P0734 on Mercury: How to Diagnose & Fix
Gear 4 Incorrect Ratio
1 Mercury model · 1 documented issue · 1 critical
P0734 on Mercury by model: symptoms, diagnosis and fix
Manufacturer bulletins on file with NHTSA describe several different CD4E transmission faults on 1999-2002 Cougars: no forward engagement (NHTSA items 605229 and 6091763), the ring gear side-loading the forward-clutch snap-ring retainer on 1999 models (item 607645), intermittent loss of first and second gears with code P0758 or P0740 (item 619324), and harsh or delayed shifts after speed-sensor service (item 630631). These faults share the CD4E transmission but do not point to one single valve-body or drum cause.
Common Symptoms
- No forward engagement in Drive (reverse may still work)
How to Fix
Preserve all transmission codes and freeze-frame data, identify which gears are lost, test line pressure and solenoid circuits, verify fluid level and condition, and inspect the transmission tag and prior repairs. Follow the symptom- and build-specific Ford procedure before choosing a sensor, solenoid, valve-body repair, hard parts or a remanufactured unit. Do not buy a transmission or rebuild kit until the failed part has been identified.
Sources for this issue (2)
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 Mercury?
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 Mercury it is documented across 1 model: Cougar.
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 Mercury?
Repair costs vary widely with the root cause and Mercury model; each issue on this page carries its own estimate where one is published.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.