this is a auto b18a1 and there is a plug right on the side of the thermostat housing. what is this plug and where does it go

 

Pic would help a lot.

Any info on this in some more detail? Are you doing a swap, is the auto tranny still on the engine, what car is this going into, does the engine run, are you getting a CEL, what code... etc?!?!?

 

I got you....
That plug is you radiator fan switch. On the 92+ motors the sensor is relocated from the back back of the motor, above the oil filter.

 

oh ok this is a auto b18a with a cable LS tranny and was swapped into a 91 civic SI.
we are getting CEL 5,7,10 I think maybe the TPS and the MAP need to be swappped around, and the IAT(#7) I don't know what the deal with this one. Also the temp gauge doens't work and I know that the single wire plug and two wire connector under the dizzy are both plugged in. I fig. that the plug on the thermostat housing had something to do with it. But if that is only the fan switch then the problem lies somewhere else.