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…
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
- 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
- Clogged/restricted DEF filter or supply line — Restriction in the DEF filter or lines reduces delivered pressure to the injector.1
- Frozen or contaminated DEF — Frozen/contaminated reductant can block flow and cause low pressure readings.1
- Leaking DEF lines/connectors or injector leak-back — External or internal leaks in the DEF delivery circuit lower system pressure.1
- Faulty DEF pressure sensor or wiring — Incorrect pressure feedback can set P20E8 even with normal hydraulic pressure.1
- 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
- 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.
- Inspect DEF tank level and quality; confirm proper DEF, no contamination, and no ice formation in cold ambient conditions.
- Visually inspect DEF supply lines, connectors, filter, and injector for leaks, cracks, or signs of crystallization.
- 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.
- Check DEF pressure sensor signal and reference/ground circuits for plausibility and wiring integrity.
- If pressure remains low with no external leaks, evaluate DEF supply module/pump performance and DEF filter restriction per manufacturer test criteria.
Specs and test values
| Check | Normal reading | When to measure | Source |
|---|---|---|---|
| DEF urea concentration | 32.5% (check with a DEF refractometer) | Sample from the tank | Ram service bulletin 25-005-14 (Diesel Exhaust Fluid) |
| DEF freeze point | About 12°F (−11°C); freezing alone is normal and the system is built to thaw it | Cold weather | Ram service bulletin 25-005-14 (Diesel Exhaust Fluid) |
Common mistake to avoid
Adding just a little DEF to clear the 5 mph warning. Ram’s bulletin says at least 2.5 gallons must go in before the system resets the message. Source: Ram service bulletin 25-005-14 (Diesel Exhaust Fluid).
Parts and tools you may need
- Scan tool with Cummins/SCR live data
- Digital multimeter
- DEF pressure test adapter per service info
- Basic hand tools for DEF filter/lines inspection
Tools you’ll need
- OBD-II scan tool — Read P20E8 freeze-frame and monitor commanded vs actual reductant pressure and pump duty cycle to confirm low DEF pressure.
- digital multimeter — Check DEF pressure sensor reference, ground, and signal voltages for plausibility and wiring integrity.
- fuel pressure gauge — Verify actual reductant delivery pressure against spec during pump operation to identify supply module or filter restriction.
- inspection mirror — Inspect DEF lines, connectors, filter, and injector for leaks or crystallization without removing components.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Wiring or connector repair | $10–$100 | 1–4 hrs | DIY with tools and a manual |
| DEF system drain, flush and refill (contaminated fluid) | $50–$200 | 1–3 hrs | DIY with tools and a manual |
| DEF pump / supply module | $400–$1,200 | 1–3 hrs | DIY with tools and a manual |
| DEF level/temperature/quality sensor (tank header) | $300–$1,200 | 1–3 hrs | DIY 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 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.
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
Repair videos: P20E8 on the 6.7 Cummins '13+
Third-party videos, not ours — picked for this engine. Confirm anything against the factory service information for your truck.
Spotted an error, or something missing? Tell us
Run a 6.7 Cummins (SCR / DEF)? This goes straight to the editor — it is not published.