P21C9

Code

Generic P — Powertrain

Reductant Control Module Power Relay/Relays Control Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 21 EN: 39 RU: 31

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Page language: EN

Causes

Faulty reductant control module power relay (stuck or internally shorted)
Short to battery voltage on the relay control wire
Open or high-resistance ground or supply to the relay coil or module
Damaged, corroded, or disconnected wiring/connector (pin corrosion, broken conductor)
Faulty reductant control module (driver output transistor/IC)
Blown or incorrect fuse supplying relay power

Symptoms

Malfunction Indicator Lamp (MIL) or DEF system warning illuminated
Reductant/DEF pump, heater, or dosing valve inoperative or intermittent
Reduced or inhibited regeneration events (if system relies on reductant)
Possible limp mode or reduced engine performance depending on system strategy
Stored manufacturer-specific reductant control module fault codes

What to check

Obtain freeze frame/live data and scan for related manufacturer codes and freeze frame parameters
Visually inspect relay, fuse, wiring harness, and connectors for damage, corrosion, or heat discoloration
Verify correct fuse(s) for reductant relay are present and intact
With ignition ON (but engine off), backprobe relay control and supply pins to measure voltages
Check continuity and resistance of control and ground wires between relay and RCM
Inspect for aftermarket devices or recent repairs in the DEF/reductant circuit path

Signal parameters

Battery supply voltage at relay feed: approx. 11–14.5 V (key ON/engine running)
Relay control/driver voltage (expected): near 0 V when held low by module (ground-driven) or battery voltage when supply side; a persistent >8–12 V on a control/feedback line can trigger this code
Relay coil resistance (typical): a few ohms to tens of ohms depending on relay — compare to spec or identical relay
Ground continuity: near 0 Ω between relay ground pin and chassis ground
Switching behavior: relay control may be steady or pulsed; confirm expected duty cycle with manufacturer data if available

Diagnostic algorithm

Read and record all codes and freeze frame parameters; clear codes and re-run to confirm repeatability.
Visually inspect relay, fuse, connector, and wiring for burns, corrosion, pin damage, or poor routing. Repair obvious issues.
Identify relay pinout (supply, control, output, ground). With key ON, measure supply at relay feed and control/feedback pin voltage. Note values with relay commanded ON and OFF via scanner or key cycle.
Test relay coil resistance against specification or swap with identical known-good relay from a noncritical circuit and retest the fault. Replace relay if defective.
Check continuity and resistance between the relay control pin and the reductant control module output pin. Repair broken/open/high-resistance wiring or poor grounds.
Probe for a short to battery: with circuit disconnected, measure resistance from control wire to battery positive; a low resistance indicates a short. Isolate and repair short.
If wiring and relay check good, monitor the module output with oscilloscope or DVOM while commanding the device; unusual high/static voltage indicates internal module driver failure — consider module replacement.
After repairs, clear codes and perform actuations and road-test to confirm that the code does not return. Re-scan for additional faults and confirm system function.
Note: follow manufacturer service manual for safe handling of reductant/DEF components and to avoid contamination.

Likely causes

Wiring short to +12V on the relay control/feedback circuit
Failed relay coil or relay contacts causing abnormal feedback voltage
Connector corrosion at the relay or RCM causing high resistance and voltage anomaly
Internal RCM driver failure causing incorrect output voltage

Fault status

⚠️ Status

Reductant control module power relay control circuit voltage higher than expected. Possible short to battery, relay or module fault, or wiring/connectivity issue. Inspect relay, fuses, wiring, and module outputs.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours