Powertrain, generic (extended)

P21C8 Reductant Control Module Power Relay/Relays Control Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21C8 means the control line of the DEF module's power relay, or relays, sits too close to ground. No manufacturer documents P21C8 publicly. Ford's 2023 diesel summary only shows the open-circuit sibling P21C7, judged by comparing the commanded relay state with the voltage on the switched side; a low-side code follows the same idea with a pin voltage below the expected window. What a short to ground does depends on the driver: if the module switches the coil on its ground side, the relay may stay energised and drain the battery after switch-off; if it switches the supply side, the relay never pulls in and the SCR system loses power. The engine module pair P0685/P0686 is the closest analogy. P21C9 is the high-side fault.

Likely causes

Control wire chafed and shorted to ground
Shorted relay coil
Moisture in relay socket or module connector
Output stage in the controlling module faulty

Symptoms

  • Check engine light on
  • DEF system without power or permanently powered
  • Flat battery after the vehicle has been parked

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

P21C7P21C9P20EBP0686

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