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 suggests a short to ground, open circuit, or faulty component, potentially leading to inadequate DEF delivery and increased emissions.
01. Most Common Causes
- Faulty reductant metering unit heater
- Open or shorted heater control circuit wiring
- Damaged connector or poor electrical connection
- Failed PCM (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- 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 reductant metering unit connector and wiring for damage, corrosion, or loose connections.
Step 3: Using a multimeter, measure resistance of the heater element; compare to manufacturer specs (typically 2-10 ohms).
Step 4: Check for voltage at the heater control circuit with the ignition on; should be battery voltage. If low, test for shorts or opens in the wiring.
Step 5: If wiring and heater are good, suspect a faulty PCM and perform further diagnostics.
04. Commonly Affected Makes
05. 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 cause the SCR system to malfunction, leading to increased emissions and potential damage to the catalytic converter. In some cases, the vehicle may enter limp mode, limiting speed and performance.
How much does it cost to fix?▼
The cost to fix P20B7 typically ranges from $50 to $150 for diagnosis and repair, depending on the cause. Replacing a faulty heater element or repairing wiring is usually under $100, while PCM replacement can be more expensive.