Thermostat displays E0 and the American Standard air conditioner will not respond to temperature changes

Safety first: Turn off power to the air handler and outdoor unit at the breaker panel before removing any panels. Low-voltage circuits can still short and damage boards even if they are not dangerous to touch.

• Check thermostat wiring: Remove the thermostat from its wall plate. Verify that R, C, Y, G, and W wires are firmly clamped and not corroded or broken. Tighten any loose screws.
• Inspect air handler control board fuse: Open the indoor unit's electrical panel. Locate the 3A or 5A blade-style low-voltage fuse on the American Standard control board. If it is blown (melted link), replace it with the same rating only. Do not bypass the fuse.
• Check low-voltage cable: Follow the thermostat cable into the air handler. Look for staples driven through the cable, rodent damage, or melted insulation. Repair damaged sections with proper low-voltage splice connectors or replace the cable.
• Reset system: After correcting any wiring or fuse issues, restore power at the breaker, then power-cycle the thermostat (remove batteries or disconnect for 30 seconds if applicable). Recheck for the E0 code.
• If E0 persists: The Comfort Control thermostat or the indoor control board may be defective. At this point, professional diagnosis with a multimeter is recommended to avoid damaging the control electronics.