سایش کلاچ یا نوار دنده عقب

The clutch pack or band providing reverse, worn so it cannot hold. Reverse often uses an element used in no forward gear, which is why it can fail alone — a transmission that drives forward perfectly and slips or refuses in reverse is describing exactly that.

The module sees it as a ratio error in reverse only, and stores accordingly. Rule out hydraulics before wear: low fluid, a stuck solenoid or low line pressure produce the same symptom and are far cheaper. Fluid condition is the evidence — burnt-smelling fluid and friction material on the sump magnets confirm wear and say the repair is internal, which is the point at which the conversation becomes about whether the transmission is worth rebuilding.