Thermostat displays F2 and may disable compressor operation; cooling or heating (for heat pump) will not start.

Safety first: Turn off power to both indoor and outdoor units. Low‑voltage shorts can blow fuses or damage boards.

• Check low‑voltage fuse: On the Payne furnace/air handler control board, locate the 3–5A blade fuse protecting the 24V circuit. If blown, replace with the same rating after finding and correcting any shorts.
• Inspect Y wiring: Verify the Y (and Y2 if present) wires are secure at the thermostat and indoor control board, with no bare copper touching other terminals or metal.
• Test contactor coil (advanced): At the outdoor unit, with power off, disconnect the two low‑voltage wires to the contactor coil and measure resistance. A very low reading (near 0Ω) indicates a shorted coil; replace the Payne contactor.
• Bypass thermostat (advanced): With power off, temporarily jumper R to Y at the indoor control board. Restore power; if the outdoor unit starts and runs normally, the thermostat or Y wire is suspect. Remove the jumper after testing.
• Replace thermostat if needed: If the system runs with an R‑Y jumper but F2 persists and the compressor will not start via thermostat commands, replace the Payne smart thermostat.