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P0731 to P0736: incorrect gear ratio, gear by gear
Which gear the code names tells you which clutch or band is slipping. That is a genuinely useful piece of information before anyone opens the gearbox.
The transmission control module knows what ratio each gear should produce. It measures input and output shaft speeds, divides one by the other, and compares. When the answer is wrong for a specific gear, it names that gear.
| Code | Gear |
|---|---|
| P0731 | 1st |
| P0732 | 2nd |
| P0733 | 3rd |
| P0734 | 4th |
| P0735 | 5th |
| P0736 | Reverse |
The general version: P0730 incorrect gear ratio.
Why naming the gear helps
Inside an automatic gearbox, each gear is produced by applying a specific combination of clutches and bands. Different gears share some elements and not others.
That means the pattern of codes narrows the failing element:
| Pattern | Suggests |
|---|---|
| One gear only | The element unique to that gear |
| Two gears that share an element | That shared clutch or band |
| Several gears | A pressure or fluid supply problem |
| All gears | Fluid, pump or a sensor rather than clutches |
A single-gear code is the best case, because it is specific. A code covering every gear usually means a hydraulic problem rather than mechanical wear — and hydraulic problems are more often fixable without a rebuild.
Slip or measurement error?
The other thing to establish before assuming the worst.
The ratio is calculated from two sensors. If either is faulty, the calculation is wrong and the gearbox is fine.
- Input speed sensor: P0716 and P0717.
- Output speed sensor: P0720 to P0722.
If either sensor has its own code alongside, fix that first and re-test. The ratio codes may disappear entirely.
What genuine slip looks like
Log engine RPM, input speed, output speed and vehicle speed through a full acceleration.
| Observation | Meaning |
|---|---|
| RPM rises smoothly with speed, drops at each shift | Healthy |
| RPM flares between gears | Slip during the shift |
| RPM climbs while speed holds | Slip in the engaged gear |
| Slip only in one gear | That gear’s element is worn |
| Slip that worsens when hot | Classic wear pattern |
The hot-versus-cold behaviour is diagnostic in itself. Clutch material that holds cold and slips hot is worn material. Fluid that is degraded loses its friction properties as it heats.
Gears slipping on an automatic.
Before concluding it needs a rebuild
Fluid level and condition
The most common cause of every automatic transmission complaint, and the cheapest to address.
Low fluid causes pressure loss and slip. Burnt fluid — dark, smelling scorched — has lost its friction modifiers and cannot hold clutches properly.
Most modern gearboxes have no dipstick and the level is set at a specified fluid temperature, so this check needs a working temperature sensor and the correct procedure: P0710 to P0713 transmission fluid temperature sensor and automatic transmission ATF diagnostics.
Solenoid faults
A pressure control solenoid that cannot deliver the commanded pressure produces slip that looks exactly like worn clutches.
P0745 to P0747 pressure control solenoid A and P0750 shift solenoid diagnostics.
Solenoids are dramatically cheaper than a rebuild, and they fail often enough to be worth eliminating first.
The valve body
Wear in the valve body bores causes internal leakage and pressure loss. Often repairable or replaceable as a unit without a full rebuild.
Adaptation values
Modern gearboxes adapt clutch apply pressures over time. A gearbox with adaptations at their limits is compensating for wear, and reading those values shows how far along the wear is.
That data lives in the transmission module rather than in generic OBD-II: P0700 transmission control system malfunction.
The honest conversation
If fluid is correct, solenoids test good, and one specific gear slips under load and worsens when hot, then the element for that gear is worn and the gearbox needs opening.
That is a legitimate conclusion — but it should be reached in that order, not offered as the opening diagnosis.
Where to start
- Read all codes — sensor codes alongside change everything.
- Check fluid level and condition properly.
- Log RPM against vehicle speed through acceleration in each gear.
- Note whether it is hot-only.
- Test the solenoids before opening anything.
- Read adaptation values from the transmission module.
Log engine RPM against vehicle speed through each gear, once cold and once fully warm — an ELM327 adapter and the free tier of the app are all it takes. The difference between those two runs is the clearest evidence of wear you can gather yourself.
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