What it means
This code indicates a high voltage condition in the reductant metering unit heater control circuit.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant metering unit heater circuit powers the heater that keeps the DEF dosing block warm. P20B8 means the module detects higher-than-expected voltage in that circuit, which may result from a short to battery power or a high-resistance connection.
Common causes
Most likely first.
- Short to B+
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Record freeze-frame data and note whether the high reading occurs with the heater commanded or while it is off.
- Inspect the metering unit heater connector for corrosion, loose pins, or wiring pinched against a power source.
- Trace the heater harness for insulation damage that could bridge the circuit to battery voltage.
- With the circuit isolated as specified by the manufacturer, test for unwanted voltage and excessive resistance at connectors.
- If the wiring and heater pass, verify the module output using the manufacturer's diagnostic procedure.
Common fixes
- Repair a heater wire shorted to battery power.
- Clean or replace corroded high-resistance connector terminals.
- Repair pinched or damaged heater wiring and secure its routing.
- Replace the heater or repair the module only when testing confirms the component is at fault.
Can I keep driving?
You may be able to drive briefly, but the metering unit may not heat correctly in cold weather, which can disrupt DEF dosing and lead to emissions warnings or a no-start countdown. Do not ignore the warning if temperatures are near freezing.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.