Powertrain, generic · Transmission

P0729 Gear 6 Incorrect Ratio

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

The transmission module divides input (turbine) speed by output speed and compares the result with the ratio of the commanded gear; a deviation means slip – a clutch or brake not holding, too little pressure – or a wrong gear. Thresholds: Ford more than 30 rpm off for 1.8 s three times (diesel 2023: over 20 % for 12 s), GM's 2006 tables and the Allison 1000 converter slip of 200–230 rpm for 2.25 s after one trip, Saab's Aisin boxes over 20 % twelve times without MIL. P0729 concerns sixth gear. Saab's Aisin six-speed also checks whether the actual ratio matches fourth or fifth – the sixth gear clutch is not holding and the box drops into another gear. Ford stores it only if the fault does not fit a particular solenoid (otherwise P0751 ff). On GM manual transmissions (LT4) it checks the gear position sensor instead.

Likely causes

Fluid level too low or fluid worn or contaminated
Clutch for sixth gear worn (linings, piston seal)
Sticking shift or pressure valve, leaking valve body gasket, blocked passages
GM manual transmissions only: gear position sensor faulty or not learned

Symptoms

  • Slipping or flare in sixth gear, engine speed higher than expected
  • Harsh or delayed shifts
  • Limp mode (Aisin: second gear and reverse only), transmission warning
  • Check engine light depending on manufacturer

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–3,500
Time1–14 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

transmission clutchestransmission valve bodytransmission fluidtransmission control module

Related codes

P0730P0735P0736P073BP0717P0722

References: saabwisonline.com, gsi.ext.gm.com, newtonnet.co.uk, gsi.ext.gm.com.