P21C7: Reductant power-relay monitoring checks both commanded directions

P21C7 is reductant control-module power relay/relays control circuit/open. Ford’s diesel summary lists two command-versus-voltage mismatches under this code. An “open” title alone does not describe the complete monitored behavior.

The voltage should agree with the power-stage state

The first listed condition is power stage on, heaters off, and heater voltage at 0 V. The second is power stage off, heaters off, and heater voltage above 0 V. The monitor therefore evaluates both missing voltage when commanded and unexpected voltage when switched off.

Ford gives a maximum 5,000 ms detection duration for the relay-control group. A useful assessment has to identify which command state applied; one voltage snapshot without its state cannot distinguish the two branches.

Preserve the individual relay-control codes

P21C4 uses a more specific off-state voltage boundary elsewhere in the same table, while P21C2/P21C3 concern current in their stated test state. These codes should not be collapsed into one assumed open relay or one universal threshold.

The source does not locate the fault in a relay, supply, wiring branch, or controller. It also provides no safe relay-bypass test. Confirm the vehicle’s exact circuit arrangement and follow its diagnostic procedure. P21C7 contributes a state-consistency finding, but the final component decision needs more evidence than the broad relay/open label.

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.