P205B — Reductant Tank Temperature Sensor Circuit Range/Performance — 6.7 Cummins (SCR / DEF)

Diagnostic reference for P205B on 6.7 Cummins SCR/DEF 2013–present. Focuses on diagnosing the DEF tank temperature sensor circuit range/performance issue…

Updated October 15, 2026

What it means

The ECM sees the DEF tank temperature sensor signal outside the expected range or not performing as expected; the reading is implausible. On the 6.7L Cummins the DEF temperature sensor is part of the tank header assembly.¹

Most likely causes

1. DEF tank temperature sensor internal failure or drift — Sensor element in the tank header reads out of range or intermittently drops out, causing implausible temperature values.¹
2. Connector or wiring fault at the DEF tank header — Corrosion, moisture intrusion, chafing or loose terminals at the tank header connector produce erratic resistance/voltage to the ECM.¹
3. Wiring harness chafe/ground issue between tank header and ECM — Damaged or shorted wiring in the DEF tank harness leads to out-of-range signals.¹

How to diagnose

1. Verify code and freeze frame; confirm P205B is current and note ambient/DEF temperature conditions.
2. Inspect DEF tank header connector and sensor pigtail for corrosion, moisture, bent pins, and proper seating; look for signs of DEF crystallization.
3. Check wiring from tank header to chassis harness for chafing, heat damage, and secure routing; inspect grounds related to the DEF module.
4. Monitor live DEF tank temperature PID with a scan tool during a key-on/engine-off to key-on/engine-running sequence; compare to ambient temperature and look for implausible jumps or flat-lining.
5. Perform a wiggle test on the tank header connector and harness while observing the temperature PID for dropouts or sudden shifts.
6. If available, compare DEF temperature reading to an independent temperature measurement near the tank to assess plausibility.
7. Check for related DEF system codes (e.g., heater or level faults) that may indicate broader tank header/module issues.
8. Clear codes after inspection and road test under varied ambient conditions to confirm the fault does not return.

Specs and test values

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 PIDs
• Digital multimeter
• Inspection light
• Dielectric grease
• Contact cleaner

Tools you’ll need

• OBD-II scan tool — Reads P205B and live DEF tank temperature PID to confirm implausible readings and monitor changes during wiggle tests.
• digital multimeter — Checks sensor supply voltage, ground continuity, and resistance of the DEF tank temperature sensor circuit for opens/shorts.
• inspection mirror — Lets you see the DEF tank header connector and wiring routing in the tight underbody area for corrosion, DEF crystallization, and chafing.
• contact cleaner — Removes DEF residue and moisture from the tank header connector to restore reliable signal before condemning the sensor.

Rough cost and difficulty

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 the temperature PID remains implausible after connector/harness inspection, or if the tank header requires removal for testing, have a qualified diesel technician evaluate the DEF tank header assembly and ECM inputs. Do not attempt high-pressure fuel or emissions-system repairs without proper training and equipment.

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