Reductant Heater Control Circuit High
The P20C8 code indicates that the powertrain control module (PCM) has detected a high voltage condition in the reductant heater control circuit. This heater is part of the selective catalytic reduction (SCR) system, which injects diesel exhaust fluid (DEF) into the exhaust stream to reduce NOx emissions. A high voltage signal suggests a short to power, open circuit, or faulty heater element, preventing proper DEF heating and potentially leading to increased emissions.
01. Most Common Causes
- Faulty reductant heater element
- Short to power in the heater control circuit
- Open or damaged wiring/connectors
- 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: Use a scan tool to confirm P20C8 and check for related codes.
Step 2: Visually inspect the reductant heater and its wiring for damage, corrosion, or loose connections.
Step 3: Measure resistance of the heater element; compare to manufacturer specs (typically 0.5-2 ohms).
Step 4: Check for voltage at the heater connector with key on; should be battery voltage. If high or no voltage, trace circuit for shorts or opens.
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 or limp mode.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for diagnosis and repair, but can be higher if the heater assembly or PCM needs replacement.