CEL & AT light on, not shifting to 4th
 

My car is Mazda 3 2007 with triptronic gears. The problem started with CEL & AT coming alive while driving and the gears are not shifting to 4th. When I run the scanner with my mechanic, it shows that the Transmission Range Sensor is having a fault. I went for a 2nd opinion & they say that the ECM is having a fault & a replacement is required.

When this fault comes I stop the vehicle & switch off the engine and when start back the AT light goes away. Also the shifting comes back to normal on most occasions. Recently I changed ATF but no good.

I have seen the same fault has been discussed but nobody has found a solution for this.

Appreciate anyone of you can help me to figure out the issue & find a solution.