensor works then stops.
  3. Check reductant system performance data: DEF tank level, reductant quality counter, dosing quantity actual vs desired, and reductant pressure. P204F indicates reductant system performance issues on L5P.
  4. Review reductant pressure and delivery: confirm pressure meets target and no low-pressure faults. P20E8 is reductant pressure too low on this platform.
  5. Inspect DEF/SCR electrical circuits and harness for damage, corrosion, or signs of overheating/melting; DEF heater short issues have been reported to cause harness melt and limp mode on diesel trucks.
  6. Check aftertreatment temperatures and exhaust flow: verify DOC/DPF/SCR temps reach expected light-off during operation; excessive soot load or flow restriction can affect SCR efficiency. Platform uses DOC/DPF/SCR.
  7. Assess recent DEF quality/fill history and tank heater operation; contaminated or frozen DEF and heater faults affect dosing accuracy.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by root cause; NOx sensors, DEF dosing components, wiring repairs, and aftertreatment service can differ widely. Diagnosis should confirm the specific failed component before replacing parts.

When to see a pro

If efficiency remains low after confirming sensor rationality and reductant delivery, or if wiring harness damage/DEF heater faults are suspected, have a qualified diesel aftertreatment technician perform pinpoint tests. Do not perform step-by-step repair of high-pressure fuel or safety-critical systems. Refer to GM service information for approved diagnostic procedures.

Sources

  1. Curated DTC seed — L5P Duramax (curated) ↩︎ ↩︎

  2. Curated DTC seed — L5P Duramax (curated) ↩︎ ↩︎

  3. NHTSA owner report 11661486 ↩︎

  4. Curated DTC seed — L5P Duramax (curated) ↩︎ ↩︎

  5. NHTSA owner report 11456577 ↩︎

  6. NHTSA owner report 11457481 ↩︎

  7. NHTSA owner report 11635352 ↩︎