What Does P20B5 Mean?
The powertrain control module (PCM) has detected an open circuit or electrical fault in the reductant metering unit heater control circuit. This heater is part of the diesel exhaust fluid (DEF) system that prevents freezing and ensures proper SCR operation.
Common Causes
35%
Damaged or corroded wiring harness between PCM and reductant metering unit heater
30%
Faulty reductant metering unit heater element
20%
Blown fuse or failed relay in heater control circuit
10%
Failed PCM or DEF control module
5%
Connector terminal corrosion or poor ground connection
Diagnostic Steps
1
Step 1: Visually inspect all wiring and connectors at the reductant metering unit for damage, corrosion, or disconnection
2
Step 2: Check fuses and relays related to the DEF heater circuit using wiring diagram
3
Step 3: Test circuit continuity from PCM connector to heater element connector using multimeter
4
Step 4: Measure resistance of heater element (typically 2-6 ohms when cold) and compare to specifications
5
Step 5: If all circuits test normal, perform PCM voltage supply test to heater control circuit under command
Estimated Repair Cost
$150 - $800
Parts + labor, varies by vehicle and location
The powertrain control module (PCM) has detected an open circuit or electrical fault in the reductant metering unit heater control circuit. This heater is part of the diesel exhaust fluid (DEF) system that prevents freezing and ensures proper SCR operation.
The most common cause of P20B5 (Reductant Metering Unit Heater Control Circuit/Open) is: Damaged or corroded wiring harness between PCM and reductant metering unit heater
Typical repair costs for P20B5 range from $150 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle remains safe to drive but DEF system may not function in cold weather. Extended operation may trigger derate or prevent engine restart after certain conditions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P20B5 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.
