1991 Lumina Euro 3.1 Need Some Help

First of all I wanted to say I have been reading posts here for a few weeks and I must say you guys found my problem just by posting here and I fixed them...you guys know your stuff so thank you for that but now I have a list of new problems. I have been really impressed but here's my story. My mom and step dad gave me the car last year and I have never had any problems until recently. It started with not starting and not all the time so I replaced the fuel pump and then of course the new problems started. The check engine light was on and of course I took it to my neighbor parts store for the free code scanning but of course out of luck again because my car was too old for their machine to scan my Blue Betsy. I bought the new O2 sensor on a whim. Of course that didn't solve the problem but after reading here I learned how to pull the code so I sent the old man out to the car with my new information...of course he couldn't get the code...so I had him run the car until car put the Check Engine Light on and he did shut it off pulled the Code which was 23 after reading it could be either the MAT sensor or the TPS sensor I tried to price them both...but after calling the local GM dealer which I don't know if he was giving correct info he said MAT sensor's only were on the 4 cyl engines so I bought the TPS which I had him install today and he started the car and after 20 minutes no light...so away we go...then the light comes on of course the car is running great but the light annoys me and he pulls the new code which is this...

25Manifold Air Temperature (MAT) Sensor - signal voltage is lowOR Vacuum Switching Valve Circuit open or shorted to ground OR ATS Sensor - signal voltage is high

which of course if the information that the dealer gave is true I have it narrowed down to two things...but I know nothing about these things so can someone help me with these things of how to understand them or where to look or what they actually are..I know that with the fuel pressure gauge 41 lbs of pressure, just changed the plug and wires, TPS, O2 sensor. Car was missing but now isn't since we replaced the TPS today.

Welcome to AF

A code 25 is either a MAT in the 2.5, or an air cleaner temp sensor in the 3.1/3.4. GM calls it different things, just to make it confusing. Look in the top of the air cleaner housing, right next to where the rubber tube leading to the engine connects, and you'll see the sensor there. Check the wiring for cuts or corrosion before replacing the sensor. If the wiring looks good, then the sensor is most likely the problem.