P21CA

Code

Generic P — Powertrain

Reductant Control Module Supply Voltage Circuit

Brand: Generic

Type: P — Powertrain

Views: UK: 33 EN: 68 RU: 31

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Blown or corroded fuse for reductant control module power
Open, shorted, or damaged power/ground wiring or connector to reductant control module
Loose, corroded, or water-intruded connector pins at the module
Faulty power/ground relay or body control module that supplies the reductant module
Failed reductant control module (internal power regulation failure)
Low battery or poor charging system voltage causing undervoltage

Retrieve all related DTCs and freeze frame data with a capable scan tool
Check battery resting voltage and cranking voltage (battery should be ~12.4–12.8 V at rest; >8.0 V while cranking)
Visually inspect fuses and relays for the reductant module; verify fuse continuity
Inspect module connector for corrosion, bent pins, water intrusion, or pushed-out terminals
Backprobe the power and ground pins at the reductant control module while key ON and engine running to verify voltage and ground
Wiggle the harness while monitoring voltage to find intermittent faults

Supply voltage at module pin (key ON, engine off): approx. 11–14.5 V expected (nominal ~12 V)
Supply voltage while engine running: approx. 12–14.5 V (depending on charging system)
Cranking voltage: typically should remain above ~8.0 V at module connector
Ground resistance: low/near 0 Ω to chassis (normally
Intermittent voltage spikes or drops indicate wiring/connector issues

Use a scanner to read P21CA and any related codes; record freeze frame and diagnostic data.
Verify battery state of charge and charging system operation. Repair as needed before further checks.
Visually inspect fuse(s) and relay(s) protecting the reductant control module; replace any suspect components and recheck. Verify fuse continuity with meter.
Locate reductant control module connector. Inspect for corrosion, water intrusion, pushed pins, or damage. Repair or replace connector as required.
With the harness accessible, backprobe the module power and ground terminals. With key ON, confirm supply voltage is within expected range. With engine run, confirm stable voltage.
Wiggle harness and connector while monitoring voltage and data to detect intermittent faults. If voltage drops or spikes, isolate and repair wiring.
Check continuity/resistance of power feed from fused source/relay to module. Check continuity of ground(s) to chassis.
If power is switched by another control module (BCM/PCM), verify that module output or relay control is operating using a scan tool or oscilloscope as needed.
If wiring and external components test good, replace the reductant control module per service procedures. Reprogram or adapt module if required by manufacturer.
Clear codes and perform a road/operation test to confirm fault does not return; monitor reductant system operation and confirm no new codes.