P20C6: Reductant Heater D Control Circuit Performance
What to do
Ask whether other heater faults are stored at the same time. One heater code usually means that section; several at once usually means one shared supply or earth — and on a system with four heated sections that is the difference between a connector repair and several expensive parts.
Do not dismiss it because the car drives normally. The whole heating system is only as good as its weakest section — one frozen part stops reductant dosing completely, and that leads to the countdown towards a vehicle that will not restart.
Get it repaired before deep winter, since the fault has no symptoms at all in mild weather.
Ask which part of the system this heater serves on your vehicle, because the lettering differs between manufacturers and so does the cost.
What this code means
The fourth reductant heater circuit is being commanded but not performing as expected.
Heater D is usually the last section in the path — commonly the metering unit or the injector line — and it completes the set of four this block carries.
This is a good place to state what the set means together, because that is more useful than any one member. The heaters exist to keep a continuous path of liquid reductant from tank to exhaust. A single frozen section breaks the path, so the system is only as good as its weakest heater — and one failed element disables reductant dosing as completely as four would.
Which is why none of these codes should be treated as minor because the car drives normally. They are all seasonal faults with the same eventual outcome: no dosing in cold weather, then the inducement sequence, then a vehicle that will not restart.
Reading the set together is the practical skill. One heater code points at that section. Several at once point at a shared supply, earth or module output — and on a system with four heated sections, that difference is the difference between one connector repair and several expensive heated assemblies.
The section nearest the exhaust has its own complication worth checking: it runs in a hot environment, so its temperature readings are influenced by exhaust heat, and a sensor there can appear to be performing when the fluid is not actually being heated by the element at all.
Likely causes
Start here
- Faulty reductant line heater · moderate
- Corroded or loose connector · easy
- Faulty reductant temperature sensor · moderate
Less common
- Heat damage to wiring insulation · moderate
- High resistance in sensor wiring · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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