Powertrain, generic (extended)

P21CA Reductant Control Module Supply Voltage Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21CA is the general code for the supply feed of the DEF control module: the voltage arriving at the module is missing or unusable, without the module assigning it to the low or high case. Volvo and Mack trucks use it, as P21CA00, on the aftertreatment control module that runs the SCR system. Volvo article KC-2040 describes a typical pattern: a battery or ground connection slowly loosens, the module voltage sinks below about 8 V, the module cuts power to the NOx, particulate and DEF sensors, communication codes such as U029D and U02A2 follow, and the truck enters SCR inducement and derate. Volvo stresses that this rarely means a bad module and first checks terminals, batteries, charging and the in-line fuse. P21CB and P21CC are the specific low and high codes.

Likely causes

Loose or corroded battery or ground connections of the module
Weak batteries or insufficient charging voltage
Faulty in-line fuse or damaged supply wire
Control module itself faulty

Symptoms

  • Check engine light on
  • Several sensor communication codes at the same time
  • SCR inducement with power reduction

Repair estimate

DifficultyMediumDIY possible
Parts and labour€0–600
Time0.5–3 h

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

Parts involved

scr control module

Related codes

P21CBP21CCP056DP0562U02A2

References: static.nhtsa.gov, static.nhtsa.gov, law.cornell.edu.