P0729

Code

Generic P — Powertrain

Gear 6 Incorrect Ratio

Brand: Generic

Type: P — Powertrain

Views: UK: 19 EN: 29 RU: 33

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Low or contaminated transmission fluid
Faulty input/turbine or output/vehicle speed sensor (wiring, connector, or sensor)
Stuck or failed shift solenoid(s) controlling 6th gear
Valve body or hydraulic control fault (blocked passage, worn spool)
Internal mechanical damage (worn clutch pack, planetary gear set, broken drum)
Faulty TCM or incorrect calibration/programming

Symptoms

MIL (Check Engine/Transmission) illuminated
Incorrect gear engagement or no 6th gear
High engine RPM for vehicle speed (over-revving in 6th)
Harsh, delayed, or slipping shifts entering 6th gear
Transmission may enter limp/backup mode
Unusual transmission noise during 6th gear

What to check

Read freeze frame and live data with a scan tool; confirm P0729 and note conditions (speed, temp, throttle, gear commanded)
Monitor input (turbine) and output (shaft/wheel) speed sensors and calculate actual gear ratio vs target while road-testing in 6th gear
Inspect transmission fluid level and condition (color, smell, contamination)
Check connectors and wiring for speed sensors and solenoids for damage, corrosion, and secure pins
Command shift solenoid(s) with scan tool and observe response and current draw where supported
Perform hydraulic pressure tests (line and apply pressures) if electrical checks are OK

Signal parameters

Input/turbine shaft speed (RPM) — used to compute gear ratio
Output/vehicle speed sensor (shaft or wheel) (RPM or km/h/mph)
Calculated actual gear ratio (Input RPM / Output RPM) — compare to expected for 6th
TCM commanded gear (target ratio or gear number)
Shift solenoid status/duty cycle and current draw
Transmission fluid temperature (affects hydraulic behavior)

Diagnostic algorithm

Connect a capable scan tool; retrieve codes, freeze frame, and live data. Confirm code P0729 and note conditions when it set.
Perform a visual inspection of transmission fluid level, smell, and color; correct fluid level and top/replace if contaminated.
Inspect wiring and connectors for the input/turbine and output speed sensors. Repair any damaged wiring or poor connections.
Monitor input and output speed sensors while driving to reproduce the condition. Calculate actual ratio and compare to expected 6th gear ratio from service data.
If a speed sensor reads incorrectly, test sensor resistance, supply voltage, ground, and signal with an oscilloscope or multimeter; replace faulty sensor.
If speed sensors are OK, use the scan tool to command the 6th gear solenoid(s) and observe solenoid response, current, and pressure where supported.
If solenoid or control faults suspected, inspect valve body and solenoids; clean or replace solenoids and service valve body as required.
Perform hydraulic pressure tests (line, clutch apply) to verify apply pressures for 6th gear. Low pressure points to pump, valve body, or internal wear.
If hydraulic and electrical systems test good but ratio is still incorrect, consider internal inspection of clutches, bands, and planetary gears; plan for transmission removal/repair.
After repair, clear codes and road test to verify correct 6th gear ratio and no recurrence of the code.

Likely causes

Faulty or out-of-range input or output speed sensor signal (most common)
Low or incorrect transmission fluid level/quality
Failed shift solenoid or solenoid circuit for the 6th gear path
Hydraulic pressure problem in the valve body
Internal wear or damaged gear/clutch pack

Fault status

⚠️ Status

Gear 6 incorrect ratio — the transmission control module detected the measured 6th gear ratio is outside expected limits. Possible causes include faulty speed sensors, solenoid or hydraulic faults, low/contaminated fluid, internal transmission wear, or TCM control issues.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 2-6 hours