What it means
The control module monitors the electrical circuit for reductant heater D. It sets this code when it detects an open circuit, meaning current cannot flow through the monitored path as expected.
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) so it can flow through the SCR system in cold conditions. The control module monitors current through heater D’s control path and sets P20C5 when that path is open and expected current cannot flow.
Common causes
Most likely first.
- Heater winding open
- Pin pushed back out of connector body
- Cracked solder joint inside the module or sensor
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
- Review freeze-frame data and SCR module codes to confirm P20C5 and note when heater D was commanded on.
- With the system cool and powered down as specified by the manufacturer, inspect heater D’s connector and harness for a backed-out pin, broken wire, corrosion, or heat damage.
- Check connector seating and terminal retention, paying particular attention to pins pushed back into the connector body.
- Test heater D’s winding and the circuit continuity using the wiring diagram and manufacturer’s procedure to distinguish an open heater from an open harness.
- If the heater and wiring pass, check the module-side connections and output against the manufacturer’s specification for an internal open or cracked joint.
Common fixes
- Reseat a displaced connector pin and restore proper terminal retention.
- Repair a broken or open wire in heater D’s circuit.
- Replace heater D if its winding is confirmed open.
- Repair or replace the module or sensor assembly only if testing confirms an internal open.
Can I keep driving?
If the vehicle drives normally, cautious driving is generally possible, but heater D may not thaw DEF when temperatures are low and the SCR system can become unavailable. Address any no-start countdown promptly because ignoring it can eventually prevent restarting.
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.