P0784

P0784 is the 4-5 shift malfunction — TCM commanded a 4-5 upshift, didn't see the input/output ratio drop to the fifth-gear target (around 0.85:1 on a typical 6-speed's fifth, around 0.7:1 on a 5-speed's fifth), and set the code. This code only applies to transmissions with five or more forward gears, so you're looking at a modern 5/6/8/10-speed automatic, not an old 4-speed. Cheapest-first ladder: (1) Fluid level + condition. On these modern units, the fluid spec is platform-specific (Mercon LV for Ford 6R80, Dexron HP/ULV for newer GM, ZF Lifeguard 8 for ZF 8HP, Toyota WS for Aisin) and using the wrong fluid is itself a P078X cause. (2) Live data during a 4-5 shift event (highway cruise, light throttle around 55-70 mph); watch for shift flare or harsh apply. (3) Read all codes — P0784 paired with PCS solenoid codes (P0745, P0775) points at the valve body; paired with P0729 (gear ratio incorrect, fifth) confirms the fifth-element clutch is slipping. (4) Drop the pan, magnet inspect, fluid sample sent to a lab if the customer is high-mileage and considering a rebuild. The expensive-misdiagnosis caveat — and this one's load-bearing: don't condemn the trans before confirming fluid level + condition. A burned-fluid 4L60E with a P0784 needs internal repair, not a $90 solenoid; the same logic applies to every modern multi-speed automatic, where the cost of a rebuild is 3-5x what it was for the 4-speed predecessors.

Vehicle-specific patterns: 2010-2019 Ford 6R80 (F-150, Mustang, Explorer) throws P0784 from valve-body wear and lead-frame degradation; Ford sells the lead frame as a service part, $200-400, but proper diagnosis often points at the full valve-body rebuild at $1,400-2,800. 2007-present GM 6L80/6L90 (Silverado, Tahoe, Camaro SS) throws P0784 from valve-body wear and from 3-5-R clutch failure (this is the clutch pack that applies in 3rd, 5th, and Reverse on the 6L80, so a worn 3-5-R clutch presents as P0784 AND reverse slip simultaneously); rebuild is $2,400-4,000. 2009-present ZF 8HP (RAM 1500, BMW, JLR) throws P0784 from mechatronic problems and clutch wear; ZF mechatronic service or rebuild is $2,800-5,500. 2013-present Aisin AS69RC (RAM 2500/3500 HD) throws P0784 from internal failure and is a $5,500-8,000 rebuild given the part costs and labor. Estimated repair: $400 (fluid + filter + solenoid) to $8,000 (AS69RC rebuild).