Need Help...

I recently put an engine in an old 1994 Beretta Z26 I have and I can't seem to get the check engine light to turn off, I took it to a dealership and the codes they say that they are getting are for the maf sensor and the coolant temp sensor, both of which have been replace and should work well. They told me to replace the computer so I did and I still have a check engine light on... Also the car runs well except every once in a while it will just die, and the oil pressure gauge is like permanently stuck all the way to the right, but I can't seem to find the oil pressure switch to replace it. I would like to have a good idea on what I need to do to fix it rather than go stand out side and look at it for hours because it's about 10 degrees here and I hate the cold. Thanks alot guys you help would be much appreciated...

I would look at your old engine and remove the oil pressure sending unit and install it on this engine. When you unplug it the gage should go to zero.
I would drive the car until its up to operating temperature then feel both the top and bottom radiator hoses. The new thermostat may be stuck open.
For the airflow sensor make sure all connecting hoses are good and the air filter is clean.
Its possible a connector is bad or you pinched a wire when doing the engine change.
Usually if the problem didn't exist until the change over its something you did during the change over.
Make sure all ground wires you removed are cleaned and tight.

I've deduced that the coolant temperature sensor works, it does everything its supposed to but I do not know why the code comes up? Also I searched for the oil pressure sensor/switch and I can't find it, I found the area where its supposed to be but the sensor does not match the new sensor I got or the old sensor on a different engine. So I'm not sure if I've found the right sensor or not. Anyone know for sure where it is at on a 3100 v6

Some sensors are for a light and some are for a gage I think this is part of your problem. Look in your manual the one for the light is usually one wire.

Thanks for your help and...
My car has a gauge, I looked in the manual and found nothing, I purchased a Chilton Auto Manual for my car and pulled out the sensor closest to the one the book described as being the oil pressure switch/sensor, and it most definitely was not the right one, I'm actually not sure what in the world it is but its not the oil pressure sensor/switch. So now I'm looking all over the car to find it but still no success, any chance it could be a sensor plugging directly into the top of the engine block on the drivers side below where the air intake connects with the manifold? I found a sensor there but no idea what it does although I think its not the right one because the new sensor I got at the auto store has 3 wires and possible one on the engine has 2? Any ideas anyone?

According to Mitchell manual. Low oil level switch on side of oil pan.
Fuel pump/oil pressure switch on right front of engine block.
This is for a 3.1L