Reductant Metering Unit Heater Control Circuit/Open
The P20B5 code indicates a malfunction in the heater control circuit of the reductant metering unit, which is part of the Selective Catalytic Reduction (SCR) system. This heater is responsible for warming the diesel exhaust fluid (DEF) to prevent freezing and ensure proper injection. An open circuit means there is a break in the electrical path, preventing the heater from functioning. This can lead to reduced NOx conversion efficiency and potential emissions non-compliance.
01. Most Common Causes
- Open or shorted wiring in the heater circuit
- Faulty reductant metering unit heater
- Blown fuse or relay for the heater circuit
- Corroded or damaged connector at the metering unit
02. Symptoms You Will Notice
- Illuminated Check Engine Light
- Reduced engine performance or limp mode
- Increased diesel exhaust fluid consumption
- Failed emissions test
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Visually inspect the wiring and connectors for damage or corrosion.
Step 3: Test the heater circuit for continuity and resistance using a multimeter.
Step 4: Check the fuse and relay for the heater circuit.
Step 5: If all else is good, replace the reductant metering unit heater.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as the SCR system may not function properly, leading to increased emissions and potential engine derate. Short trips may be okay, but repair should be done promptly.
How much does it cost to fix?▼
The cost typically ranges from $150 to $400, including parts and labor. The heater unit itself is around $50-$150, and labor adds $100-$250 depending on the vehicle.