P1751

P1751 is a Mitsubishi-specific code meaning the TCM detected a fault in the A/T Control Relay (the relay that supplies switched battery voltage to all transmission solenoids and the TCM solenoid driver outputs) — either the relay isn't energizing when commanded, isn't dropping out when commanded, or the feed-voltage rail is out of spec. The A/T Control Relay sits in the underhood fuse/relay block on most Mitsubishi applications and acts as the master power switch for the entire shift-control circuit; when it fails, the TCM loses the ability to actuate any solenoid and the trans defaults to limp-mode (typically locked in 3rd gear with no torque-converter lock). Cheapest-first ladder: (1) Pull the A/T Control Relay from the fuse block and swap it with an identical relay (horn, fuel pump, etc.) — if the fault clears, the relay was bad ($0 diagnostic, $15 replacement). (2) Verify switched-battery voltage at the trans connector on the solenoid feed pin with KOER — should be battery voltage (12.5-14.5V); if 0V with KOER, the relay isn't energizing or the relay-control wire from the TCM is open. (3) Backprobe the TCM relay-control pin per FSM — should be 12V key-off (relay coil de-energized) and 0V key-on (TCM pulls the coil to ground to energize); no transition means TCM driver dead or open in the control wire. (4) Check the trans-solenoid feed fuse — a blown fuse on the relay output side will set P1751 along with multiple solenoid codes simultaneously. Expensive-misdiagnosis caveat: don't replace the TCM for a P1751 without first verifying the relay coil isn't stuck or the relay socket isn't carbon-tracked from a prior overheat — Mitsubishi's relay sockets are known to char internally after a solenoid short event, and replacing the TCM without replacing the socket causes the new TCM to fail the same way within weeks.

Vehicle-specific patterns: 2002-2007 Mitsubishi Lancer 2.0L (4G94) throws P1751 from relay-socket corrosion in the underhood fuse block after water intrusion past the box seal — common on cars parked outdoors in northern climates. 2004-2012 Mitsubishi Galant 2.4/3.8L sees P1751 paired with multiple solenoid codes when the A/T relay fuse blows from a downstream solenoid short; trace and fix the short before replacing the relay or the new relay blows on first key-on. 2006-2012 Mitsubishi Eclipse 3.8L and 2004-2011 Endeavor 3.8L share the same fuse-block layout and the same relay-socket carbon-track pattern. 2007-2013 Mitsubishi Outlander 2.4/3.0L throws P1751 from a different failure mode — on CVT-equipped variants the A/T relay also feeds the stepper-motor power, and a stepper short pulls the relay coil out of range. 2001-2006 Mitsubishi Montero Sport 3.5/3.8L sees P1751 from harness corrosion at the firewall pass-through where the relay-output wire enters the cabin to reach the TCM. Estimated repair: $20 to $450.