P21C3: Reductant Heater Relay Control Circuit Low

P21C3 is the low-side fault on the control line of the DEF heater relay: the signal is pulled toward ground, for example by a chafed wire touching the frame or a shorted relay coil, so the relay no longer follows the command. Ford's 2023 diesel summary uses the same continuous check as for P21C2, but here the relay stage is on, every heater has been told to stay off, and still more than 6 A of heater current is measured; the fault is confirmed within 5 s. Such a reading means power reaches the heater circuit by a path the module never requested. Whether the heaters then stay dead or run without control depends on how the relay is wired. P21C2 is the open-circuit counterpart, P21C4 the high-side fault, P21C7 the module power relay.

Medium severity. Can I keep driving?Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

Symptoms

Check engine light on
DEF heating inactive or running when not commanded
Blown fuse in the heater supply

Most likely causes

Control wire chafed through and shorted to ground
Relay coil with internal short or welded contacts
Water or salt in the relay or heater connector
Output stage of the controlling module faulty

What to check, in order

Start with the simplest. You can tick off each step.

Check the AdBlue/DEF level and quality, and do not add additives or water
With the connector unplugged, check whether the signal wire is making contact with ground.
Look for white crystals around the injector and the tank
Check connectors and heaters with the scanner
Run the dosing test available on your diagnostic tool
Replace parts only when the test points to that component

Good to know

Repair difficulty: medium
It can trigger limp mode.
It affects emissions.

What each character of P21C3 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
1: Air and fuel
C3: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21C3?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P21C3?

The most common causes are: control wire chafed through and shorted to ground; relay coil with internal short or welded contacts; water or salt in the relay or heater connector.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.