P0734: Gear 4 Incorrect Ratio

P0734 sets when the input-to-output shaft speed relationship measured in fourth does not match what fourth should produce. The reason this code deserves a different diagnostic approach from the lower gears is that on most vehicles fourth is where the torque converter clutch is commanded to lock, and a converter clutch that is not holding produces exactly the same measurement as a gear that is not holding.

The torque converter is a fluid coupling. In normal operation there is always a small amount of slip across it, because that is how fluid couplings work. To eliminate that loss at cruising speed, a clutch inside the converter is applied, mechanically tying the engine directly to the transmission input. When it works, engine speed and input shaft speed become effectively identical. When the clutch is worn or the fluid has lost its friction properties, the converter clutch slips instead of locking, engine speed sits above where it should be, and the module reads the resulting mismatch. It may store a converter clutch code, or it may store this one, because from the ratio calculation's point of view a slip is a slip regardless of where in the driveline it happened.

That matters commercially as well as technically, because the torque converter is not part of the gearset. It is a separate assembly, it is not repaired by anything done to clutch packs or bands, and on the majority of vehicles reaching it means removing the transmission — which puts the labour in the same territory as a rebuild even though the failed part is completely different.

There are clear symptoms that separate the two branches, and they cost nothing to look for. A slipping converter clutch produces a fine, rapid shudder at light throttle and steady cruise, most often felt somewhere between roughly 40 and 60 miles per hour, which drivers usually describe as driving over a rumble strip or as the road surface having changed. Crucially, it stops the instant you touch the brake pedal or open the throttle hard, because both of those commands unlock the converter clutch. A gearset slip does not behave that way. A gearset slip gets worse under load, not better.

Fourth gear has a second characteristic that separates it from the lower gears, and it is the mirror image of the second-gear argument. Fourth is engaged comparatively rarely — it takes one shift to get there and it stays there — but the vehicle then spends most of its operating life in it, because that is what motorway driving is. So the fourth-gear elements accumulate very few apply cycles and an enormous amount of time under sustained load and sustained heat. Failures here tend to look like gradual thermal degradation rather than mechanical wear from repeated engagement.

Finally, this is the only gear in the family with a free isolation test built into the vehicle. Most vehicles that have four speeds also have a way for the driver to lock out overdrive, either a button on the shifter or a selectable position. Drive normally for a few days with overdrive disabled. If the code stays away, fourth has been confirmed as the problem gear with no tools and no cost. If it returns anyway, the fault is not confined to fourth and the picture has just got broader.