P21C4: Reductant Heater Relay Control Circuit High
What to do
This is one of the cheaper faults in the reductant system, so it is worth fixing promptly rather than waiting — a relay or its socket is inexpensive, and everything that heats the reductant stops working when it fails.
Ask for the relay socket to be inspected as well as the relay. These carry heavy current, and the socket terminals heat and spread over years — a new relay pushed into a burnt socket fails again.
Get it done before deep winter. In mild weather nothing happens at all; the first hard frost freezes the fluid and starts the countdown towards the vehicle refusing to restart.
Mention any recent work in the fuse or relay box, since a relay not fully seated produces this exactly.
What this code means
The control circuit for the reductant heater relay is reading higher than commanded.
A relay sits between the control module and the heaters for a straightforward reason: current. Heating a tank of reductant and several metres of line draws far more than a module output can switch directly, so the module drives a relay coil and the relay carries the load.
This code is about the coil circuit, not the load. The heaters may be perfect and the relay contacts fine — the module simply cannot drive, or cannot verify, the coil.
Reading high means a short to a supply voltage or an open circuit, and the module cannot distinguish them from inside. Measuring at the relay socket with the relay removed separates them in a minute.
The open case is the more likely, and relay sockets are where it happens. A relay that has been carrying heavy current for years heats its socket terminals, which spread and oxidise until the coil circuit no longer makes reliable contact. Look for discoloured plastic around the terminals rather than only testing continuity.
This is the cheapest fault in the reductant heating family and the one most worth catching early, because everything downstream of it — tank heater, line heaters, the lot — stops working when the relay does.
The consequence is seasonal and silent: nothing in summer, frozen fluid and an inducement countdown at the first hard frost.
Likely causes
Start here
- Open circuit in relay control wiring · moderate
- Corroded or loose connector · easy
- Failed reductant heater relay · easy
Less common
- Heat damage to wiring insulation · moderate
- Wiring shorted to voltage · moderate
Symptoms
Components involved
Information, not instruction
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