What it means
This code indicates a high voltage condition in reductant heater D's control circuit.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant heater D warms DEF (AdBlue) to keep it flowing in cold conditions. The control module monitors heater D’s control-circuit voltage and sets P20C8 when it is higher than expected, which can result from a short to battery power or an abnormal 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
- Check freeze-frame and SCR live data for heater D’s command and circuit voltage when P20C8 set, and note any related codes.
- With the system cool and powered down as specified by the manufacturer, inspect heater D’s wiring and connector for pinching, corrosion, damaged pins, or contact with a B+ source.
- Use the wiring diagram to identify the control circuit and test it for an unintended connection to battery power using the manufacturer’s procedure.
- Check the connector terminals for corrosion or poor contact that may distort circuit readings, and repair any damaged connections.
- If the wiring passes, test heater D and the control-module output against the manufacturer’s specification to locate an internal fault.
Common fixes
- Repair a control wire shorted to battery power or pinched against a B+ source.
- Clean or replace corroded or damaged heater D connector terminals.
- Repair damaged wiring and secure it away from power wires, hot surfaces, and moving parts.
- Replace heater D or repair the control module only when testing identifies it as faulty.
Can I keep driving?
Cautious driving is generally possible if the vehicle runs normally, but heater D may not thaw DEF in cold weather and an emissions warning or starting countdown may follow. Have the fault addressed before a countdown expires, since the vehicle may then refuse to restart.
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.