Just got a pre test done on my 98 LS. The motor has 148K miles and it’s completely stock w/ the exception of a resonator delete. Could you guys take a look at my scanned emission test results? What would cause my NO (PPM) readings at 15 mph to be abnormally high, then drop off at 25 mph? What is the life expectancy of an OEM cat?

I’ve already replaced the following items and I always run 91 octane:

O2 sensor
Plugs
Cap & rotor
Timing belt & water pump
Fuel filter

 

Why would you run 91 octane on a stock B18B? That certainly isn't going to help with a smog check...

High NOx readings are due from high combustion temps, which can be from running lean, incorrect timing, faulty EGR, or a few other things. It could also be a dead/dieing cat.