What it means
The control module monitors whether reductant heater D responds as expected when commanded. It sets this code when the heater’s measured performance, such as its warming response, falls outside the calibrated range; the code does not by itself identify the cause.
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 usable in cold conditions. The control module compares heater D’s response to its command, including the expected warming behavior, and sets P20C6 when that response falls outside the calibrated range; the code alone does not prove the heater itself has failed.
Common causes
Most likely first.
- Heater D partly failed
- Corroded or loose electrical connections
- Damaged wiring
- Poor heat transfer or excessive heat loss
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 live data for heater D’s command and response, along with coolant or ambient temperature and related DEF-system codes.
- Inspect heater D’s connector and harness for looseness, corrosion, chafing, or damage that could reduce power or interrupt feedback.
- Check whether heater D receives the commanded supply and control signals using the wiring diagram and manufacturer’s test procedure.
- Evaluate heater D’s electrical condition and warming response under the conditions specified by the manufacturer, comparing results with its requirements.
- If the heater and wiring pass, investigate heat transfer, mounting, and excessive heat loss around heater D before considering a module fault.
Common fixes
- Clean and secure loose or corroded heater D connections.
- Repair damaged wiring or connector terminals in heater D’s circuit.
- Replace heater D if testing confirms weak or out-of-range heating performance.
- Correct poor heater mounting or heat loss if inspection identifies it as the cause.
Can I keep driving?
Cautious driving is usually possible if there are no other drivability problems, but heater D may not thaw DEF in cold weather and SCR emissions control can be reduced. Heed any DEF warning or no-start countdown, since continued operation may lead to a starting restriction.
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.