P0735: Gear 5 Incorrect Ratio

P0735 means the shaft speed relationship measured while fifth was commanded did not match what fifth should produce. Two things about this particular gear shape the entire diagnosis, and neither applies to the gears below it.

The first is that fifth is barely ever used. It is only commanded above a threshold road speed, with light throttle, once the transmission is warm and the vehicle is cruising steadily. That is a narrow set of conditions and none of them occur on a short drive. So a vehicle brought in with this code and given a ten-minute road test around local roads has been exercised in first through fourth and never once put into the gear the code concerns. The report that comes back — no fault found, code did not return — is accurate and completely uninformative. Reproducing this fault requires a sustained run at motorway speed with light throttle and the fluid fully up to temperature, watching live data. If nobody has done that, the fault has not been tested for.

The second is that a fifth-gear slip produces almost no drivability complaint. There is very little torque going through the transmission when the vehicle is cruising, so a marginal element that would slip obviously in first will slip almost imperceptibly here. What the driver notices, if they notice anything, is engine speed sitting a few hundred revolutions higher than it used to at the same motorway speed, and fuel economy quietly getting worse. Nothing bangs, nothing flares, nothing lurches. These codes are typically found during a scan for something else entirely. It is worth stating plainly that the absence of symptoms is not evidence of the absence of a problem, because that assumption is how a top-gear slip gets ignored until it becomes something larger.

There is a caveat that has to come before any parts are ordered. Fifth is not a consistent physical thing across designs the way the lower gears broadly are. It only exists on transmissions with five or more forward ratios, and on those it is produced in different ways — sometimes by an additional planetary set, sometimes by a second overdrive element, and on some designs by an add-on overdrive section behind the main gearbox. Confirm from service data how many forward gears your transmission actually has and which element serves fifth before anyone quotes for a part. A generic answer to this code does not exist.

The genuinely good news, and it is worth saying because transmission codes rarely carry any, is that a fault confined to fifth narrows the suspect list considerably rather than widening it. Every element used in first through fourth has just demonstrated that it works. Whatever has failed is either the element unique to fifth or the solenoid that commands it, and both of those are a smaller, shallower and cheaper proposition than a fault in a gear the vehicle depends on to move off. If the ratio codes had covered several gears the outlook would be much worse. Confined to top gear, this is the best version of this family of codes to get.