Just bought a nice little civic with a d16a9 ZC motor swap. It runs well and has the correct ECU, but it is eating through gas. I live in a city where there's a speed bump every 10 yards but I'm getting 200 miles per tank (11.9 gallons) while driving very cautiously. It does smell lime it's running rich.

As I began to review the engine, I cannot find a O2 sensor anywhere. It does have aftermarket headers and a full exhaust.

After a lot of googling, the consensus is it has an O2, it has a CO2 or it has nothing. I am at a complete loss. Mechanic says he'd need to buy the sensor, rewire it, and install a cat just to get it back up to code, but that seems wrong.

Any help would be appreciated.