P20EB

Code

Generic P — Powertrain

Reductant Control Module Power Relay De-Energized Performance - Too Late

Brand: Generic

Type: P — Powertrain

Views: UK: 27 EN: 43 RU: 27

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Page language: EN

Stuck or welded relay contacts (relay mechanically or electrically failing)
Relay coil circuit shorted to power or ground preventing release
Faulty relay (coil or internal contacts)
Poor or intermittent ground at relay or RCM
Corroded, loose or damaged connector or wiring in relay control or supply circuits
Low battery or unstable supply voltage during relay operation

MIL / Check Engine Light illuminated
Reduced or no reductant dosing (SCR system fault)
Exhaust aftertreatment warnings (e.g., SCR/DEF system)
Possible limp-home mode or reduced engine performance depending on vehicle strategy
Erratic operation of reductant heater, pump, or valves
Possible difficulty starting or unexpected battery drain if relay stuck closed

Read freeze-frame and full code list with a capable scan tool; note conditions when code set
Visual inspection: relay, relay socket, fuses, wiring harness, and RCM connector for corrosion, heat damage, or loose pins
Verify battery voltage and charging system health (key on and during cranking/engine run)
Check fuse(s) for RCM power feed and replace if corroded or open
Backprobe relay coil control terminals to observe control signal (voltage or ground) while reproducing condition
Backprobe relay power contact terminals to verify battery feed and switched feed to RCM when energized and after command is removed

Battery voltage (key ON, engine OFF; during engine running): expected ~12–14.5 V
Relay coil control voltage or ground command: 0 V/near-battery voltage or pulsed signal depending on driver type
Voltage at relay coil when commanded OFF (should be released/no drive)
Switched output voltage from relay to RCM when energized (should be battery voltage)
Time between command-off and actual relay contact open (ms) — compare to factory spec or expected immediate release
Relay coil current draw when energized (mA) — abnormal high or zero indicates fault

Retrieve P20EB and any related codes; record freeze-frame data and attempt to reproduce fault conditions.
Visually inspect relay, relay socket, wiring harness and RCM connector for damage, corrosion, or water ingress; repair as needed.
Verify fuses and battery/charging system voltages; restore proper supply if low or unstable.
With a multimeter/oscilloscope backprobe relay coil control and power terminals. Observe command signal while commanding relay ON/OFF from the scan tool or during normal operation.
If relay does not release when command removed, remove relay and bench test/replace relay. Re-test with known-good relay.
If relay behaves correctly off-vehicle, verify wiring between relay and RCM for shorts to battery or to ground, opens, or high resistance. Repair harness as required.
If wiring and relay are good, monitor RCM output driver. If the module is continuing to drive the relay when it should be OFF, check module grounds and power, clear codes, and re-test.
If module output is internally stuck ON or intermittent after harness and supply checks, verify for available software/firmware updates. Replace RCM only after confirming wiring and relay are good and following manufacturer procedures.
After repair, clear codes and complete several drive cycles/regeneration cycles to confirm fault does not return.

⚠️ Status

RCM power relay did not de-energize within the expected time window; relay remained energized longer than allowed or control did not remove drive.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours

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P20EB — Reductant Control Module Power Relay De-Energized Performance - Too Late