P21CB reports low supply voltage at the reductant control module. Understand the difference between local module supply and vehicle-wide voltage.
Meaning, symptoms, causes and checks
P21CB concerns reductant control-module supply voltage. The monitor reports supply voltage below its accepted range for the named circuit or device. The monitored supply is assigned to the reductant module. The wording alone does not show whether a vehicle-wide or a local supply problem exists.
Affected system: Reductant dosing and SCR aftertreatment
Symptoms
- Warning light may illuminate
- Related system function may be impaired
- Vehicle-specific symptoms vary
Possible causes
- Low or unstable supply voltage, fuse or relay fault, poor ground, or wiring damage
- Related wiring, connector, power or ground fault
- Vehicle-specific mechanical or control issue
What to check
- Record stored and pending codes and freeze-frame data
- Measure battery and charging voltage under load and compare with scan data; test the named circuit, fuse, relay, grounds and connectors.
- Confirm measured values against the vehicle service information
- Retest under the conditions that set the code
- Measure battery and charging voltage under load and compare with scan data; test the named circuit, fuse, relay, grounds and connectors.
- Repair the verified cause and confirm the code does not return
Also written as: P21CB-00, P21CB00 (some scan tools add the fault-type byte, e.g. -00).