Neither indoor nor outdoor units will turn on.

Indoor Unit - Rheem model RHSL-HM3617JA
Outdoor Unit - Rheem model 13PJL36A01

Troubleshooting thus far (I have electronics background, though not in HVAC):

1) All Circuit Breakers are on, both indoor and outdoor units are getting 240VAC.

2) On thermostat I tried both heat and cool, neither worked.

3) On thermostat I turned the fan to "ON" but fan did not come on.

4) I have more than one unit, so I swapped the thermostats (they are identical) but nothing changed. Broken HVAC still broke with known good thermostat. Good HVAC still good with suspect thermostat 24 hours later. Thermostat eliminated as culprit.

5) Outside unit - took off panel, no visible burnt circuits, lose wires, etc. I pressed the Contactor test button it started up, turned off when released.

6) Inside unit - took off panel, again no visible issues. Metered the transformer - 240VAC input, 28VAC output. Supposed to be 24VAC output, so I replaced the transformer. New one has output of 27.5VAC, not that big of difference, so I'm eliminating the transformer as the culprit.

After researching HVAC systems, many people have issues with just 1 unit. The inside unit can run while the outside is broke, and vice versa. This means one or the other circuit board (other than the thermostat) tells both units to turn on. Question is - which one?

submitted by /u/Red_Rocket
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