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Home / DTC / P1801 — Automated-M/T Fail(relay ON)

P1801 — Automated-M/T Fail(relay ON)

Detailed page for trouble code P1801.

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P1801

MITSUBISHI P — Powertrain

Automated-M/T Fail(relay ON)

Brand: MITSUBISHI
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Faulty transmission/MT (manual) control ECU (TCM/MT ECU)
  • Stuck, welded, or failed transmission control relay
  • Blown fuse or poor power supply to relay/TCM
  • Open, shorted, or corroded wiring/connectors in relay control or ECU power/ground circuits
  • Failed shift actuator, clutch actuator or solenoid drawing excessive current
  • Faulty gear position or speed sensor input to transmission ECU

Symptoms

  • MIL (check engine) and/or transmission warning lamp illuminated
  • Transmission stuck in single gear, limp-home mode, or refuses to shift properly
  • Loss of drive or unpredictable shifting
  • Noises from actuator/gearbox when relay is activated
  • Inability to command transmission actuators from a scan tool

What to check

  • Read stored codes and freeze frame data with a diagnostic scanner; record conditions when fault set
  • Check battery and charging system voltage (engine running and key ON)
  • Inspect fuses and the transmission/relay fuse/relay socket for corrosion or heat damage
  • Visually inspect wiring and connectors to MT/TCM, relay, and actuators for damage or corrosion
  • Backprobe relay control and power circuits to confirm voltages when relay is commanded ON/OFF
  • Command relay and actuators with a scan tool (if supported) and observe operation

Signal parameters

  • Battery voltage with key ON and engine running: ~12.0–14.5 V
  • Relay control voltage (from ECU): 0 V (OFF) and battery voltage (~12 V) when commanded ON
  • Relay coil resistance: manufacturer-specific; typical relay coils ~50–200 Ω (verify with OEM spec)
  • Actuator/solenoid operating voltage: near battery voltage when activated
  • Actuator current draw during operation: compare to expected range (excessive current indicates binding/short)
  • CAN bus status: normal message traffic present between TCM/ECU and PCM/instrument cluster

Diagnostic algorithm

  1. Retrieve P1801 and any related codes; note freeze frame and conditions when set.
  2. Verify battery and charging system are within spec; a weak battery can cause control faults.
  3. Inspect fuses and the transmission relay (visual damage, heat, corrosion); replace or swap with identical relay to test.
  4. With a scanner, command the transmission relay/actuators ON and OFF; observe relay clicking and actuator movement. Record voltages and currents.
  5. Backprobe relay power and control terminals while commanding ON: confirm battery feed and ECU control signal present.
  6. Inspect wiring and connector continuity between relay, MT/TCM and actuators; repair any damaged wiring or corroded pins.
  7. Measure actuator motor/solenoid resistance and current draw; remove mechanical binding if actuator draws excessive current.
  8. If wiring, power and actuator tests are good but fault remains, consider replacing the transmission control ECU (verify with manufacturer tests) or consult factory service information.
  9. Clear codes and perform road test to confirm repair; if intermittent, consider wiring harness stress points and re-check after reproduction.
  10. If unable to diagnose, capture live data and consult manufacturer-specific troubleshooting and wiring diagrams.

Likely causes

  • Relay contacts stuck closed or relay coil control circuit shorted to ground/power
  • Intermittent or low battery/charging voltage when relay is commanded ON
  • Corroded connector or poor ground at MT ECU or relay socket
  • Internal fault in the transmission control ECU detected while relay energized

Fault status

⚠️ Status
Automated manual transmission failure detected while control relay was ON. System entered fail-safe/limited mode. Inspect relay, power/ground, wiring, actuators and transmission control ECU.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

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406

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