Code
P21C7
Generic
P — Powertrain
Reductant Control Module Power Relay/Relays Control Circuit/Open
AI status
Completed
Completed
100%
Causes
- Blown fuse for reductant control or associated relay coil supply
- Open or damaged wiring in relay control or power feed circuit
- Corroded or loose connector at the reductant control module or relay
- Failed power relay (stuck open or coil open)
- Faulty reductant control module (internal open)
- Poor ground at module or relay
Symptoms
- MIL (check engine light) illuminated
- Reduced or no reductant pump/injector operation (DEF system inactive)
- DEF or SCR system warning lamp on dashboard
- Possible engine derate or regen inhibition due to DEF failure
- No relay click when reductant system is commanded
What to check
- Read and record freeze-frame and all stored DTCs with a capable scan tool
- Visually inspect fuses, relay, connectors, and harness for damage or corrosion
- Check for presence of relay click when the reductant system is commanded via scan tool or key cycles
- Backprobe relay coil pins and control pin at reductant control module while commanding the relay ON
- Measure battery voltage at relay power feed and continuity to reductant control module power pin
- Verify good ground at module and relay ground pin(s)
Signal parameters
- Battery feed (relay supply): ~12 V with key ON (9–14 V acceptable)
- Relay coil control (PCM/ECM output): toggles between near-battery voltage (open) and ~0 V when switched to ground (or vice versa depending on design)
- Relay coil resistance: typically a few ohms to a few tens of ohms; infinite/resistance out-of-range indicates open coil
- Continuity: power feed and ground to reductant module should show low resistance (
- Open-circuit indication: infinite/OL on continuity or voltage missing at expected pin
Diagnostic algorithm
- Verify DTC P21C7 is current and note freeze-frame data; clear codes and re-run to confirm repeatability
- Inspect fuses and relay for the reductant/DEF system; replace suspect fuse or swap relay with identical known-good relay and re-test
- Visually inspect wiring harness and connectors between PCM/ECM, power relay, and reductant control module for corrosion, damage, or disconnection; repair as needed
- Backprobe the relay coil power and control pins. With key ON or while commanding the reductant system ON from a scan tool, verify battery voltage at the relay supply and that the control pin changes state (to ground or battery) to energize the relay
- If control pin does not change, check continuity between PCM/ECM control output and relay coil; if open, trace and repair wiring or connector fault
- Measure coil resistance of relay out of circuit; if coil is open, replace relay and retest
- Verify module power and ground at the reductant control module connector; repair grounds or power feed if missing
- If wiring and relay check OK but control output never toggles, investigate PCM/ECM driver circuit — consult manufacturer service info before replacing ECM
- After repairs, clear codes, perform active tests (command reductant pump/injector/relay) and road/test to confirm proper function
Likely causes
- Blown fuse or open wiring to the relay coil
- Failed relay (coil open)
- Disconnected/corroded connector at reductant control module or relay
- Open control output from PCM/ECM
Fault status
Status
Reductant Control Module Power Relay/Relays Control Circuit — Open (P21C7)
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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