What Does P20B8 Mean?
The PCM has detected abnormally high voltage or current in the reductant metering unit heater control circuit. This may indicate a short to voltage or a control circuit fault.
Common Causes
35%
Short to voltage in heater control circuit wiring
30%
Failed reductant metering unit heater element with internal short
20%
Faulty PCM or DEF control module driver circuit
10%
Damaged connector causing cross-circuit short
5%
Failed relay stuck closed providing constant voltage
Diagnostic Steps
1
Step 1: Disconnect reductant metering unit heater connector and check for voltage at harness side with key on (should be 0V with heater off)
2
Step 2: Measure heater element resistance for short to ground or low resistance condition
3
Step 3: Inspect wiring harness for chafing, pinch points, or damage causing short to power
4
Step 4: Check for proper PCM control signal operation using scan tool command function with heater disconnected
5
Step 5: Test relay for stuck closed condition and verify proper control from PCM
Estimated Repair Cost
$150 - $850
Parts + labor, varies by vehicle and location
The PCM has detected abnormally high voltage or current in the reductant metering unit heater control circuit. This may indicate a short to voltage or a control circuit fault.
The most common cause of P20B8 (Reductant Metering Unit Heater Control Circuit High) is: Short to voltage in heater control circuit wiring
Typical repair costs for P20B8 range from $150 to $850, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Generally safe to drive but poses potential fire risk if short circuit is severe. May cause excessive current draw and battery drain.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P20B8 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions Control 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.
