nissan sentra

Could anyone here help me? I recently had the engine of my nissan sentra (1993) replaced with one which had fewer miles. The new engine makes the car drive a lot better. The problem is - my car started to not crank the day I took it from the shop. I would have to pump the gas pedal to get the engine to turn over. I had to tow it back to the shop the next day since the engine had gotten to the point that it would simply not turn over at all.
I never had any problems cranking my car at all with my old engine. The car shop said that a sensor (the one that controls the fans) can be taken out and then the car will start. This sensor can't be left out of the car since it does control the fans. They have checked and cleaned every connection which they would have touched when installing the new engine. Now they are saying that it is a wiring problem. Can anyone shed some light on what might be happening? Thanks so much.

the year of the car would be awesome help for us.... im assuming he means the coolant temp sensor, and it will affect fuel delivery to the point of a no-start..... also with the motor coming out and a new one going in there could be any kind of wiring fault if someone pinched a wire or stripped one, maybe a connector that got left off