What Does P20C4 Mean?
The PCM has detected a voltage signal higher than expected in the control circuit for reductant heater C. This indicates a potential short to voltage or open circuit in the heater control system.
Common Causes
35%
Short to voltage in heater control circuit wiring
25%
Open circuit in heater C ground path
20%
Failed reductant control module or driver circuit
15%
Faulty reductant heater C with internal open circuit
5%
Damaged connector terminals causing high resistance
Diagnostic Steps
1
Step 1: Visually inspect wiring harness for chafing against sharp edges or contact with hot exhaust components
2
Step 2: Disconnect heater C connector and measure resistance to ground and to voltage on control circuit wire
3
Step 3: Check ground circuit continuity from heater to chassis ground and clean ground connection points
4
Step 4: Measure heater element resistance with connector disconnected (should be within specifications)
5
Step 5: Monitor control circuit voltage with scan tool commanding heater on/off while observing for proper switching
Estimated Repair Cost
$200 - $900
Parts + labor, varies by vehicle and location
The PCM has detected a voltage signal higher than expected in the control circuit for reductant heater C. This indicates a potential short to voltage or open circuit in the heater control system.
The most common cause of P20C4 (Reductant Heater "C" Control Circuit High) is: Short to voltage in heater control circuit wiring
Typical repair costs for P20C4 range from $200 to $900, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle is safe to drive but may enter limp mode. DEF system may not function properly in cold weather, leading to potential no-start conditions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P20C4 to identify the root cause.
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Quick Info
Category
Powertrain
System
Diesel Emissions System (SCR/DEF)
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
