My 850 fail smog check because of very high pollution on NO at low speed (15MPH). On 25MPG NO was high but acceptable. HC and CO were perfectly low. O2=0.
My guess is that I have top engine deposits. Is it right guess? What other things could be defective to cause this problem?
Thanks,
Sarkis.

 

What year is your car?
If it's '95 or prior, you can check for codes stored without a scanner.
Checking for codes would be the first step.

 

There is no error codes on my car. It is 97 so it has ODBII so if there were any error codes they would be reported to state and I would get failure poing there too.
I get noses on slow sppeds (probably back fire) expecially when decelerating downhill wiht engine breaking.