Transmission issue

Hi, my rx 300 sometimes not engaging in gear 4. Any possible cause will be appreciated. The filter was replaced in October last year. Thanks.

It won't normally engage 4th gear if the engine is cold or you are driving very slow. If the problem occurs when you are driving normally, then it may be your engine coolant temperature sensor is bad. If this is the case, your computer thinks that your engine is always cold and doesn't allow up-shifting to fourth gear. Test your coolant temperature sensor.

Also, unfortunately, the RX300 gearbox has a tendency to break at semi-high mileage. How many has yours got?