Thermostat display shows E0 and will not communicate with the HVAC equipment or mobile app.

Safety first: Turn off power to the air handler/furnace and outdoor unit at the breakers before touching any wiring. Verify power is off with a non-contact voltage tester.

Step-by-step checks:

• Inspect ABCD wiring: Remove the thermostat from the wall plate. Verify the ABCD terminals on the Rheem smart thermostat match the same color and position on the indoor unit's control board (A→A, B→B, C→C, D→D). Look for loose, broken, or pinched wires.
• Check polarity and pairing: Ensure the same conductor color is used for each letter at both ends. Reverse polarity on the communication bus can trigger E0.
• Test continuity: With power still off, disconnect the ABCD cable at both ends and ohm each conductor to confirm no opens or shorts between conductors or to ground.
• Reseat connections: Reinsert each conductor fully under the terminal screws on both the thermostat sub-base and the indoor unit control board. Tighten firmly but do not overtighten.
• Power cycle system: Restore power at the breakers, wait 2–3 minutes, and check if the E0 code clears.
• If E0 persists: Temporarily connect the thermostat to the indoor unit with a short known-good 4-conductor cable at the air handler. If communication works, the in-wall cable is faulty. If not, the thermostat communication interface or the indoor control board may be defective and should be evaluated by a qualified technician.