Powertrain, generic (extended)

P21CB Reductant Control Module Supply Voltage Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21CB says the DEF control module receives too little supply voltage. GM measures it as a difference to system voltage: in the 2018 diesel tables a gap above 3 V between battery voltage and the supply of the SCR power module, seen in 40 of 50 samples taken every 100 ms with the powertrain relay in range and cranking excluded, sets the code; the 2025 tables apply the same 3 V gap to the permanent DEF supply against the gateway voltage for 3 s and classify it as Type B. Ford's 2023 diesel summary uses an absolute limit for its reductant pump controller, below 8.5 V within 5 s. Causes lie between battery and module: resistance in fuses, terminals or ground. P21CA is the umbrella code, P21CC the overvoltage case.

Likely causes

Voltage drop at fuse, terminal or ground of the module feed
Weak battery or low charging voltage
Damaged or corroded supply wire to the module
Internal voltage measurement of the module faulty

Symptoms

  • Check engine light on
  • DEF dosing or heating drops out intermittently
  • Further undervoltage or communication codes from the SCR system

Repair estimate

DifficultyMediumDIY possible
Parts and labour€0–500
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

P21CAP21CCP056DP0562P20FF

References: gsi.ext.gm.com, gsi.ext.gm.com, fordservicecontent.com.