There's no real easy answer. Knowing the exhaust data will help a good
technician know where to look. It's possible that changing the oil will
help by removing HC from the PCV system.
Is your check engine lamp on? If so, diagnose that first. If not,
things to make sure of:
1. Your car is running properly. If it isn't performing properly, it's
likely putting out excessive emissions, too. If it doesn't feel like
running properly, start there.
2. Vehicle is properly preconditioned (properly warmed up). This cannot
be stressed enough. Sometimes even waiting in line idling will be enough
to make a vehicle fail. If I have to wait, I usually keep my engine
revved around 2k rpm or so, with a few breaks.
3. The thermostat opens at the proper temperature. If your thermostat
opens too soon, the vehicle will likely never reach proper temperature.
If your engine gets too hot, it will result in excessive NOx.
4. You have the correct PCV valve in the car and it's working properly.
A PCV with too large a port or that is stuck open will cause excess HC.
5. There are no exhaust leaks. This can cause you to fail because
neither the oxygen sensor nor the machine can read your exhaust properly.
There are many other things that can cause a problem, but they become
increasingly difficult to check. If all the above checks out, it's
probably time for professional help.