Code
P0731
HUMMER
P — Powertrain
Incorrect 1st Gear Ratio
AI status
Completed
Completed
100%
Causes
- Low or contaminated transmission fluid
- Faulty/dirty input or output speed sensor
- Electrical problem (open/short/corroded wiring or connector) to speed sensors or solenoids
- Defective shift solenoid or valve body
- Worn or damaged 1st gear clutch pack, band, or planetary gear set
- Torque converter damage or excessive slip
Symptoms
- MIL/Check Engine light illuminated
- Harsh, delayed, or erratic 1st gear engagement
- Engine revs higher than expected in 1st gear (apparent slipping)
- Poor acceleration from a stop or reduced drivability in low gears
- Transmission may default to limp-in/second gear or display limited functionality
- Reduced fuel economy
What to check
- Read freeze frame and full scan-tool freeze data for rpm, turbine/shaft speeds, gear commanded and gear actual
- Clear codes and attempt to reproduce with live data logging
- Compare engine RPM to transmission output (vehicle speed) to calculate actual gear ratio vs expected
- Inspect transmission fluid level, color and odor (burnt smell)
- Visually inspect wiring and connectors for speed sensors, shift solenoids, and TCM for corrosion or damage
- Check for related codes (speed sensor, solenoid circuits, TCM faults)
Signal parameters
- Engine RPM (crankshaft)
- Transmission input/turbine speed (if available)
- Transmission output/vehicle speed sensor (VSS)
- Calculated gear ratio = engine RPM / output shaft RPM (or scan-tool supplied ratio)
- TCM commanded gear and actual gear PID
- Shift solenoid status/duty (%) and voltage
Diagnostic algorithm
- Connect a professional scan tool and record freeze frame and live data while reproducing fault; note commanded gear vs actual gear and relevant sensor readings
- Calculate the actual 1st gear ratio from engine RPM and output shaft (VSS) RPM and compare to factory spec/scan-tool expected value
- Visually inspect and wiggle-test wiring and connectors for the input/turbine and output/VSS sensors and repair any damage or corrosion
- Check transmission fluid level, color and smell; change fluid and filter if contaminated or unknown service history, then retest
- Test speed sensors (resistance/signal output) and replace if out of spec; confirm correct sensor operation with oscilloscope or live data
- Check shift solenoid operation: command solenoids On/Off with scan tool and verify changes in pressure or gear engagement; inspect valve body for sticking solenoids or debris if solenoid faults present
- If electrical and hydraulic checks pass, perform a pressure test to confirm line/shift pressures within spec (may require manufacturer test ports and gauge)
- If pressure and solenoid operation are correct, suspect internal mechanical failure (worn clutch pack, band, planetary gears, or torque converter); further diagnosis may require partial or full transmission teardown or removal for inspection
- Consider TCM software update, relearn/adaptation reset, or TCM replacement only after all sensors, wiring and hydraulic/mechanical checks are completed
- After any repair, clear codes and perform a road test with live-data logging to confirm correct 1st gear ratio and that the code does not return
Likely causes
- Low fluid level or degraded fluid causing slip or slow actuator response
- Bad/contaminated speed sensor or connector producing incorrect RPM/vehicle speed signals
- Sticking or failed shift solenoid or valve body passage preventing proper engagement
- Worn 1st gear clutch pack or damaged planetary gears producing an actual ratio error
- TCM software bug or lost adaptation values after repairs
Fault status
Status
P0731 — Incorrect 1st Gear Ratio. TCM detected mismatch between actual and expected 1st gear ratio; MIL set.
Repair difficulty: Medium
Diagnostic time: 2-6 hours
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Repair manuals
Manual library for HUMMER
138
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