P0734 signals that the transmission control module detected an improper gear ratio when the vehicle was in fourth gear. The code is triggered when the actual ratio does not match the expected ratio for that gear.
The solenoid that controls engagement of fourth gear may be stuck, shorted, or have failed electrically, causing the PCM to detect an incorrect ratio.
Worn clutch plates or insufficient pressure in the fourth‑gear clutch can prevent proper ratio, triggering the code.
Insufficient fluid level or fluid that is dirty and lacks proper hydraulic pressure can affect shift quality and cause ratio errors.
Damaged wiring, corroded connectors, or poor ground can interrupt signal to the shift solenoid, leading to incorrect ratio detection.
A rare PCM software or internal failure could misinterpret sensor data, but this is usually considered only after other causes are ruled out.
It is possible but uncommon; most often the issue stems from the shift solenoid, mechanical wear, or fluid problems.
Only after mechanical faults are confirmed; many cases are resolved by replacing a solenoid or performing a fluid service.
Driving may be safe for short distances, but prolonged operation can lead to further transmission damage.
It appears relatively often on vehicles that use electronic shift control, especially when fluid maintenance is neglected.