The DTC code P0734 OBD2 is stored in vehicles with automatic transmissions due to problems shifting to or driving in gear number 4. The automatic transmission controlled by the vehicle's computer uses multiple gear ratios to get higher vehicle speed while maximizing engine output.
The transmission ratio is the difference between the input speed and the output speed. Generally, there are speed sensors on the input and output of the transmission. The ECM (Engine Control Module) uses these sensors to compare speeds and make sure the gear ratio is right. If the ECM determines that the transmission ratio is out of the specified value, the DTC code P0734 is set.