Thermostat shows E4 and cooling or heating stages do not energize correctly; may indicate a shorted output.

Safety first: Turn off all HVAC breakers. A shorted low‑voltage circuit can damage the transformer or thermostat.

• Visual inspection at thermostat: Remove the thermostat and inspect the Y, Y2, W, W2, and G terminals. Ensure no bare copper is touching adjacent terminals or the backplate. Trim and re‑terminate any frayed wires.
• Check at indoor control board: Inspect the corresponding Y, W, and G terminals on the Payne furnace/air handler control board. Look for melted insulation or signs of arcing. Correct any shorts and secure wiring.
• Isolate circuits: With power off, disconnect the suspect output wire (e.g., Y1) at the thermostat. Restore power and command cooling. If E4 does not appear with the wire disconnected, the short is downstream (wiring or outdoor unit). If E4 appears even with no load connected, the thermostat output is likely damaged.
• Test outdoor contactor coil (advanced): At the outdoor unit, disconnect the low‑voltage leads to the contactor coil. Measure coil resistance; a reading near 0Ω indicates a shorted coil. Replace the Payne contactor if defective.
• Correct accessory wiring: Ensure any humidifier, dehumidifier, or ventilation devices are wired only to the designated accessory terminals and not piggy‑backed onto Y or W in a way that exceeds current ratings.
• Replace damaged components: If the thermostat output is shorted internally, replace the Payne smart thermostat. If the wiring or contactor is shorted, repair/replace those components.