Diesel Trouble Codes

P20E8 — Reductant Pressure Too Low (L5P Duramax)

Reductant Pressure Too Low

Updated September 2, 2026

What it means

The PCM has detected DEF (reductant) delivery pressure below target in the SCR system. On the L5P Duramax, DEF/SCR codes apply and P20E8 is a valid reductant pressure code for this platform.12

Most likely causes

  1. DEF dosing unit / injector clogged or leaking — Restricted or leaking dosing valve reduces system pressure; typical reductant delivery fault on L5P SCR system.1
  2. DEF supply pump or pressure sensor fault — Low pump output or erroneous pressure sensor reading causes pressure-too-low detection in the SCR/DEF system.1
  3. DEF tank heater / wiring harness issue causing limp/derate — DEF heater short or harness melt can affect DEF system operation and induce limp mode on diesel trucks.34
  4. Contaminated or frozen DEF — Poor quality or frozen reductant impairs pump flow and pressure; generic reductant delivery pressure low condition.1

How to diagnose

  1. Verify freeze frame and confirm P20E8 is present with SCR/DEF active; check for accompanying reductant codes P204F, P203F, P20EE, P2BAD, P2201.
  2. Inspect DEF tank, lines, and dosing unit for leaks, crystallization, or contamination; check DEF quality and freeze status.
  3. Check DEF supply pump electrical supply and pressure sensor signal plausibility; compare commanded vs actual reductant pressure.
  4. Inspect DEF tank heater and wiring harness for shorts, melting, or damage; repair harness faults that can derate the truck.
  5. Clear codes, perform a reductant system pressure test/activation with scan tool and re-evaluate pressure achievement.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by market and extent of DEF system damage; wiring harness faults may involve backordered components.

When to see a pro

If wiring harness melting or tank heater short is suspected, or if repeated derate/limp mode occurs, have a qualified diesel emissions technician inspect and repair the SCR/DEF system.

Sources

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

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

  3. NHTSA owner report 11456577 ↩︎

  4. NHTSA owner report 11457481 ↩︎

Watch a repair

Third-party videos, not ours — for illustration. Confirm anything against the factory service information for your truck.

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