P0733: Gear 3 Incorrect Ratio

P0733 reports that while third was commanded, the measured relationship between input shaft speed and output shaft speed was not what third should produce. What makes this gear different from its neighbours is how third is achieved mechanically on a great many transmissions, and that difference gives you both a free verification test and a much sharper way to read companion codes.

On most traditional four-speed and many five-speed automatics, third is direct drive: a ratio of exactly one to one. It is not produced by gear teeth at all. Instead, two members of the planetary gearset are locked together so the entire assembly rotates as a solid unit and the input shaft turns the output shaft at identical speed. That single fact is worth more than any diagnostic chart, because it means you do not need a specification to check the code. Put input and output shaft speed side by side in live data, get the transmission into third on a road test, and read the two numbers. If they are the same, third is holding. If input speed is meaningfully higher than output speed, something in there is slipping and the code is telling the truth. No other gear in this family can be checked that way, because every other gear requires you to look up a ratio and do the arithmetic.

The second consequence of direct drive is more subtle and it changes what you should suspect. Locking a planetary set solid requires two elements applied at the same time, not one. Those elements are not exclusive to third — each of them is also used to produce other gears. So a slip in third does not identify a single component the way a slip in a reduction gear might. Instead it hands you a logic problem, and the way to solve it is to look at what else is stored. If P0733 is the only ratio code present, suspect the element that third uses and its neighbours do not. If P0733 arrives together with a ratio code for another gear, stop thinking about third specifically — the fault is almost certainly the element those two gears have in common, and a repair aimed at third alone will not touch it.

Third also sits at a particular kind of shift. On many designs the two-to-three change is clutch-to-clutch: one element is released at the same moment another is applied, and the overlap between them is controlled by timing and pressure rather than by any one-way device. That makes third-gear complaints far more likely than other gears to present as a flare between shifts rather than a steady slip once the gear is engaged. A brief rise in engine speed during the shift, followed by a normal, solid third gear, is a timing and pressure symptom, not worn friction material. On vehicles with adaptive shift learning, that is worth acting on before anything else: a transmission that has adapted its shift timing around a previous problem, a fluid change, or a battery disconnection can produce exactly this, and resetting the adaptive values and letting it relearn costs nothing and occasionally clears the whole complaint.