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DTC codes / Mitsubishi codes / P1751

P1751 · MitsubishiA/T Control Relay

P1751 on Mitsubishi vehicles indicates that the transmission control module (TCM) has detected a fault in the automatic transmission control relay circuit, which supplies power to the solenoids and TCM itself. The TCM monitors the relay's output voltage and sets this code when the expected voltage is not present with the relay commanded on, or when the circuit is open or shorted.

SystemPowertrain
Code typeMitsubishi specific
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • The check engine light or transmission warning light illuminates on the dashboard.
  • The transmission may enter limp mode, sticking in a single gear or refusing to shift.
  • Harsh or delayed gear engagements, especially when cold.
  • No overdrive or torque converter clutch lockup operation.
  • In some cases, the vehicle may crank but fail to start if the relay also supplies power to other circuits.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.

Common causes, most likely first

  1. Failed automatic transmission control relay (internal coil or contact wear).
  2. Blown fuse or fusible link supplying the relay.
  3. Corroded, loose, or damaged wiring or connectors at the relay, TCM, or transmission solenoids.
  4. Open or shorted wire in the relay control circuit between the TCM and relay.
  5. Faulty TCM driver circuit for the relay.
  6. Poor ground connection for the relay or TCM.
  7. Less commonly, a shorted solenoid or internal transmission harness fault loading the circuit.

How to diagnose and fix it

  1. Verify the battery is fully charged and check all fuses related to the transmission control system, including under-hood and under-dash boxes.
  2. Locate the automatic transmission control relay (often in the engine bay fuse box) and swap it with an identical known-good relay to see if the code changes.
  3. Inspect the relay socket and nearby wiring for corrosion, loose pins, or chafed insulation.
  4. With a multimeter, check for battery voltage at the relay's power terminals with the key on, and check the ground circuit for continuity.
  5. Backprobe the relay control wire at the TCM and command the relay on with a scan tool to verify the TCM is grounding the circuit.
  6. If the control circuit is intact but the relay does not click, replace the relay; if the relay clicks but voltage does not pass, check the relay contacts and load circuit.
  7. Clear the code and test-drive the vehicle through all gears to confirm the repair and ensure the code does not return.

Relay circuit checks before TCM replacement

  • A common misdiagnosis is replacing the TCM when the actual fault is a $10 relay or a corroded fuse box terminal that interrupts power to the transmission solenoids.
  • The transmission control relay often shares power with other circuits, so a failure can cause unrelated symptoms like a no-start or intermittent dash lights, leading technicians away from the transmission.
  • Intermittent P1751 codes are frequently caused by a relay that tests fine when cold but fails when hot; always test the relay under load or swap it with a known-good unit.
  • Some Mitsubishi models use a relay integrated into the TCM or underhood fuse box, making diagnosis more involved and sometimes requiring replacement of the entire assembly.
  • A shorted solenoid or damaged internal transmission harness can overload the relay circuit and set P1751 without blowing a fuse, so check solenoid resistance if the relay and wiring test good.

Can I keep driving with P1751?

If the transmission is in limp mode or shifting erratically, avoid highway driving and seek repair promptly to prevent transmission damage. If only the warning light is on and the vehicle drives normally, you can drive cautiously to a workshop, but avoid heavy loads or towing.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Mitsubishi vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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