I have a 91 Escort GT with 238k miles on it, all original parts. The last thousand miles the performance has been slipping from about 30mpg down to 24mpg. I decided to replace the front O2 sensor and when I disconnected it, I decided to see if my Check Engine Light would turn on. I started the car and No Check Engine Light. The bulb is not burned out, I see it when the ignition is in the On position. So, now I think my computer EEC IV is bad. How do I test it?

sometimes time veicle have to sense a fault on system 3 or more times before it set the check engine light and to sense that the ignitionneed to be cycle again at least 3 times

This link is one of several on the subject. The test routine include tests that affirm if the EEC-IV is OK or not.
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http://www.corral.net/tech/maintenance/eecivtest-koeokoer.html#koeo