My test pipe came with a bung on top, and I've never thrown a code. I think that will fix your code but unless your A/F is hooked up to a wide-band there's no telling what it will read.

 

the OBD1 setup is simpler, the O2 is before the cat, so even if you dont have a cat it doesnt matter.

get a bung and put the sensor back, so the ECU can make adjustment to your fuel maps.

 

As long as your sensor is good, it needs to be in the path of the exhaust. once your computer begins to read oxygen again the code should reset itself. The zirconia heated oxygen sensor works by comparing ambient oxygen content to exhuast oxygen content creating voltage in the sensor that the ECM reads and adjusts your fuel trim from rich to lean about once every second and a half. It operates in analog voltage less than one volt, but you really need to have it hooked up. Your car will run leaner and you'll get better gas mileage and emissions as a result.

 

The stock cat had the sensor plugged directly into the middle of the cat, I'm goin to get a hole drilled and tapped so I'll post results soon.