1993 Altima, 188K, my "check engine" light has come on twice the past 2 weeks (occured same day, same place). Oil, water, transmission fluids check out fine, start driving again and the indicator turns off when I drive by the same area and stays off. Faulty light or another underlying problem?

The check engine light is working correctly. It has nothing to do with water, oil, or overheating. The purpose of the check engine light is to tell you that your computer controlled engine is not operating optimally. Your engine is probably wasting fuel and creating excessive emissions. The computer has received input from one of the sensors that is outside normal parameters. Your light does not stay on. That shows the problem is what is known as a soft code. Most of the time the input messages from your engine sensors are within normal parameters so the light turns off. It could be that under the same circumstances you refer too when the light came on each time, that your oxygen sensor is sending the out of bounds message. An O2 sensor or MAF Sensor on a 93 may be contaminated, worn out, and needs to be replaced. Or maybe the engine needs a tune-up. The computer stores a code that will help diagnose the cause. The cause is likely to continue and turn your CK ENG light on more often. Your next step is to have the code that is stored in your ECU (computer) read. AutoZone will read it without charging you. You can follow instructions to read it yourself. Or you can take it to a repair shop.

I called AutoZone and they don't check the codes- they'll loan you the equipment to do it-- so I took it to PepBoys since I found a coupon, however the guy said that to run the diagnostic that the light had to stay on (the light comes on for about 15 minutes, but then goes off and stays off). PepBoys said it should be OK.