Failed emissions 95 GMC 1500 Pick-up 5.7 Liter K motor. Fails Hydrocarbon at low idle, but easily passes high idle. Could this be a computer, catalyst, or egr hose?

SOURCE: ASM 2525 NO ppm TAILPIPE EMISSIONS failure

have you had the the engine codes pulled and reset?
and if so what are your engine codes?
i dont know of any car that will pass emissions test with the engine light on.
we cure your trouble that caused your engine light to come on and you will pass your test.
later
paul

SOURCE: Emission system problem

fill your muffler with SOS pads just before your test a wire coat hanger can remove them after the fact..
good luck its how i got my prelude to pass inspection in vancouver bc canada

SOURCE: rough idle

well you need to hook it to a computer and determine if o2 sensors are working correctly and make sure enough fuel pressure

SOURCE: failed emissions....high nox

Sticking egr will raise everything except nox. Bad o2 bad cat hard to give an accurate diagnosis with out checking it

SOURCE: Failing NJ State Inspection for Low Idle Emissions

Based on what your write up says it would have to be a timing issue or Mass Air flow Sensor (MAF) .If your getting 20 HG's at idle steady there can't be a vacuum leak.Your a little on the rich side so its probably a dirty MAF.Try cleaning it first and see what you get . If that doesn't work then reset timing.
Keep me posted on this .