i have a 93' civic hatch, with a d16z6, and i went to get it smoged and it failed at 15 mph but passed at 25 mph, if anyone can help me or tell me what id need to do id really appriciate it thanks for ur time.

 

any mods? how many miles?

 

did you eheat the cat enough wut exactly failed at 15 nox? co? hc?

 

NO failed max is 720 and at 15 it measured 783, and at 25 mph it measured at 116, the only mod is a dc header

 

how old is the cat and also how old is O2? did you do a tune up? did you drive the car for a while to get the cat hot? I have noticed if you have the old dc headers with the O2 bung on the runner as opposed to the collector I have seen cars not pass like that.

 

a good header will make no go up. the less exhaust gasses going back into the motor the higher the no and reversely like more backpressure (more exhaust gas going back into motor) makes engine temps lower decreasing no. kinda weird rite but dc header isnt all that great meaning it should be ok for smog lol but if you got the stock header throw it on and c wut happens.

 

i dont have the stock header, but the car wasnt running very long before i smoged it maybe 15 min, how long should i let it warm up, so the cat is heated up enough

 

I would drive on it for at least a half hour and vary the conditions to generate heat. cats need to get hot to work well