Steve Turner wrote:
A couple of months ago the heater quit coming on in my upstairs Trane
XB80 HVAC unit. It's been off warranty for about 2 years, and I am
enough of a do-it-yourselfer that I'm comfortable with simple
repairs. I found that the LED on the control board was not lighting
up, and subsequently found that I was not getting 24V output from the
115V-24V transformer. Sure enough, the 24V circuit was open, so I
found a local replacement transformer (not the exact form factor, but
same specs) and things worked fine after I replaced it. Now, I have
the very same problem (only this time it's the A/C that's trying to
come on; not that it matters), and once again 24V is missing on the
output side of the transformer. This time the 24V circuit has
continuity, but the 115V circuit is open. Of course, I can replace
the transformer AGAIN, but I'd sure like to know what could be
causing this problem... Any ideas would be appreciated. Thanks.

TWO is (probably) a coincidence; THREE would be a trend.