What Does P20B7 Mean?
The PCM has detected abnormally low voltage or current in the reductant metering unit heater control circuit. This indicates insufficient power delivery to the heater element.
Common Causes
35%
High resistance in power supply or ground circuit to heater
25%
Poor battery voltage or failing alternator affecting heater circuit
20%
Corroded or loose electrical connectors at reductant metering unit
15%
Failed or weak relay supplying power to heater circuit
5%
Shorted heater element creating low voltage condition
Diagnostic Steps
1
Step 1: Measure voltage at heater connector with key on, heater commanded on (should be near battery voltage)
2
Step 2: Perform voltage drop test on power and ground circuits while heater is operating (should be less than 0.5V total)
3
Step 3: Check battery state of charge and load test charging system output
4
Step 4: Inspect and clean all connectors in heater circuit, check for terminal spread or corrosion
5
Step 5: Test relay operation and check for voltage drop across relay contacts
Estimated Repair Cost
$100 - $700
Parts + labor, varies by vehicle and location
The PCM has detected abnormally low voltage or current in the reductant metering unit heater control circuit. This indicates insufficient power delivery to the heater element.
The most common cause of P20B7 (Reductant Metering Unit Heater Control Circuit Low) is: High resistance in power supply or ground circuit to heater
Typical repair costs for P20B7 range from $100 to $700, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle is safe to operate but DEF system may not function properly in cold weather. May result in emissions system derate over time.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P20B7 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.
