I have an 85 pickup with the 22re, after market headers, new after market o2 sensor and a good cat (just had it checked). When I drive on the freeway at sustained rpms for about 5 mins the check engine light will come on. The light will stay on until I take my foot off of the gas, it then goes off and comes right back on again 5 mins later. I had a couple exhaust leaks that I thought were causing the problem; well I just had them all fixed and now the light does not come on as often, but it still comes on sometimes.

I've been told that some times just having an after market o2 sensor can make the light trip. Is that legit? Has anyone ever come across a problem like this?

Also, the truck is not having any drivability issues (my mechanic friend says it's running great

 

You need to jump the diagnostic plug and see which code its showing then go to the list 4Crawler has and see what your code means.
I have always used aftermarket O2 sensors and never had a code from them. Usually used Bosch.

 

Cool. Thanks I'll try that and post back.