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It sounds like your cat is plugged, catalytic converter that is....
This could cause your check engine light to come on and set a cat efficiency code.
I have never had a communication problem using a Snap-On or Modus Scanner on a '95 S-10 with a 4.3 Vortec and an OBD II style connector. There may be an electrical problem in the wiring between your vehicle computer and the OBD II connector.
I think your going to have to have this problem diagnosed by a qualified tech, as it sounds like there are several things going on.
One thing you can check is all your fuses. There are several ECM (vehicle computer) fuses. I'm not certain if a blown fuse would cause your communication problem, but it wouldn't hurt to check.
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