P1791

P1791 is a Mitsubishi-specific code meaning the TCM and ECM disagree about the engine coolant temperature signal — the ECM reads ECT directly from the coolant-temp sensor and broadcasts the value to the TCM over a hardwired signal line (pre-CAN platforms) or the data bus (CAN-equipped platforms); P1791 sets when the value received by the TCM is implausibly different from the value the ECM is using, or the signal is missing entirely. The TCM needs ECT for shift-timing decisions (cold-shift schedule vs warm-shift schedule) and torque-converter lockup enable, so this fault forces limp-mode shifts. Cheapest-first ladder: (1) Verify the underlying ECT sensor first — pull live data and compare actual ECT (from the ECM) against a known-good thermometer at the upper radiator hose; if the ECM's value is wrong, fix the sensor and the P1791 clears as a side effect ($0, 10 min). (2) Inspect the hardwired ECT-to-TCM signal wire (typical on F4A4/F5A5 applications) — it runs from the ECM to the TCM and corrodes at the firewall pass-through; a 5-volt bias should sit on the signal pin at KOEO and drop linearly as engine warms ($0 with a DVOM). (3) On CAN-equipped Mitsubishi (post-2008 Outlander, Lancer Evo X, etc.), check for U-codes (U0100, U0101) alongside P1791 — bus dropouts will manifest as P1791 because the TCM never receives the ECM's ECT broadcast. (4) Inspect TCM ground at the cabin grounding stud — a corroded ground floats the TCM's reference and biases every analog input including the ECT pickup. Expensive-misdiagnosis caveat: don't replace the TCM for P1791 before verifying the ECM is reading correct ECT and the signal wire between them is intact — Mitsubishi shops have replaced $900 TCMs only to find the actual problem was a $25 ECT sensor that was reporting one wrong value to both modules; the TCM was just dutifully flagging that something upstream was sending nonsense.

Vehicle-specific patterns: 2002-2007 Mitsubishi Lancer 2.0L (4G94) commonly throws P1791 from ECT-sensor wiring corrosion at the thermostat housing — the connector is mounted in the splash zone behind the timing belt cover and catches water from the wheel well. 2004-2012 Mitsubishi Galant 2.4/3.8L throws P1791 alongside P0115/P0117 ECT codes when the sensor itself fails open. 2006-2012 Mitsubishi Eclipse 3.8L and 2004-2011 Endeavor 3.8L sees P1791 from the same ECT sensor corrosion pattern. 2007-2013 Mitsubishi Outlander 2.4/3.0L (CAN bus) throws P1791 typically as a downstream effect of a U-code bus dropout — fix the bus first, P1791 clears. 2001-2006 Mitsubishi Montero Sport 3.5/3.8L throws P1791 from harness chafe where the ECT signal wire crosses the engine lift bracket. Estimated repair: $25 to $500.