I just bought a 1998 honda civic dx coupe with a early model 1.6 L engine. It isnt passing smog and saying senor 2 is failing. i have looked under the car and only find one o2 sensor in front of the exhaust manifold, please help i have no idea were the other sensor is. i have looked at the cat convertor and no luck, i know this is a new swap engine with d16y8 engine. so is it safe to say there is only one o2 sensor?

attaching my diagram pics of the engine

 

Obd2 needs 2 sensors, one before and one after the cat, maybe they removed the second one while doing the swap but the plug on the car should still b there. The plug will b (standing at the front of car looking to the back) left side of the head under the distributor slightly to the front. Both sensors need to b a 4 wire sensor. It looks like the pic of the d16 shows a single wire sensor which will not work.

 

Your original D16Y7 engine has a manifold/cat combination. One O2 sensor pre cat to determine fuel curve. The other is post cat to monitor emissions.

If someone removed the manifold/cat combo and installed a regular exhaust manifold/downpipe combo, they probably opted out of installing a cat, and hence opted out of installing a secondary O2 sensor.

There's probably an empty 4 pin plug dangling with nothing plugged in near the exhaust manifold.

 

Should look like this, however the engine you're picturing above is a D16Y8 not the Y7 that came in the car