Powertrain, generic (extended)

P21C3 Reductant Heater Relay Control Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

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

Symptoms

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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€30–500
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant heater relay

Related codes

P21C2P21C4P21C7P20BB

References: fordservicecontent.com, law.cornell.edu.