P204C: Reductant Pressure Sensor Circuit Low

P204C means the DEF pressure sensor output has fallen below its valid range, which points to an open signal wire, a short to ground or a missing 5 V supply. On the 6.6 L LML, GM fails anything under 0.41 V lasting 0.4 s, GM's 2018 diesel table a raw value under 5 % of the supply, and the 2025 DEF controller less than 0.45 V in 6 of 100 samples, each Type A. Ford's 2023 diesels set it under 0.2 V. On 2017 to 2019 Super Duty trucks with the 6.7 L, Ford (TSB 19-2315) found water entering connectors C3613 and C4851 of the reductant harness and replaced harness, pump/heater assembly or tank. Deutz uses failure type 11 for it on the EMR5. Lacking a pressure value, the controller suspends dosing.

Medium severity. Can I keep driving?Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

Symptoms

MIL on
DEF dosing stopped, DEF warning messages
Further reductant codes stored at the same time

Most likely causes

Water intrusion in the reductant harness connectors (Ford TSB 19-2315)
Signal wire open or shorted to ground
Missing 5 V reference to the sensor
Failed sensor in the pump/heater assembly

What to check, in order

Start with the simplest. You can tick off each step.

Check the AdBlue/DEF level and quality, and do not add additives or water
With the connector unplugged, check whether the signal wire is making contact with ground.
Look for white crystals around the injector and the tank
Check connectors and heaters with the scanner
Run the dosing test available on your diagnostic tool
Replace parts only when the test points to that component

Good to know

Repair difficulty: medium
It can trigger limp mode.
It affects emissions.

What each character of P204C means

P: Powertrain: engine and transmission
2: Generic code: same on every make
0: Subsystem depends on the code range
4C: Specific fault in that system

Frequently asked questions

Can I keep driving with code P204C?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P204C?

The most common causes are: water intrusion in the reductant harness connectors (Ford TSB 19-2315); signal wire open or shorted to ground; missing 5 V reference to the sensor.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.