P0731 means the transmission control module commanded first gear and measured a ratio between input shaft speed and output shaft speed that did not match first gear. The family runs P0730 (general incorrect ratio) and P0731 through P0736 for gears one to six, with higher gears on newer transmissions in the P07xx and P08xx blocks. The module does not measure the gear directly; it divides two sensor readings and compares the result with a table.
Severity: moderate to high. Can you keep driving? To a shop, gently. An incorrect ratio in first gear usually means slip in the element that holds first, which generates heat and wear fast. If the vehicle flares from a stop or takes off with high rpm and little motion, have it towed.
What the measurement can and cannot tell you
Two speed sensors and a table. Anything that changes either speed reading, or lets the transmission turn at a ratio other than commanded, sets the code:
- Slip in the first-gear clutch or band, from wear, low pressure or low fluid.
- A stuck valve or a hydraulic leak that fails to apply the element fully.
- A one-way clutch that does not hold, letting first gear freewheel.
- A speed sensor reading wrong or a damaged tone ring, which makes a healthy transmission look like it is slipping.
- A range control problem that puts the transmission in a different gear than the module believes.
That last one is the GM case. Bulletin 18-NA-083 (May 2019) for the 2017 Buick LaCrosse and 2017 to 2019 Cadillac XT5 with the automatic transmission: a Service Transmission message and check engine light with P0731 and the rest of the ratio codes (P0729 through P0735), P0722, P07A2, unable-to-engage codes in the chassis module, and P1769 (transmission range sensor to range control module correlation), caused by misalignment of the coupler between the transmission range control module and the transmission. The transmission was not in the range the module thought it was, so every ratio was wrong. The repair realigns the coupler; nothing inside the transmission is touched.
Causes, in the order they are found
Low or burnt fluid. The first check on any ratio code.
A range control or shift linkage misalignment, as in GM's case, when several ratio codes and range codes set together.
A speed sensor or tone ring fault, when the ratio codes come with speed sensor codes (P0715 through P0722).
A stuck valve or a clogged solenoid screen starving the first-gear element.
A worn clutch, band or one-way clutch, when the fluid is dark and the slip is progressive.
The TCM, last.
A diagnostic order
- Record every code. P0731 alone points inside; with all the ratio codes and a range correlation code it points at the range control or linkage; with speed sensor codes it points at a sensor.
- Check the fluid by the manufacturer's procedure, then color and smell.
- Watch input and output speed and the calculated ratio live on a road test. A ratio that wanders under load is slip; a ratio that is wrong but steady is a range or sensor problem.
- Check the range control coupler and linkage on vehicles with a shift-by-wire range module, and the manual lever alignment on cable-shift vehicles.
- Check the speed sensors and tone rings if their codes are present.
- Line pressure test if the fluid, range and sensors pass; low pressure in first gear points at the valve body or the pump.
- Repair what measured wrong, run any required adaptive relearn, clear and verify from several stops at operating temperature.
Frequently asked questions
Can I drive with a P0731 code?
To a shop, gently, if the vehicle still takes off normally. If it flares from a stop or moves slowly with high rpm, first gear is slipping and continued driving burns the clutch; have it towed.
Does P0731 mean my transmission is failing?
Not necessarily. The code means the measured ratio was wrong, which slip causes, but so does a misaligned range control coupler (GM's 2017 to 2019 LaCrosse and XT5 case), a bad speed sensor or low fluid. Check those before assuming internal wear.
Why do I have P0731 with P0732, P0733 and more?
Because a fault that affects every gear, such as the transmission being in a different range than the module believes, or a speed sensor reading wrong, sets a ratio code for each gear it tries. One wrong gear is slip; every gear wrong is range or sensor.
How much does it cost to fix P0731?
Fluid is the least expensive outcome. A coupler or linkage realignment is an hour or two. A speed sensor is an inexpensive part. A valve body is several hours, and internal clutch work is a rebuild.
Related codes
- P0730 / P0732 to P0736: general and other gear ratio codes.
- P0715 to P0722: input and output speed sensor circuits.
- P1769 (GM): transmission range sensor to range control module correlation, stored with the ratio codes in GM's case.
- P0741 / P0742: torque converter clutch performance and stuck on, a different slip measurement.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.