Reductant Metering Unit Heater Control Circuit High
The P20B8 code indicates that the powertrain control module (PCM) has detected a high voltage condition in the heater control circuit for the reductant metering unit, which is part of the selective catalytic reduction (SCR) system. This heater is used to warm the diesel exhaust fluid (DEF) to prevent freezing and ensure proper injection. A high voltage reading typically points to a short to power, a faulty heater element, or a problem with the PCM or wiring.
01. Most Common Causes
- Short to power in the heater control circuit
- Faulty reductant metering unit heater
- Damaged wiring or connectors
- Failed PCM (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Reduced engine performance or limp mode
- Increased emissions
- Possible DEF system warnings
03. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the code and check for related codes.
Step 2: Visually inspect the wiring and connectors for damage, corrosion, or shorts.
Step 3: Test the heater circuit resistance and voltage with a multimeter.
Step 4: If wiring is okay, test the reductant metering unit heater directly.
Step 5: Replace the heater or repair wiring as needed, then clear the code and test drive.
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 DEF system may not function properly, leading to increased emissions and potential engine derate. You should address the issue promptly.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for parts and labor, depending on whether it's a simple wiring repair or heater replacement.