Powertrain, generic (extended)

P21C9 Reductant Control Module Power Relay/Relays Control Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21C9 is stored when the control line of the DEF module's power relay, or relays, carries more voltage than the driver state allows, typically because the wire touches a battery feed or the driver can no longer pull it down. No manufacturer documents P21C9 publicly. The closest published logic is Ford's 2023 diesel summary for P21C7, which flags voltage on the switched side while the output stage is off; a high-side code adds the specific diagnosis of a short to supply on the control pin. Depending on the driver layout the relay then either never releases, keeping pump and heaters live with the engine off, or never closes. P0687 is the equivalent code for the engine module relay, and a late drop-out after key-off is reported as P20EB.

Likely causes

Control wire shorted to a battery-voltage circuit
Relay contacts welded, switched side stays live
Driver in the controlling module shorted

Symptoms

  • Check engine light on
  • DEF pump or module audible or active with the engine off
  • Battery drained overnight

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

scr control modulepower relay

Related codes

P21C7P21C8P20EBP0687

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