Possible reasons for the absence of 4th gear in the Subaru Forester automatic transmission:
Mechanical problems:
* Worn or damaged 4th gear planetary gear
*Damage to overrunning clutch
* Clogged valve body or 4th gear valve
*Low transmission fluid level or contamination
Electrical problems:
* 4th gear solenoid valve malfunction
* Gear selector position sensor failure
*Short circuit or broken wiring
Software problems:
* Errors in the software of the automatic transmission control unit
* Incorrect transmission calibration
Other reasons:
* Worn friction or clutch discs
* Torque converter malfunction
Diagnostics:
To diagnose the exact cause of the lack of 4th gear, you must carry out the following steps:
* Read fault codes using a scanner
* Check the level and condition of the transmission fluid
* Inspect the mechanical components of the automatic transmission
* Test the operation of solenoid valves and sensors
* Update the automatic transmission control unit software (if necessary)
Repair:
Depending on the cause of the problem, repairs may include:
* Replacement of damaged automatic transmission parts
* Cleaning or replacing the valve body
* Repair wiring or replace solenoid valves
*Software update
* Automatic transmission adjustment
It is recommended that you contact a qualified mechanic for accurate diagnosis and repair of the automatic transmission.