P0734

P0734 sets when the TCM commands 4th gear (typically overdrive on a 4-speed, direct or first overdrive on a 5-6 speed) and the calculated ratio is outside the expected 4th-gear window by 5-7% for the dwell time. On a 4-speed, 4th is overdrive (typical 0.69-0.72:1) and the apply element is almost always the overdrive band or overdrive clutch, both of which are wear items, P0734 in this context is the textbook symptom of overdrive-band burn or overdrive-clutch-pack failure. On 5-6 speeds, 4th is typically a direct or near-direct ratio and the apply elements vary. Cheapest-first: ATF level and condition at spec temp, then line pressure under load (4th gear pressure is often lower than lower gears, but should still hit 80-120 PSI at light throttle), then scan-tool ratio monitoring during a 3-4 upshift, the ratio should drop from 1.00 (or thereabouts) to the 4th-gear value cleanly without flare. A flare (engine RPM jumps before the ratio settles) confirms slip in the apply element. Expensive misdiagnosis: do not authorize a rebuild without first checking for a stuck TCC solenoid, a stuck-on torque-converter clutch in 3rd combined with a healthy 4th can mimic a P0734 if the TCM logic confuses TCC state with gear state, also a $60 TCC solenoid replacement sometimes clears the code on platforms where the TCC solenoid and 4th-gear apply share fluid paths.

Vehicle-specific patterns: 1994-2007 Ford 4R70W/4R75W (F-150, Mustang, Crown Vic) is famous for overdrive-band burn, the OD band wears as a maintenance item and replacement at 150K is preventive; 2001-2007 Chrysler 42RLE (Dakota, Liberty, 300, Charger) sees P0734 from OD clutch-pack wear, Chrysler TSB 21-001-07 covers the updated clutch-pack hardware; 2005-2010 Honda Odyssey/Pilot 5AT (BGRA, BYBA) has known 4th-gear apply issues tied to torque-converter lockup damage propagating to the 4th-clutch pack, Honda Service Bulletin 11-033 covered affected VINs with extended warranty. Estimated repair: $350 to $4,500.