My A/C unit is completely separate from the heater with a different thermosat. The A/C thermosat needed new batteries. I put in new batteries and nothing the screen is blank so I changed the batteries again still nothing. What can I do before calling an HVAC?

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    did it show a display specifically saying the battery was low, or did it just stop working (and you assumed that meant it needed new batteries)? Are you entirely 100% sure you put the batteries in the right direction? It matters. – Harper - Reinstate Monica Jun 14 '17 at 19:31
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    If you provide the make/model of the thermostat, maybe someone will be familiar with an idiosyncrasy or reset button or something. There really isn't much here to go on. Can we assume it was previously working, and you put in known-good batteries in the right direction? Like any electronic device, it could have failed. – fixer1234 Jun 14 '17 at 19:36
  • Look very closely for a reset button, could be just a small hole about .7mm. Also check the manual for a restart sequence. – Retired Master Electrician Jun 14 '17 at 21:07
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    Try a full power cycle of the HVAC system - usually a cutoff switch on/near the air handler. – manassehkatz-Reinstate Monica Jun 15 '17 at 4:27

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