Powertrain, generic (extended)

P21C7 Reductant Control Module Power Relay/Relays Control Circuit/Open

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21C7 flags a break in the control line of the power relay, or group of relays, that supplies the DEF control module and the loads behind it. Ford's 2023 diesel summary evaluates it inside the same continuous heater relay check as P21C2 to P21C4, so on that design the relay also feeds the heating circuits. The code sets within 5 s if the output stage is on but the heater supply reads 0 V, or if the stage is off and voltage is still present, meaning the commanded and the actual relay state disagree. Without supply, DEF heating and possibly dosing stop, and SCR warnings follow. P21C8 and P21C9 are the low and high faults of this line; P20EA and P20EB judge when the relay drops out after the key is turned off.

Likely causes

Relay defective or missing from its socket
Open control wire or loose terminal between module and relay
Blown fuse in the relay or module feed
Driver in the controlling module faulty

Symptoms

  • Check engine light on
  • DEF heating and dosing do not work
  • SCR warning and countdown to restricted starting or speed

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–500
Time0.3–2 h

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

Parts involved

scr control modulepower relay

Related codes

P21C8P21C9P20EAP20EBP21C2P0685

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