Tempstar air conditioner indoor unit display shows E0 and system will not respond to thermostat commands

Safety first: Turn OFF power to both the indoor air handler and outdoor condenser at the breaker panel and the service disconnect before removing any panels. Verify power is off with a non-contact voltage tester.

Step-by-step checks:

• 1. Inspect thermostat wiring: Remove the indoor unit control panel. Verify R, C, Y, G, W wires are firmly connected and not shorted. Look for bare copper touching other terminals or metal.
• 2. Check low-voltage fuse: On the indoor control board, locate the 3–5A blade fuse (often purple). If blown, replace with the same rating only. If it blows again, there is a short in the low-voltage circuit that must be found before further operation.
• 3. Inspect communication cable: If your Tempstar system uses a communicating thermostat or 4-wire communication harness, verify the plug is fully seated on the indoor board and that the cable to the outdoor unit is not damaged or pinched.
• 4. Reset power: After checking connections, restore power at the breaker and disconnect. Wait 5 minutes, then call for cooling at the thermostat and see if the E0 clears.
• 5. If E0 persists: The indoor control board or thermostat may be defective. At this point, further diagnosis with a multimeter and Tempstar service literature is required and is best handled by a qualified HVAC technician.