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 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. A high voltage reading suggests a short to power, a faulty heater element, or a problem with the control module.
1. Most Common Causes
- Shorted heater element in the reductant metering unit
- Short to power in the heater control circuit wiring
- Faulty reductant metering unit
- Corroded or damaged connector at the metering unit
- Failed PCM (rare)
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Reduced engine performance or limp mode
- Increased diesel exhaust fluid consumption
- Failed emissions test
- Possible DEF system warning message on dashboard
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to retrieve the code and freeze frame data.
Step 2: Visually inspect the reductant metering unit connector and wiring for damage, corrosion, or shorts.
Step 3: Measure the resistance of the heater element; compare to manufacturer specifications (typically 2-10 ohms).
Step 4: Check for voltage at the heater control circuit with the ignition on; should be battery voltage when commanded on.
Step 5: If wiring and heater are okay, test the PCM output or replace the reductant metering unit.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances with this code, as it can lead to reduced engine performance, increased emissions, and potential damage to the SCR system. The vehicle may enter limp mode, limiting speed and drivability.
How much does it cost to fix?▼
The cost to fix P20B8 typically ranges from $150 to $400, including parts and labor. The reductant metering unit itself can cost $100-$300, plus diagnostic time and labor for replacement.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.