Not a part of this discussion but would like to ask a question.

Why not just purchase a roll of 2 wire bell wire, disconnect the existing low voltage wiring at the control panel and at the contactor in the compressor unit. Temporarily replace LV wire with the bell wire and see what happens?

If your present LV wire happen to run through the attic, squirrels love the stuff.

Assuming I have a multimeter,
It not possible to check continuety on my LV from both ends of the Y or G line. Leads are not long enough. Is there any other way to check integrity of the wire with multimeter?

Thanks,

Your not going to be checking integrity, but rather shorts to other lines.

The most likely short is between Y and C. All of the other wires shorted together really don't cause any problems except odd behavior. Common is likely at ground potential and it's supposed to be.

Again assuming Y is the problem

A short from "Y" to ground or "Y" to C are most likely.

It's probably not G, if you can use the FAN position of the thermostat and the fuse doesn't blow.

Thus with power off, you can go to the furnace and measure the resistances from "Y" to every other terminal and the furnace case (ground).

You could disconnect the LV wire at both ends and then check for continuity between the two wires. Continuity would indicate a short between the two.

I have the same problem. However, after reading online forums, I tested for and found a short between Y and COM.

Any suggestions on what I should do next?
Is this because of a bad component on the board or another bad part?

Hi g33k86. Next step is to run new themostat wire. Most likely the bad wire is no fualt of any other components, just a bad wire. It happens. I would suggest running a new five wire stat wire. Unless you happen to have an extra wire in your current wiring that is not being used, then you could combine y and c, use it as one wire, and use the extra wire for the y, or c, which ever you choose.