What Does P204C Mean?
The ECM has detected a voltage signal from the reductant (DEF/AdBlue) pressure sensor that is below the normal operating range. This indicates a potential short to ground, sensor failure, or wiring issue in the diesel emissions aftertreatment system.
Common Causes
35%
Faulty reductant pressure sensor with internal short or failure
30%
Damaged wiring harness or connector corrosion causing short to ground
20%
Chafed or pinched sensor wiring creating intermittent ground
10%
ECM internal fault or poor ground connection
5%
Water intrusion in sensor connector causing low voltage reading
Diagnostic Steps
1
Step 1: Visually inspect reductant pressure sensor connector for corrosion, moisture, or damaged pins
2
Step 2: Backprobe sensor connector and measure voltage signal with key on, engine off (typically 0.5-4.5V range)
3
Step 3: Check wiring harness continuity between sensor and ECM, and test for short to ground using multimeter
4
Step 4: Monitor live sensor data while wiggling harness to identify intermittent shorts
5
Step 5: If wiring is intact, replace reductant pressure sensor and retest system
Estimated Repair Cost
$150 - $600
Parts + labor, varies by vehicle and location
The ECM has detected a voltage signal from the reductant (DEF/AdBlue) pressure sensor that is below the normal operating range. This indicates a potential short to ground, sensor failure, or wiring issue in the diesel emissions aftertreatment system.
The most common cause of P204C (Reductant Pressure Sensor Circuit Low) is: Faulty reductant pressure sensor with internal short or failure
Typical repair costs for P204C range from $150 to $600, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle may enter reduced power mode (limp mode) or experience derate after extended operation. Not immediately unsafe but should be repaired promptly to avoid being stranded.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P204C to identify the root cause.
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Quick Info
Category
Powertrain
System
Selective Catalytic Reduction (SCR) Emissions System
Difficulty
Type
Generic (SAE)
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