O2 sensor mystery

[paranoidpete23]

I have a 90 civic hatch with a d16 engine and trans from a '95. obd0->obd1. when i got the thing, it already had the swap done, but it was firing code 42 (primary o2 sensor) and sometimes code 2 (i dont even have a secondary!). ive replaced the sensor twice and it still fires the same code! I've checked all the wiring with Rywire schematics and everything is hooked up properly. to add mystery to misery, the engine light goes on at random intervals, after 1 mile or 150 miles. What could be causing this problem?

[CTgoldhatch]

sort in the wire

[paranoidpete23]

dunno, the wiring is all new. and i even troubleshot the thing with a friend's ICU. I could rewire the whole lot, i guess. it runs and idles fine, fast, and good mileage, but i failed emissions, so something is wrong.