I just acquired a 98 Honda Civic Ex 4DR automatic and it seems to run and accelerate ok with no check engine lights and it idles pretty good. I only has 160,580 miles on it. However, I put some Guarantee to Pass Smog in the gas tank and drove it for 20 minutes and then took it in for a smog test. Well it pass most of the test except for the Nox test NO (PPM). The reading for the 15mph 1950rpm test read 2490 ppm and max is 487. The reading for the 25mph 1762rpm test read 2087 ppm and max is 774. The spark plugs look good and the air cleaner looks pretty clean. It looks like they replaced the distributor cap because it has an aftermarket cap (clear). I see the catalytic converter is hooked up but not sure if it was hollowed out. The check engine light comes on for a second when you first turn the key so the light is working but there is no check engine light that stays on. I plan on changing plugs and air filter anyway because it is pretty cheap. Would guaranteed to pass smog fuel additive cause it to fail NOx test? Please advise.

 

If it has an EGR system (California model) the EGR passages may be clogged. You could disconnect one end of the cat to see if hollowed out. Is the fuel pressure regulator stock?

Failing NOx but passing the others generally means it's running too lean. That is if you are testing with regular gas. I don't know what "guaranteed pass smog" will do.

 

It does not have an EGR on it. I found that the radiator cap bad but I am not sure if that would make a difference on the NOX test. Although the NOX test is a test for how hot the cylinders are burning. I will replace the cap and thermostat to see if it makes a difference. It looks like it opens up at 150 deg F. That is when the fan comes on and when I start to feel the return hose to get warmer. I will test it gain when I get the thermostat in and put fresh fuel. There is a half tank still left in it. Do you know if heet will help it pass smog particularly the NOX test?