95 2.2L Service Engine Soon

My truck has 175K miles on it and the service engine light comes on usually after 15 minutes of driving (but not always). I have replaced the O2 sensor, checked the Intake Air Temperature and Engine temperature sensors for correct resistance at various temperature and plotted versus the correct reading for each thermoresistor. The truck idles fine and runs good up to 80 mph (I just don't drive any faster since it is turning 3900 rpm at 80). Any other ideas? Please don't reply with "read the code" since you have to pay to have the code read on this truck due to the ODB I/II state of this model year.

Thanks!

Check the EGR valve

Thanks, I had the emissions tested today and the NOx was high, a problem associated with the EGR valve.

If you have an AutoZone nearby, they will check the codes for free.

If you have an AutoZone nearby, they will check the codes for free.
the egr is stikin

Autozone will not test anything older then '96, it has to be an ODBII, or at least at the Autozone in my town.

It turned out to be the EGR valve, NOx dropped from 1400 at high speed to around 200 and the service engine light now no longer come on.