OBD-II trouble code
P0731: Gear 1 Incorrect Ratio
The transmission's actual gear ratio in first gear doesn't match what it should be — the PCM compares input and output shaft speeds and sees the wrong relationship. The list includes internal slipping (worn clutches or bands), low or degraded fluid, a shift solenoid or valve-body problem, and a speed-sensor issue. None of those is a counted share of cases. It can come with slipping, harsh shifts, or a drop into limp mode, and warrants prompt attention to avoid further transmission damage.
Quick facts
- System
- Powertrain
- Category
- Transmission
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $200 – $8,000
- DIY difficulty
- Advanced DIY
Browse every code in P0700–P0799, or start from the full code library.
What does P0731 mean?
An automatic transmission's computer knows what gear ratio it should be in by comparing the input (turbine) shaft speed to the output shaft speed — each gear produces a specific, known ratio between the two. P0731 sets when, in first gear, the measured ratio doesn't match the expected value. In other words, the transmission was commanded into first, but the math says the gears aren't actually delivering a first-gear ratio. This is a result/performance code: it tells you the transmission isn't achieving the correct gear, which is a performance signal rather than a simple sensor circuit fault, and it can mean something inside isn't holding properly.
The causes span hydraulic, mechanical, and electronic. Low or degraded fluid is the check that does not require opening the transmission, because it changes the pressure that applies the clutches and bands. It is not a counted share of P0731 cases. Worn friction elements — slipping clutches or bands that can't hold first gear — are a mechanical cause, as is a sticking valve-body valve or a shift solenoid that isn't routing pressure correctly. A failing input or output speed sensor can also produce a false ratio reading, so the sensors are worth verifying before assuming internal damage. When the transmission can't achieve the commanded ratio, the TCM may respond by limiting shifting or dropping into a limp/failsafe mode to protect the unit.
For the driver, P0731 can show up as slipping in first gear (engine revs climbing without matching acceleration), harsh or flaring shifts, hesitation pulling away from a stop, and a limp mode that holds a fixed gear. The car can still move, but keep the trip short. Continued driving while a gear is slipping generates heat and accelerates wear, so it's worth addressing promptly. Diagnosis starts with fluid level and condition, verifies the speed sensors and reads live data for the actual ratio, and then evaluates the valve body, solenoids, and internal condition.
Common causes
- Low or degraded transmission fluid affecting apply pressure
- Worn or slipping first-gear clutches or bands
- Sticking valve-body valve misrouting pressure
- Shift solenoid not applying the correct element
- Faulty input or output speed sensor giving a false ratio
- Internal hydraulic leak reducing clutch apply pressure
- General internal wear in the transmission
Symptoms
- Check engine light with P0731 stored
- Slipping in first gear — revs rise without matching acceleration
- Harsh, flaring, or delayed shifts
- Hesitation when pulling away from a stop
- Limp / failsafe mode holding a fixed gear
- Reduced performance and fuel economy
Diagnostic steps
- 1.Check transmission fluid level and condition early — low or burnt fluid is the check that does not require opening the case — but budget properly for the check. Where there is a dipstick, follow the manufacturer's procedure, which states whether the engine is running, the gear, and the fluid temperature. Many transmissions now have no dipstick: level is set through a fill plug or standpipe inside a specified temperature window, which means a lift, a running engine and fluid hot enough to burn.
- 2.Read live data and compare input vs. output speed to see the actual first-gear ratio. If the transmission is sitting in failsafe it may refuse to select first at all, in which case there is no ratio to observe until the code is cleared and the condition reproduced. Have a passenger read the tool or log the data for review rather than watching the screen while driving.
- 3.Verify the input and output speed sensors are reading accurately, since a bad sensor can fake an incorrect ratio.
- 4.Inspect for internal slipping symptoms and check for other transmission codes (solenoids, pressure).
- 5.Evaluate the valve body and shift solenoids for sticking or incorrect pressure routing.
- 6.If fluid, sensors, and solenoids check out, the cause is likely internal wear requiring transmission inspection or rebuild.
Repair cost
$200 – $8,000
P0731 names first gear. A sensor or solenoid that misreports first is not the rebuild band. RepairPal's transmission fluid change average, updated 3 September 2026, is $294–$356, with vehicle examples from about $162 to about $463. AutoZone's published band for a simple solenoid replacement is $300–$900, parts and labor. AutoZone's published band for torque converter replacement is $600–$1,200, parts and labor, because the transmission comes out. AutoZone's published band for a transmission rebuild is $1,800–$3,500, and for a full replacement $4,000–$8,000. The printed pair spans the fluid-service examples through a replacement. It is a planning span, not one invoice.
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DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.