Diesel Trouble Codes

P20E8 — Reductant Pressure Too Low (6.7 Cummins (SCR / DEF))

Diagnostic reference for 6.7L Cummins SCR/DEF 2013–present. P20E8 is a DEF/SCR reductant pressure low code on this platform and is not a fuel-rail pressure…

Updated September 2, 2026

What it means

The engine control module has detected DEF (reductant) delivery pressure below target in the SCR system. On 6.7L Cummins SCR/DEF 2013–present this is a DEF/SCR reductant pressure low fault, not a fuel-rail pressure code.1

Most likely causes

  1. DEF supply module/pump degraded or failing — Low or erratic reductant pressure from the DEF pump/supply module is a typical failure mode for P20E8 on this platform.1
  2. Clogged/restricted DEF filter or supply line — Restriction in the DEF filter or lines reduces delivered pressure to the injector.1
  3. Frozen or contaminated DEF — Frozen/contaminated reductant can block flow and cause low pressure readings.1
  4. Leaking DEF lines/connectors or injector leak-back — External or internal leaks in the DEF delivery circuit lower system pressure.1
  5. Faulty DEF pressure sensor or wiring — Incorrect pressure feedback can set P20E8 even with normal hydraulic pressure.1
  6. DEF heater or SCR controller issue affecting pressure control — Less common; heater/controller faults can contribute to pressure-related DTCs on the SCR system.1

How to diagnose

  1. Verify freeze-frame and confirm P20E8 is active with SCR/DEF system enabled; check for accompanying reductant codes such as P204F, P203F, P20EE, P2BAD, P2201.
  2. Inspect DEF tank level and quality; confirm proper DEF, no contamination, and no ice formation in cold ambient conditions.
  3. Visually inspect DEF supply lines, connectors, filter, and injector for leaks, cracks, or signs of crystallization.
  4. Using a scan tool, monitor commanded vs actual reductant pressure and DEF pump duty cycle during a key-on/engine-running test to identify low pressure or pump command anomalies.
  5. Check DEF pressure sensor signal and reference/ground circuits for plausibility and wiring integrity.
  6. If pressure remains low with no external leaks, evaluate DEF supply module/pump performance and DEF filter restriction per manufacturer test criteria.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by model year and configuration; diagnosis should confirm the specific failed component before replacement.

When to see a pro

If reductant pressure faults persist after basic checks, or if SCR controller/DEF heater diagnostics are required, have the system evaluated by a qualified diesel technician with SCR service capability. Do not attempt high-pressure DEF line service without proper depressurization procedures.

Sources

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

Watch a repair

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

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