P0731 means the transmission control module has detected that the actual gear ratio in first gear does not match the expected ratio calculated from input and output speed sensor signals. This indicates that the transmission is not properly engaging or holding first gear, often due to fluid, solenoid, or internal mechanical issues.
Insufficient fluid level or dirty/degraded fluid can prevent proper hydraulic pressure, causing the transmission to slip or not hold first gear.
A stuck or electrically failed solenoid can block the flow of fluid needed to engage the first‑gear clutch pack, resulting in an incorrect ratio.
Mechanical wear of the components that lock first gear can cause slippage, so the actual ratio deviates from the expected value.
If either sensor provides incorrect speed data, the TCM calculates a wrong gear ratio even when the transmission is mechanically sound.
Although rare, a corrupted TCM/PCM program or internal hardware fault can misinterpret sensor data and set P0731; this is considered only after external causes are ruled out.
No. PCM/TCM failure is uncommon for this code; most cases are due to low/dirty fluid, a faulty shift solenoid, or internal transmission wear.
Driving may be possible but could cause further damage to the transmission; it is advisable to have the issue diagnosed and repaired promptly.
If the code is caused by low or contaminated fluid, a fluid change (and possibly a filter replacement) can resolve it. If the problem lies elsewhere, further diagnosis is required.
Only if internal components such as clutch packs or bands are worn. Many P0731 cases are resolved with external repairs like fluid service, solenoid replacement, or sensor/wiring fixes.