P20C9

Code

Generic P — Powertrain

Reductant Control Module Requested MIL Illumination

Brand: Generic

Type: P — Powertrain

Views: UK: 27 EN: 68 RU: 45

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

RCM-detected reductant system fault (injector, pump, heater, sensors)
Loss or corruption of CAN/LIN communications between RCM and PCM/ECM
Intermittent or low supply voltage or poor ground to the RCM
Wiring harness or connector corrosion/damage
Faulty RCM internal failure or software/calibration mismatch
Secondary aftertreatment faults (NOx sensors, SCR catalyst) causing RCM request

Symptoms

MIL (check engine light) illuminated at driver's request
Stored and pending DTCs in RCM and possibly PCM/ECM
Possible reduced power / limp mode depending on vehicle strategy
Emissions system warnings or DPF/SCR-related derate messages
Visible leaks, crystallized/red residue of reductant near fittings

What to check

Read and record all stored, freeze frame and pending codes from PCM/ECM and RCM using dealer-level or bi-directional scanner
Check for active communication with the RCM (CAN/LIN status, module present)
Inspect battery voltage and charging system (measure at battery and RCM connector) under key on and cranking
Visually inspect wiring, connectors, and harness routing to RCM, reductant pump, injector, and sensors for damage or corrosion
Check reductant level, quality (contamination), and presence of crystallized deposits at joints and filters
Perform manufacturer-recommended actuator tests (pump run, injector pulse) if supported

Signal parameters

CAN communication: module present/absent and error count (bus error frames)
Battery supply voltage at RCM: typically ~12–14.5 V (key on/engine run)
RCM power/ground circuit continuity and voltage drop under load
Reductant temperature sensor voltage or resistance (manufacturer range)
Reductant level sensor resistance or level percentage
Dosing valve/pump command duty cycle and measured current draw

Diagnostic algorithm

Connect a capable scan tool and retrieve codes from PCM/ECM and RCM. Save freeze frame and event data.
Note any RCM-specific DTCs that identify the subsystem (pump, injector, sensors) and prioritize those for repair.
Verify RCM is communicating on the vehicle network. If RCM is not present, check power, ground, and CAN termination.
Measure battery voltage at the RCM connector with key on and engine running. Check for excessive voltage drop and poor grounds.
Visually inspect all reductant system wiring, connectors, and mechanical components for damage, corrosion, fluid ingress, or crystallization.
Check reductant level and quality; top up with correct fluid if low. Replace filters and clean fittings if contaminated.
Perform bi-directional tests: run pump, command dosing valve, and monitor current/response. Compare to manufacturer expected values.
Test related sensors (temperature, level, conductivity, NOx) for correct resistance/voltage and response to temperature or level changes.
If communication faults persist, trace CAN/LIN wiring, check termination resistances, and repair any shorts/opens.
If module hardware and wiring check good but fault persists, verify software calibration levels and update RCM/ECM software if available.
Clear codes and test drive or run required self-tests/drive cycles. If fault returns, replace faulty component(s) per manufacturer procedure.

Likely causes

Failed reductant dosing valve or pump (mechanical or electrical)
Reductant temperature, level, or conductivity sensor out of range
Open/short in CAN bus or high bus error counts
Corroded/wet connectors at RCM or urea system components
Battery voltage problems or intermittent power supply to module
RCM software version incompatible with ECM or recent module replacement

Fault status

⚠️ Status

RCM requested MIL illumination due to detected reductant system or communication fault. The condition requires diagnosis of the reductant dosing system, sensors, wiring, power/ground, and module communication before repair.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours