installed my ecobee premium but now the a/c won't turn off, it just runs constantly

Diagnosis

The Ecobee Smart Thermostat Premium uses a 24V AC switching circuit to trigger your HVAC system. When the A/C runs constantly regardless of the set temperature, it indicates a "closed circuit" on the Y1 (cooling) terminal. This means the thermostat is either sending a constant 24V signal to the compressor or the circuit is being bypassed entirely, tricking the air handler into thinking the cooling call is active. This typically happens for one of three reasons: a physical short circuit at the backplate where a stray copper strand from the Y1 wire is touching the R (power) terminal, a misconfiguration in the "Equipment" settings where the thermostat believes it is in a "Test" mode or "Fan Always On" state, or a failure in the internal relay of the thermostat itself. You can confirm this by pulling the thermostat off the wall plate; if the A/C shuts off immediately upon removing the device, the issue is the thermostat's configuration or wiring. If the A/C continues to run while the thermostat is detached, the problem is a short in the wall wiring or a stuck contactor at the outdoor unit.

How to Fix It

Follow these precise steps to isolate and resolve the constant cooling issue:

1. Shut off the power to your HVAC system at the main breaker panel to prevent electrical arcs or blowing the 3-amp or 5-amp fuse on your furnace control board.
2. Gently pull the Ecobee Premium off its wall plate. Inspect the Y1 (yellow) and Rc (red) terminals. Ensure no stray copper filaments are bridging the gap between these two pins. If you see any, trim the wire back and re-strip it to exactly 1/4 inch.
3. Push the wires firmly into the terminals. Ensure the insulation is not trapped inside the clamp, but that no bare wire is exposed outside the terminal block.
4. Snap the thermostat back onto the base and power the system back on.
5. Navigate to the main screen and go to Main Menu > Settings > Installation Settings > Equipment > Cooling. Ensure the "Cooling Stage 1" is correctly identified as your A/C and not a heat pump.
6. Check the Fan settings under Main Menu > Settings > Installation Settings > Equipment > Fan. Ensure it is set to "HVAC Managed" rather than "Thermostat Managed" if you have a high-efficiency blower, as some systems trigger cooling when the fan is forced on.
7. Verify that "Cooling" is not set to "Always On" in the test menu. Go to Main Menu > Settings > Installation Settings > Test > Cooling and toggle it off to ensure the relay releases.

If That Didn't Work

• Check for a Stuck Contactor: Go to your outdoor condensing unit. If you hear a humming sound but the fan isn't spinning, or if the unit is running despite the thermostat being off, the 24V contactor (the relay switch) may be physically welded shut. This is a mechanical failure of the A/C unit, not the Ecobee. You will need to replace the contactor part (typically a 24V single-pole contactor).
• Inspect the C-Wire Power Extender Kit (PEK): If you used the PEK because you didn't have a C-wire, check the wiring at the furnace control board. If the Y and G wires are swapped at the PEK, the system can enter a feedback loop that keeps the cooling stage active. Re-verify that the PEK wires match the labels exactly.
• Test with a Jumper Wire: With the thermostat removed, use a small piece of insulated wire to jump the R terminal to the Y1 terminal on the wall plate. If the A/C starts and then stops when you remove the jumper, the wiring is fine, and the Ecobee unit itself may have a defective internal relay and requires a warranty replacement.
• Verify Threshold Settings: Check Main Menu > Settings > Installation Settings > Thresholds. If the "Cooling Differential Temp" is set too low (e.g., 0.5°F), the system may cycle so rapidly it appears to be running constantly. Set this to 1.5°F or 2.0°F for stability.

Heads Up

• Always check the 3A or 5A fuse on your furnace/air handler board after a wiring error; if the A/C won't turn on at all after your troubleshooting, you likely blew this fuse.
• Avoid using "wire nuts" inside the wall cavity; use the provided crimp connectors to ensure the Y1 signal is clean and not intermittent.
• If you have a heat pump, ensure you haven't accidentally configured the system as "Conventional" in the settings, as this will cause the auxiliary heat and cooling to conflict.
• Regularly clean your A/C condenser coils; a system that cannot shed heat efficiently may trigger "long run" cycles that look like a thermostat failure but are actually a mechanical efficiency issue.