Digital thermostat mounted on a concrete wall
Generic photo: Alexander Mass · Pexels License

Thermostat calls but heat or cooling does not start

Split the problem in two: is the thermostat sending 24 V on the call wire, and is the equipment responding to it? A jumper test at the board answers that in minutes.

Single source ~40 min moderateBrands: All

Before you start

  • Jumper only low-voltage terminals (R to W, Y or G); never touch 120/240 V terminals
  • Do not jumper R to C: it shorts the transformer
  • Gas-side and refrigerant-side faults are refer-out: diagnose only

Diagnostic steps

Unlock all 5 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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Thermostat calls but heat or cooling does not start · OnCall Tech