So I replaced my clutch and noticed that the O2 sensor in front of the straight pipe does not have a socket nearby to plug into!?!? But the rear one does. Now all of a sudden my air/fuel ratio gauge is not but working. Do they use one or two o2s? My evo is tuned.

 

you have a front O2 sensor that plugs directly into your O2 housing coming off your hotside of the turbine housing. You must plug it in or itll run like junk, as you're experiencing.

 

I have an aftermarket tomei o2 housing with an o2 sensor. All my 400whp are running perfect with no hiccups. I only have 3 02 sensors but only two are connected . My only problem is my air/fuel gauge is on but not working. I don't know where the connection is.

 

I have all 3 in mine with no issues.

 

I have three but only two are used.

 

To which o2 sensor does the air/fuel ratio connect to? Or where does it wire up to?

 

Since it's an aftermarket gauge and hence an aftermarket sensor, there is no clear universal answer. You could ask the shop that installed it the first time or just spend some quality time under your car and find the bung for it.