When your Ecobee thermostat screen goes black or keeps rebooting, it's losing its 24V power supply from your HVAC system. This typically happens when your furnace overheats due to a clogged filter, triggering the high-limit safety switch that cuts power to the thermostat.
Quick answer
- Check your furnace filter first — clogged filters cause overheating that trips safety switches
- Verify C-wire connections — loose common wire causes power loss and boot loops
- Test voltage at Rc and C terminals — should read ~24V AC when working properly
- Wait 15 minutes after fixing — safety switches need time to auto-reset
Symptoms
- Thermostat screen is completely black with no display
- Screen shows Ecobee bee logo on startup, runs for a few minutes, then goes black
- Problem occurs primarily when heat runs for 10+ minutes
- AC works normally but heating causes reboots and shutdowns
- Intermittent flickering or random restarts during operation
Quick checks
Test the power source: Remove the Ecobee from the wall plate and use a multimeter to check voltage between Rc and C terminals — should read 24V AC.
Inspect the filter: Pull your furnace filter and check if it's gray, clogged, or hasn't been changed in months.
Check wire connections: Examine the C-wire terminal for loose connections — gently tug the wire to ensure it's properly seated.
Step-by-step fix
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Check the furnace filter — locate your filter (usually in the furnace cabinet or return air duct) and inspect for dirt buildup. If gray or clogged, replace immediately.
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Reset the high-limit switch — after replacing the filter, wait 15 minutes for the furnace safety switch to automatically reset and restore power.
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Verify C-wire connection — pull the Ecobee off the wall plate and check that the C-wire is fully inserted into the terminal. Push firmly until it clicks.
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Test voltage levels — use a multimeter to measure AC voltage between Rc and C terminals. You should see approximately 24V AC.
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Check the furnace fuse — if voltage reads 0V, locate the 3A purple fuse in your furnace control board and replace if blown.
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Inspect condensate drain — during cooling season, check the AC drain pan for standing water that could trigger the float switch and cut power.
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Reinstall and test — mount the Ecobee back on the wall plate and test both heating and cooling cycles for stable operation.
If it still isn't working
PEK troubleshooting: If you're using the Power Extender Kit, verify all four wires inside the furnace are mapped correctly and the PEK ground connection is secure.
Professional diagnosis needed: If you have a clean filter but the limit switch continues tripping, you likely have a failing blower motor or blocked ductwork requiring HVAC service.
Warranty replacement: If the furnace provides stable 24V power but the Ecobee still won't turn on, the unit itself may be defective and need replacement.
FAQ
Why does my Ecobee only die when heating runs? The high-limit safety switch in your furnace cuts all power to the thermostat when it detects overheating, which typically happens during heating cycles with restricted airflow.
Can I bypass the safety switch to keep my thermostat on? Never bypass furnace safety switches — they prevent dangerous overheating that can crack the heat exchanger and cause carbon monoxide leaks.
What if I have Rh but no Rc wire? The Ecobee screen needs power from Rc specifically, but most systems internally bridge Rh and Rc. Verify your system has this bridge or install a jumper wire.
How often should I replace my furnace filter? Replace standard filters every 1-3 months, or high-efficiency filters every 6-12 months, depending on usage and air quality.