Reductant Metering Unit Heater Control Circuit Low
The P20B7 code indicates that the powertrain control module (PCM) has detected a low 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 essential for preventing diesel exhaust fluid (DEF) from freezing in cold temperatures. A low circuit voltage can result from a faulty heater element, damaged wiring, poor connections, or a defective PCM. When this code is set, the SCR system may be disabled, leading to increased emissions and potential engine derate.
1. Most Common Causes
- Faulty reductant metering unit heater
- Open or shorted heater circuit wiring
- Poor electrical connection at heater or PCM
- Failed PCM (rare)
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Reduced engine power (derate)
- Increased diesel exhaust fluid consumption
- Possible no-start in cold weather
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Visually inspect the reductant metering unit heater connector and wiring for damage or corrosion.
Step 3: Measure resistance of the heater element; compare to manufacturer specs (typically 1-5 ohms).
Step 4: Check for voltage at the heater connector with ignition on; should be battery voltage.
Step 5: If voltage is low, test circuit continuity and check for shorts to ground or power.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances with this code, as the SCR system may be disabled, leading to increased emissions and possible engine derate. In cold weather, the DEF may freeze, causing further damage.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for diagnosis and repair, depending on whether the heater unit needs replacement or just wiring repairs.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.