1990 Ford Aerostar Temp Sensor for mixture

I have a 1990 ford Aerostar that just failed the emissons test. I gave it a tune up right before the test. After the test I checked the compression which is about 120 PSI in all cilynders. Also checked the vacume which is in the green and stays very stedy. The cat also seems to be flowing free and working properly. But recently the water temprature guage in the dash quit working, so I replaced it with an after market guage and sensor. This means that the factory sensor and wire are no longer being used.
My question is: do you know if having a disconnected factory water temp sensor is tricking the computer into thinking that the engine is cold, and therefore causing a rich mixtrue?