P0730: Incorrect Gear Ratio
What to do
This code does not mean the transmission is worn out, whatever else you are told. It means a measurement disagreed with an expectation, and several cheap things produce that. Before agreeing to a rebuild, ask three questions: were both transmission speed sensors verified, was line pressure actually measured with a gauge, and does the fault affect every gear or only some? A workshop that has answers to all three has done the diagnosis. One that quotes for a rebuild without them has not.
In the meantime, drive gently and avoid towing. If the vehicle is genuinely slipping, continuing to drive it generates heat that damages the fluid and the clutches further, and that damage does not reverse.
What this code means
The transmission module divides input shaft speed by output shaft speed, compares the answer with the ratio the gear it commanded should produce, and stores this code when the two disagree. Read literally, that is all P0730 says: the transmission is not turning its input into its output at the rate expected.
It is routinely read as something else entirely — "the gearbox is worn out" — and that reading is expensive, because a rebuild is one of the most costly repairs a vehicle can need and it is not the most likely explanation of this measurement.
Start with the two sensors. The ratio is not measured directly; it is calculated from two speed signals. If either sensor is wrong, a perfectly healthy transmission reports a ratio fault. Confirming both speed signals are believable is the first step, costs nothing, and is skipped remarkably often.
Then fluid and pressure. Clutches are applied hydraulically, so they hold only if there is enough pressure behind them. Low fluid, degraded fluid, a restricted filter, a tiring pump or a worn valve body all reduce the pressure reaching a clutch, and a clutch that is not fully applied slips — which is exactly the ratio error being reported. Every one of these is repairable at a fraction of the cost of a rebuild, and a worn valve body in particular is both common on high-mileage automatics and fixable with a sleeve kit.
Then the solenoids. If the module commanded fourth and the solenoid did not move, the transmission is in third. The ratio genuinely does not match the command — and nothing whatever is worn.
Only then, clutch wear. By which point the fluid, the pressure and the solenoids have been eliminated, and the diagnosis is supported rather than assumed.
One question separates these families faster than any test: does the vehicle feel like it is slipping? A ratio code with clean, decisive shifts that simply land in the wrong gear points at a solenoid — the transmission is doing something definite, just not what was asked. A ratio code with engine speed flaring and acceleration going dull points at slip, and only then is clutch wear a live candidate. Even then, low pressure causes slip too, and it is checked with a gauge in an hour.
Finally, which gears fail is diagnostic in itself. A ratio fault confined to one gear implicates that gear's clutch or its solenoid. Faults across several gears point at something shared — pressure, fluid, the pump. And because planetary gearsets reuse the same elements in different gears, a fault in a specific subset of gears identifies which element is at fault far more precisely than a general complaint does.
Likely causes
Start here
- Low or degraded transmission fluid · easy
- Faulty turbine speed sensor · specialist
- Faulty output speed sensor · moderate
- Failed shift solenoid · moderate
- Worn valve body · specialist
- Low transmission line pressure · specialist
Less common
- Restricted transmission fluid filter · moderate
- Incorrect transmission fluid specification · moderate
- Slipping torque converter clutch · specialist
- Worn transmission fluid pump · specialist
- Worn clutch packs · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about P0730.