What Does P20C8 Mean?
The PCM has detected a voltage signal higher than expected in the control circuit for reductant heater D. This indicates a potential short to voltage or open circuit condition in the heater control system.
Common Causes
35%
Short to voltage in heater D control circuit wiring
25%
Open circuit in heater D ground path
20%
Failed reductant control module driver circuit
15%
Faulty reductant heater D with internal open
5%
High resistance in connector terminals
Diagnostic Steps
1
Step 1: Check for chafed or damaged wiring that may be contacting voltage sources or other circuits
2
Step 2: Disconnect heater D connector and measure voltage on control circuit (should be 0V with module commanding off)
3
Step 3: Verify ground circuit continuity from heater to chassis ground and clean all ground connections
4
Step 4: Measure heater D element resistance with connector disconnected (compare to specifications)
5
Step 5: Monitor control circuit with scan tool commanding heater on/off and observe for proper voltage transitions
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 D. This indicates a potential short to voltage or open circuit condition in the heater control system.
The most common cause of P20C8 (Reductant Heater "D" Control Circuit High) is: Short to voltage in heater D control circuit wiring
Typical repair costs for P20C8 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 reduced power mode. DEF heating system failure in cold climates can result in frozen DEF and vehicle 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 P20C8 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.
