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Field pattern: thermostat replaced, now nothing works or the wrong thing runs (fuse, system type, O/B)

Almost every post-install failure comes from one of four things: a 24 V short during install that blew the board fuse, a missing C wire, the wrong equipment type in the installer menu, or the O/B (reversing valve) setting reversed. The wrong turn is assuming the new thermostat is defective or that the furnace failed the same day; check the install before ordering anything.

Cross-checked ~40 min easyBrands: All

Before you start

  • Turn off power to the furnace or air handler before moving thermostat wires; touching R to C live can blow the low-voltage fuse
  • Line voltage is present inside the furnace or air handler; verify zero voltage before working there
  • Do not jump the thermostat terminals at the furnace except as a brief, deliberate test

Diagnostic steps

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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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Field pattern: thermostat replaced, now nothing works or the wrong thing runs (fuse, system type, O/B) · OnCall Tech