Know anything about Honda engines?
 

I am thinking about buying a honda from A college student in houston but he has listed "The Bad: I took it to get inspected but it failed so I took it to the mechanic who said the computer wasn't hooked up. I'm guessing the computer was from the original engine so you'll have to replace it. I did install a brand new O2 sensor but of course it doesn't make a difference without the computer." In the add right above this he says the engine runs strong, and lists all the modifications. My question is, does the computer not being hooked up affect the performance of the engine, or does that only affect the emissions tests?

Be more specific about Year and Model,size of the engine.

Yes it will affect the performance of the engine. If the original engine was replaced, the computer should have been reconnected. If the engine and computer are incompatible, then you will need a new computer. Without a passed emissions test, you will not be able to put the car on the road. Get the vehicle, engine and computer serial numbers and go to a Honda dealership that does its own service and see what they can tell you about fixing the situation.