Powertrain, generic (extended)

P21CC Reductant Control Module Supply Voltage High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21CC is the overvoltage counterpart of P21CB: the DEF control module sees a supply above what its electronics are designed for. Ford's 2023 diesel summary checks the reductant pump controller in a continuous loop and sets P21CC once its supply exceeds 19 V, deciding within 5 s. On a 12 V vehicle such values usually come from a charging system whose regulator has failed, from a jump start with a 24 V source, or from load dumps on a loose battery terminal; less often the module measures its own input wrongly. To protect pump and heaters the module usually stops working, so dosing pauses. GM's published diesel tables list only the low case. P21CA is the general supply code, and P0563 shows whether the engine module sees the same excess.

Likely causes

Alternator voltage regulator overcharging
Jump start or charging with excessive voltage
Loose battery terminal causing voltage spikes
Module's own voltage measurement faulty

Symptoms

  • Check engine light on
  • DEF dosing paused, SCR warning
  • Battery or charging warning, bulbs unusually bright

Repair estimate

DifficultyMediumDIY possible
Parts and labour€0–700
Time0.3–2.5 h

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

Parts involved

scr control module

Related codes

P21CAP21CBP056DP0563

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