What it means
This code indicates a low 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 supplies the heater that warms the DEF dosing block. P20B7 means the module detects lower-than-expected voltage in this circuit, often due to a short to ground or high resistance at a corroded connection.
Common causes
Most likely first.
- Short to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
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
- Save freeze-frame data and determine whether the low reading occurs continuously or only when the heater is commanded.
- Inspect the metering unit heater connector for corrosion, moisture, loose terminals, or damaged insulation.
- Trace the heater harness for chafing or a wire contacting a grounded part near the dosing unit.
- With the circuit isolated per the manufacturer's procedure, test for a short to ground and check the connector and wiring for excessive resistance.
- If the circuit checks out, verify the module's heater output using the manufacturer's diagnostic procedure.
Common fixes
- Clean and secure corroded or loose heater connector terminals.
- Repair wiring shorted to ground or with damaged insulation.
- Replace a connector that cannot maintain reliable contact.
- Replace the heater or repair the module only if circuit tests identify it as faulty.
Can I keep driving?
The vehicle may run normally at first, but the dosing unit may not warm in cold weather, risking DEF flow problems, emissions warnings, or a no-start countdown. Address it before freezing conditions or if the countdown begins.
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.