Diesel Trouble Codes

P20E8 — Reductant Pressure Too Low — L5P Duramax

Reductant Pressure Too Low

Updated October 15, 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.

Specs and test values

CheckNormal readingWhen to measureSource
DEF urea concentration32.5% (check with a DEF refractometer)Sample from the tankRam service bulletin 25-005-14 (Diesel Exhaust Fluid)
DEF freeze pointAbout 12°F (−11°C); freezing alone is normal and the system is built to thaw itCold weatherRam service bulletin 25-005-14 (Diesel Exhaust Fluid)

Common mistake to avoid

Replacing sensors, the doser or the SCR catalyst before testing the DEF. Old, heat-stored or diluted DEF loses strength and sets the same efficiency codes. Check it with a refractometer first. Source: Ram service bulletin 25-005-14 (Diesel Exhaust Fluid).

Parts and tools you may need

Tools you’ll need

Rough cost and difficulty

Likely repairPartsShop laborWho usually does it
DEF system drain, flush and refill (contaminated fluid)$50–$2001–3 hrsDIY with tools and a manual
DEF injector (doser)$200–$6000.5–2 hrsDIY with tools and a manual
DEF pump / supply module$400–$1,2001–3 hrsDIY with tools and a manual
DEF tank heater / heated line$100–$1,2001–3 hrsDIY with tools and a manual

Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.

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.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

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 ↩︎

Repair videos: P20E8 on the L5P Duramax

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

P20E8 code 2017 Chevy 2500/3500 Duramax what it means diagnosing & how to fix the problem — DIY Dan · 4K views