P0734 on GMC: How to Diagnose & Fix
Gear 4 Incorrect Ratio
1 GMC model · 1 documented issue · 1 critical · $1,800-$3,500 repair
P0734 on GMC by model: symptoms, diagnosis and fix
On GMT800 Sierra 1500 trucks equipped with the 4L60-E family automatic, a well-known internal failure is cracking or stripping of the reaction sun shell. Owners commonly report sudden loss of reverse, no 2nd gear, no 4th gear, or a truck that starts in 3rd gear after a bang or flare. This is widely documented in transmission rebuild literature, repair databases, and owner reports for GM trucks and SUVs using this transmission.
Common Symptoms
- loss of reverse
- no 2nd gear
- no 4th gear
- harsh bang followed by missing gears
- transmission starts in 3rd gear
- slipping on acceleration
How to Fix
Diagnosis usually starts with verifying gear loss patterns, checking line pressure, and scanning for ratio/performance codes before condemning the unit. Repair requires transmission removal and overhaul or replacement, typically installing an upgraded hardened sun shell such as the commonly recommended 'Beast' style shell along with inspection of clutches, band, pump, and valve body wear items.
Sources for this issue (1)
P0734 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0734 mean on a GMC?
P0734 stands for "Gear 4 Incorrect Ratio." This code means the Transmission/Powertrain Control Module detected an incorrect gear ratio in 4th gear. The module continuously compares input shaft speed to output shaft speed; if the ratio in 4th gear doesn't match the expected value, it sets P0734. This usually indicates the transmission is slipping or not fully engaging 4th gear due to a hydraulic, friction, or solenoid problem. Symptoms include slipping, flaring RPM, harsh or missed shifts, and possibly limp mode. On GMC it is documented across 1 model: Sierra 1500.
What are the most common causes of P0734?
The most common causes of P0734 are: Low or degraded/burnt transmission fluid, Slipping clutch packs or worn friction elements in 4th gear, Stuck or faulty shift solenoid, Valve body / internal hydraulic leak or restriction, Worn or failing torque converter, Faulty input or output speed sensor, Internal mechanical wear (bands, planetary gears). The specific cause varies by vehicle.
How much does it cost to fix P0734 on a GMC?
Published repair estimates on GMC range from $1,800 to $3,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0734 on a different make?
View P0734 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.