P20B9 — Reductant Heater A Control Circuit/Open — 6.7 Cummins (SCR / DEF)
Diagnostic reference for 6.7 Cummins (SCR/DEF) 2013–present.
What it means
Generic definition: ECM detected an open or fault in the DEF (reductant) heater A control circuit. Platform note: On 6.7L Cummins check DEF heater relay, fuse, connector and wiring, then the heater in the tank header; it matters most in cold weather.1
Most likely causes
- Open or high-resistance in Heater A control circuit (relay, fuse, wiring, connector) causing ECM to see an open — Typical on this platform: check DEF heater relay, fuse, connector and wiring, then the heater in the tank header.1
- Failed/defective DEF tank heater element (Heater A) in the tank header1
- Corrosion/moisture intrusion at DEF tank header connector leading to intermittent open1
How to diagnose
- Verify freeze-frame conditions (ambient temperature, recent cold start) because heater faults are most relevant in cold weather.
- Inspect DEF tank header connector and wiring for corrosion, moisture, or damage; confirm secure seating.
- Check heater control relay and fuse for continuity and proper operation per service information.
- Measure Heater A control circuit for opens/high resistance between ECM and heater per service information; do not perform step-by-step repair of safety-critical systems.
- If circuit integrity is confirmed, evaluate heater element resistance/continuity at the tank header per service information; do not perform step-by-step repair of safety-critical systems.
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
- Digital multimeter
- Scan tool with freeze-frame
- Service information wiring diagram
Tools you’ll need
- digital multimeter — Measures continuity, resistance, and voltage on the DEF heater A control circuit, relay, fuse, and heater element to identify opens or high resistance.
- OBD-II scan tool — Retrieves P20B9 freeze-frame data and monitors DEF heater control status to confirm cold-weather conditions and circuit faults.
- inspection light — Provides focused illumination to inspect the DEF tank header connector and wiring for corrosion, moisture, or damage.
- terminal release tool set — Allows safe removal and inspection of connector terminals at the DEF tank header to check for corrosion or poor seating.
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 tank heater / heated line | $100–$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 wiring faults are present near high-pressure fuel or emissions components, or if a control-circuit repair requires ECM access, have a qualified diesel technician perform diagnosis and repair.
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: P20B9 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.