P21C2: Reductant Heater Relay Control Circuit/Open

What does P21C2 mean?

P21C2 concerns the control line of the relay that supplies current to the DEF heaters. The control module drives the relay coil through an output stage and checks whether the circuit reacts; an open circuit means the coil, its wiring or a connector has lost continuity, so the heaters cannot be energised. Ford documents the code in its 2023 diesel summary: the check runs continuously while heating is requested, and with the relay stage on and every heater switched off, a heater current below 0.2 A sets P21C2 within at most 5 s. P21C3 and P21C4 are the low and high faults of the same line, P21C7 covers the power relay of the module itself. In frost the fluid may stay frozen, so dosing starts late and DEF warnings can follow.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• DEF system warning, especially at low ambient temperature
• Frozen DEF thaws slowly, dosing begins late after a cold start

Common causes

• High Relay coil burnt open or relay not seated in its socket
• Medium Broken control wire between module and relay
• Medium Corroded or loose terminals at relay box or module connector
• Low Faulty output stage in the controlling module

Likelihood is a general estimate across vehicles; your make and model may differ.