Possible reasons for the absence of 4th gear in a Nissan Maxima automatic transmission:
Mechanical problems:
* Damage or wear to 4th gear clutch sets or steel discs
* Defective piston or 4th gear control solenoid
* Leakage of hydraulic fluid in the transmission control system
* Biting or breaking of internal transmission parts such as planetary gear or torque converter
Electrical problems:
* Faulty gear selector position sensor
* Damaged or loose electrical wiring supplying the 4th gear control solenoid
*Transmission control module (TCM) malfunction
Other reasons:
*Low transmission fluid level
* Contaminated or old transmission fluid
* Bugs in TCM software
Diagnosis and elimination:
Diagnosing and eliminating this problem requires a comprehensive approach, which includes:
1. Checking the level and condition of the transmission fluid: Make sure that the fluid level is normal and that the fluid is clean and does not have a burning smell.
2. Diagnostic Scan: Use an OBD-II scanner to check for trouble codes that may indicate a transmission problem.
3. Checking transmission fluid pressure: Connect a pressure gauge to the transmission system and check the pressure in 4th gear. Low pressure may indicate a pump problem or leak.
4. Transmission Disassembly and Inspection: If necessary, the transmission may need to be dismantled to inspect internal components and replace faulty parts.
Precautions:
* Do not try to repair the automatic transmission yourself, as this is a complex process that requires special skills.
* Refer diagnosis and repair to a qualified mechanic who specializes in transmission service.
* Maintain your transmission regularly, including changing the transmission fluid and filter.