P0781

P0781 is the gear-specific version of P0780 for the 1-2 shift — the TCM commanded the 1-2 upshift, monitored input/output shaft ratio change, and didn't see the ratio land at the second-gear value within the allowed window (a 4L60E in second should read input/output ratio of roughly 1.62:1; if it's still at the first-gear 3.06:1 the 1-2 shift element failed). Cheapest-first ladder: (1) Verify fluid level cold and again at operating temp (most automatics are spec'd checked hot, in park, on level ground) and inspect color/smell — pink-and-burned fluid means clutch material in the fluid, no solenoid fix will help. (2) Scan-tool live data: command a 1-2 shift and watch commanded gear go from 1 to 2 while actual ratio stays at 3:1 — confirms the 1-2 element (2-4 band on a 4L60E, intermediate clutch on a 5R55) is slipping or not applying. (3) Pull the pan and ohm the 1-2 shift solenoid (typical 8-30 ohms cold; off-spec means coil failure, on-spec moves the suspect to valve-body bore wear or the band/clutch itself). (4) Drop the pan, magnet, and filter and read the debris — clean magnet + on-spec solenoid + slipping 1-2 = valve body, dirty magnet = internal repair. The expensive-misdiagnosis caveat: a 4L60E with a P0781 and bronze on the pan magnet is a planetary bushing problem masquerading as a shift fault — replacing the solenoid will get you 50-200 miles before the trans fails harder, and now the rebuild is more expensive because of the additional collateral damage.

Vehicle-specific patterns: 1995-2014 GM 4L60E/4L65E throws P0781 from 2-4 band wear (the 2-4 band is the element that applies for second gear) and from the 1-2 accumulator piston cracking — band replacement requires a unit-out rebuild on most cases, $1,800-2,800 done correctly. 1996-2008 Ford 5R55E/5R55W (Explorer, Mountaineer) throws P0781 from the intermediate band servo bore wearing oval, leaking pressure away from the band apply circuit; Sonnax oversized servo kit is $90-180 part but it's a valve-body-out job. 2003-2012 Chrysler 545RFE throws P0781 from the underdrive clutch pack glazing — full clutch replacement is rebuild territory. 2007-2017 Toyota A750/A760 (Tundra, Tacoma, 4Runner) throws P0781 from the underdrive solenoid sticking and from valve-body wear around 150k miles; Toyota valve-body service is $1,200-2,200 including reseal. Estimated repair: $400 (solenoid + filter + fluid) to $3,200 (rebuild).