The P0729 code indicates that the transmission control module (TCM) detected an incorrect gear ratio while the vehicle was in 6th gear. In this guide, we’ll show you how to diagnose and fix it using the iCarsoft CR PRO S.
Applies to: Audi, BMW, Ford, Mercedes, Volkswagen, and most automatic or dual-clutch transmissions.
What Does Code P0729 Mean?
The transmission calculates the gear ratio by comparing input and output speed sensor signals. When the ratio for 6th gear does not match the expected value for a certain time or load condition, the TCM triggers P0729 – Incorrect Gear Ratio (Gear 6).
Commonly seen in vehicles with torque converter issues, worn clutch packs, or fluid contamination. Related codes include P0730 (Incorrect Gear Ratio) and P0723 Output Speed Sensor Intermittent.
Symptoms of P0729
- Hard or delayed shifting into 6th gear
- Transmission slipping or flare at highway speeds
- Check Engine Light and/or Transmission Warning Light
- Reduced fuel economy
- Vehicle stuck in limp mode
Common Causes
- Low or dirty transmission fluid
- Faulty or contaminated transmission solenoids
- Worn clutch pack or slipping torque converter
- Faulty input/output speed sensors
- Blocked valve body passages
- Outdated or corrupted TCM software
Step-by-Step Diagnosis (Using iCarsoft CR PRO S)
- Check fluid condition: With the engine running, check ATF level and color. Dark or burnt fluid suggests internal clutch slip.
- Scan for transmission codes: Use the iCarsoft CR PRO S to read both stored and pending TCM codes — P0729 often appears alongside P0730, P0715, or P0723.
- Monitor live data: View input and output shaft speeds. In a healthy gearbox, the ratio should match the expected 6th gear value (around 0.67–0.75 depending on model).
- Perform solenoid test: Use CR PRO S’s special functions to activate each shift solenoid individually and monitor pressure change.
- Inspect wiring: Check connectors at the transmission control unit and solenoid harness for corrosion or loose pins.
- Test drive with data logging: Log speed, throttle, and gear command data. Note when the slip occurs — during cold start, load, or deceleration.
- Reflash or replace TCM: If mechanicals are fine, check for NHTSA TSBs recommending transmission software updates.
💡 Tip: If the P0729 code persists after fluid change, inspect the solenoid pack or valve body — many modern 6-speed automatics develop solenoid leakage after 80–100k miles.
Repair & Fix Options
- Replace or service the transmission fluid and filter
- Clean or replace the shift solenoid assembly
- Inspect and replace the output speed sensor if erratic
- Perform TCM software update (dealer or OE scan tool)
- In severe cases — rebuild or replace valve body / clutch pack
Perform all post-repair adaptations and road test using the iCarsoft CR PRO S.
Real-World Case
A BMW 328i (ZF 6HP) owner reported harsh 5–6 shift and code P0729. Live data showed output speed lagging 200 rpm behind input ratio at steady throttle. After a full ATF flush and solenoid replacement, the ratio normalized and code cleared permanently.
FAQ
Can I drive with P0729?
Driving short distances is possible, but continued use can cause clutch pack wear or overheating. Address the cause immediately.
Will a fluid change fix P0729?
If the cause is minor slip from old or low fluid, yes. But mechanical or solenoid faults will require further repair.
Is P0729 related to torque converter problems?
Yes. A failing torque converter clutch (TCC) can cause ratio mismatches under light load, especially in higher gears.
How can I confirm with a scan tool?
Use CR PRO S to compare Input Shaft Speed vs Output Shaft Speed at steady throttle. If ratios deviate beyond spec, the transmission is slipping internally.
Why Choose iCarsoft CR PRO S
The iCarsoft CR PRO S gives full access to transmission modules, live-data logging, and solenoid activation tests — essential for diagnosing P0729 and other gear ratio faults accurately.
