Strange emission test failure No ppm
 

My 944s2 failed the emission test at 40 Km/h 1928 Rpm.
HC ppm Limit 62 Reading 2 Pass
CO% Limit 0.35 Reading 0.00 Pass
NO ppm Limit 0700 Reading 863 FAIL

Any thoughts why it would fail The NO PPM ?

NOx is usually a CAT issue.

Try to find a stock one here or some other forum, or you will be paying $$$$$$

Cheers,

Keith

Once I failed because the tech selected 8 cyl, not 4 cyl. which skewed the results.

Thanks For the Quick reply!!

Is there any truth to taking the car for a good long drive so that the cat is very hot before you get the emission test?

yes there is. i would do that before doing the cat.

vacuum leak will cause high NOx problems. mine passed, but just barely, until i replaced ALL the vacuum lines, including the stuff under the intake manifold. after, it passed with a lot of room to spare, which makes me wish i had saved the cat i had that had been gutted....

With low CO & HC, but high NOx, you're engine is definitely running too lean.

Before assuming the cat's bad, I'd check the easy stuff first.

As mentioned before, check for vacuum leaks e.g., broken/cracked/disconnected vacuum lines.

Check O2 sensor, but a failure there usually results in overly rich mix, but it's easy to check (see procedure at Clarks Garage.)

Check proper operation of throttle position switch, also detailed at Clark's.

Check that someone didn't monkey with the air bypass screw. It's the one that's supposed to have a metal plug on top so that it's not played with. If the metal plug is in, it's probably ok. If not, adjust it to a more rich mixture. If you don't have a gas analyzer, as most of us don't, it'll be trial and error. Also detailed at Clarks.