Thermostat display shows E0 and will not communicate with the indoor unit or outdoor unit; system does not respond to mode or temperature changes.

Safety first: Turn off power to the furnace/air handler and outdoor unit at the service switch and breaker before touching any wiring. Low‑voltage circuits can still cause damage if shorted.

• Verify power reset: Turn off the HVAC breakers for 2–3 minutes, then restore power to see if the E0 clears. Sometimes a bus lockup will reset.
• Inspect ABCD wiring at thermostat: Remove the Payne smart thermostat from its wall plate. Confirm that the A, B, C, and D conductors are firmly clamped and that the insulation is stripped correctly (no copper exposed beyond the terminal). Tighten any loose screws.
• Check wiring at indoor control board: At the furnace/air handler, locate the ABCD or COMM terminal strip on the Payne control board. Confirm that each conductor color matches the same letter at the thermostat. Correct any mis‑wiring and re‑tighten terminals.
• Look for damage or splices: Trace the thermostat cable where accessible. Look for crushed cable, rodent damage, or poorly made splices. Repair with proper low‑voltage wire nuts or replace the cable if conductors are shorted or broken.
• Isolate the thermostat: If possible, temporarily connect a short piece of new 4‑conductor thermostat wire directly between the thermostat and the indoor control board. If E0 disappears, the in‑wall cable is likely defective.
• If still failing: With wiring verified, a failed communication circuit on the Payne smart thermostat or the indoor control board is likely. At this point, replacement of the thermostat or control board should be done by a qualified technician.