I've got a '93 Civic that recently failed nitrous oxide emissions (just barely). Everything else passed fine. Its a D15B8 engine (8 valves and no EGR Valve, single wire O2 sensor) There were no engine codes and the 02 sensor seems to be working (voltage was .4-.5 and it increased when I hit the gas). However, I did replace the head gasket last year for a coolant leak and did not replace the 02 sensor. I was hoping not to have to spend $200 for an O2 sensor to diagnose the problem or replace the Cat for $400-$800. Is there anything easy like running with cooler plugs or putting in a different type of fuel that will lower the NOx by a few PPM? Thanks in advance for any advice