P21C3: Reductant heater relay low-circuit detection uses excessive current

P21C3 names reductant heater relay control circuit low. Ford’s diesel monitor associates it with heater current above 6 A while the power stage is on and heaters are off. The word low therefore should not be converted into a guessed low-voltage threshold.

Current direction and command state are both important

The relay-control group lists a maximum 5,000 ms detection duration. Its general operation accompanies heater activation, but the individual P21C3 test specifies an off-heater state with the power stage on. A diagnostic interpretation needs to preserve that combination.

P21C2 uses current below 0.2 A in the same listed command state. P21C3 occupies the excessive-current branch instead. Applying the open-circuit code’s low-current test to this fault would answer the opposite electrical question.

What should be verified before replacing a relay

Establish which relay-control circuit and command states apply to the actual vehicle, then interpret available current evidence in that context. A high current during ordinary full heater operation is not automatically equivalent to the off-heater monitor described here.

Ford’s summary does not identify a short location or prove that the relay itself is defective. It gives no bypassing or external-energizing procedure. Use the correct service information to distinguish the controlled circuit, wiring, and driver possibilities. P21C3’s useful content is the unexpected current in a specific state, rather than the broad “circuit low” label alone.

Source and applicability

This explanation follows Ford’s 2023–2024 diesel OBD summary. Match the vehicle’s engine and calibration before applying these monitor values; a generic code alone does not establish fitment. Ford PDF, page 87.