I have a 92 nissan stanza. It just went for emissions and failed for high NOx. The mechanic replaced the O2 sensor and the NOx levels came down to a pass - but then the HC and CO levels (which were way below the limits before) failed. It continued to oscillate back and forth and I was able to get a conditional pass.
Has anybody else experienced this? The mechanic at the testing center said he took the car out for a while and got the Catalytic Converter up to 500 degrees (above the required 300). I will eventually need to fix this problem and I really want to avoid any unecessary repairs. Any suggestions on how to improve the emissions?
Thanks