Service engine soon light

I am new to the forum but own a 1987 Burban with 150000 miles 350 5.7 liter engine. It runs great but the service engine soon light started comming on. it does not happen everytime I drive it. I changed the o2 sensor but it came on again. i read the other threads but mine is not having that sever of the problem. i can tell when it comes on but it does not really affect the driving. Any ideas would be great.

The ECM stores a code for your light. Even if your light is off you can read the code by grounding the ALDL. If you've never cleared your codes before, pull out the fuses for the ECM (I think you have 2) for 30 seconds and then plug them back in. This will clear any old trouble codes you may have. Wait until the Check Engine Light happens again and then read your code. If you don't know how to read your codes, go to AutoZone and they'll read your code for free. Don't buy any other parts yet. Let us know what code(s) show up.