My mom got a 95 blazer a few years back it drove great. Then it stopped shifting out of third. Now it's siting Around. Her brother put a new transition and bell housing in still no luck we have taking it to multiple shops and they have told us the tyranny is just fine. Some people are suggesting that it might be a sensor but I'm leading more towards and ecu if anybody has and knowledge about what could be going on before I jump in please let me know what I should do first...

 

First step is to attach a scantool and retrieve any codes that may be present. Then go for a test drive watching the data stream and see if the computer is commanding 4th gear. Also look at the shifter position data, if it is reading drive 3 instead of OD for example, the computer thinks you want to lock out 4th and won't shift it. Once we have that info we can move to the next step, which of course varies according to what you find out.

Main data inputs to look at if no command to shift to 4th are the TPS sensor, coolant temp, trans fluid temp, and intake air temp.