P0729

P0729 sets when the TCM calculates that input shaft speed divided by output shaft speed in commanded 6th gear falls outside the expected ratio window (typically a 10-15% tolerance) for a calibrated number of samples. Cheapest-first ladder: scan live data for input/output speed sensor signals in 6th gear and look for noise, dropouts, or implausible values before condemning hard parts; check transmission fluid level and condition on a level surface at operating temp (burnt smell or brown color is a finding by itself); pull the TCM for companion codes (P0700/P0717/P0722 reframe the diagnosis); inspect the internal wire harness/pass-through connector for fluid migration that wicks into the TCM. Caveat: a true ratio fault in 6th specifically usually points to a slipping clutch pack or a stuck shift valve for that gear, not a generic 'transmission failing' verdict, so do not authorize a rebuild on a P0729 alone without verifying the slip on a road test with the scan tool graphing both speeds.

Vehicle-specific patterns: 2011-2015 Ford F-150 with the 6R80 commonly throws P0729 from a worn direct clutch pack and a cracked input drum, found on teardown after the scan tool shows 6th-gear slip rising with throttle. 2009-2014 BMW 5/7 series with the ZF 6HP19/6HP26 sets P0729 when the mechatronic sleeve and adapter plate leak internally. 2007-2013 Chevrolet Silverado 1500 with the 6L80 sets P0729 alongside harsh 5-6 shifts when the 3-5-Reverse wave plate fractures. 2010-2016 Volvo XC60/XC70 with the Aisin AW TF-80SC develops sixth-gear slip from valve body bore wear, repairable with a Sonnax oversized pressure regulator and boost valve kit. Estimated repair: $450 to $4,800.