OK just got this dc2 a few days ago...still learning from b series here. From what I understand, 96 integra Ls (what I have) came with b18b1. Well, the last owner swapped in an a1 block/ head with a p75 ecu. Now, the a1 swap is obd1 right? It was made in 93 I believe. This is where it gets complicated...

It has an aftermarket exhaust with only 1 o2 sensor hole, all the way under the intake manifold. I believe that's where the secondary o2 WOULD run. So, I have no primary o2. No hole on pipe for it either. My car is throwing a cel for the first oxygen sensor, which is missing. It just so happens I can only find one plug for an 02 sensor. Am I supposed to have two? Or do they just run out of separate locations and I'm blind?