P21CC

Code

Generic P — Powertrain

Reductant Control Module Supply Voltage High

Brand: Generic

Type: P — Powertrain

Views: UK: 35 EN: 44 RU: 41

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Short circuit to battery positive on the module power feed
Faulty internal voltage regulator in the reductant control module
Overcharging/charging system fault (alternator/regulator)
Incorrect or damaged battery/charging system (e.g., jump start or boost charger connected)
Poor or corroded ground causing erroneous voltage sensing
Blown or incorrect value resistor or component in power feed

Read and record freeze frame and stored data; note battery/charging voltages and engine conditions when fault set
Visually inspect wiring and connectors at the reductant control module for heat damage, corrosion, or shorts
Check battery voltage with ignition ON and engine running using a DMM
Inspect charging system (alternator output and regulator) for over-voltage conditions
Verify module ground integrity and resistance to chassis ground
Check for aftermarket accessories or jump-start connections that may have applied higher-than-normal voltage

Nominal supply (vehicle 12V system): ~11–14.5 V with engine running
High fault threshold (varies by manufacturer): typically >15–16 V
Low fault threshold (related):
Signal type: constant B+ or ignition-switched B+ power feed to reductant control module
Check points: module power pin voltage (ignition ON), battery terminal voltage, alternator output under load

Retrieve freeze frame and other related DTCs. Note when the code set (engine off, cranking, driving).
Perform a visual inspection of the reductant module power connector, harness routing, and ground points for damage, corrosion or obvious shorts.
Measure battery voltage at battery posts and at reductant module power pin with ignition ON and engine running. Record values.
If measured voltage at the module is greater than battery voltage, inspect wiring for a short to an auxiliary B+ source or incorrect wiring tapping.
Test alternator/regulator output. If charging system produces >15–16 V, repair charging system (alternator/regulator, voltage regulator) and clear codes.
Check module ground(s): measure resistance from module ground to chassis (
Disconnect reductant control module connector and measure voltage at the harness side. If high voltage persists with module disconnected, problem is upstream (wiring or charging system).
If harness voltage is normal when disconnected but module reports high voltage when connected, suspect internal module regulator or internal fault. Consider replacing module or performing manufacturer-specific module tests.
After repairs, clear codes, perform functional test (dosing system operation) and drive/operate vehicle to confirm code does not return.
If intermittent, consider back-probing the connector with scope to capture transient over-voltage events; consult service manual for component-specific test values.