Ecobee Not Working: OB Wiring, PEK Problems, SmartSensor & Offline Fix

Common Symptoms

• Ecobee has power and display but heating or cooling never starts
• Heat pump runs cooling in winter or heating in summer (reversed operation)
• Ecobee shows 'Heating' or 'Cooling' active but equipment does not turn on
• Fan runs but compressor never starts — outdoor unit stays silent
• PEK installed but system short cycles every 2–3 minutes
• SmartSensor shows 'Not Connected' or disappears from the ecobee app
• Ecobee app shows thermostat as offline despite thermostat appearing active
• Aux heat never turns on even in very cold weather

Most Likely Causes

1. 1

   OB Terminal Polarity Mismatch on Heat Pumps (Most Common for No Heating/Cooling)

   Heat pumps use a reversing valve to switch between heating and cooling modes, controlled by the O or B terminal on the thermostat. ecobee defaults to O (energize-on-cool), which is correct for Carrier, Trane, Lennox, Rheem, and American Standard heat pumps — the reversing valve energizes during cooling and de-energizes during heating. However, Goodman, Amana, and some older Lennox systems use B polarity (energize-on-heat) — if the ecobee is set to O but the system needs B, the heat pump will run in cooling mode when you call for heat and vice versa. Navigate to ecobee Main Menu > Settings > Installation Settings > Equipment > Heat Pump and verify the O/B polarity matches your equipment brand. Changing the wrong setting will immediately invert the behavior, which confirms the diagnosis.

2. 2

   PEK Wiring Causing Short Cycling on Multi-Stage or Heat Pump Systems

   The ecobee Power Extender Kit (PEK) intercepts the G (fan) wire at the furnace control board to create a C-wire return path on single-stage systems with no dedicated C-wire. The PEK works reliably on simple single-stage gas furnace + single-stage AC systems. However, the PEK causes instability on: (1) multi-stage heating or cooling systems (W2, Y2 terminals), (2) heat pump systems with O/B wiring, (3) systems where the G wire is used simultaneously for fan control and another function. On these systems, the PEK's signal interception creates voltage conflicts that cause short cycling, compressor lockouts, or equipment that turns on and immediately shuts off. Solution: bypass the PEK and install a dedicated C-wire from the furnace C terminal to the ecobee C terminal. If no spare wire is available in the thermostat cable, run a new 18/5 cable.

3. 3

   Minimum On/Off Time (MOT/MOFT) Protection — Not a Fault

   The ecobee's Minimum On Time (MOT) and Minimum Off Time (MOFT) settings prevent the compressor from short cycling. By default, MOT is 5 minutes (compressor must run at least 5 minutes before shutting off) and MOFT is 5 minutes (compressor must be off at least 5 minutes before starting). If the ecobee displays 'Cooling is delayed' or 'Heating is delayed,' this protection timer is active — it is not a fault. The countdown is visible in the ecobee app. You can adjust MOT and MOFT under Main Menu > Settings > Installation Settings > Thresholds if the defaults are too conservative for your system. Do not set MOFT below 3 minutes on systems with reciprocating compressors.

4. 4

   SmartSensor Not Responding — CR2032 Battery Dead or Out of Range

   ecobee SmartSensors communicate via 900 MHz RF. Each sensor uses a CR2032 lithium coin cell with a typical life of 12–18 months. When the battery dies, the sensor LED will not flash when the reset button is pressed and the sensor disappears from the ecobee app. Maximum effective range is approximately 45 feet through one standard interior wall — thick concrete walls, large metal appliances, and microwave ovens all reduce range significantly. After replacing the CR2032 (note: + side faces the label printed on the battery holder), pair via Main Menu > Sensors > Add Sensor. If the sensor was previously paired, a fresh battery typically triggers automatic re-pairing within 5 minutes.

5. 5

   Aux Heat Lockout Temperature Setting Preventing Auxiliary Heat

   ecobee allows setting an 'Aux Heat Max Outdoor Temp' lockout that prevents auxiliary (strip) heat from running above a set outdoor temperature — typically set to 35–40°F by installers to prevent expensive auxiliary heat from running when the heat pump can handle the load. If outdoor temperatures drop below the heat pump's effective range (typically 15–25°F depending on unit) and aux heat is locked out, the home may not reach setpoint even with the system running. Check this setting under Main Menu > Settings > Installation Settings > Thresholds > Aux Heat Above Outdoor Temperature. Also verify Aux Max Runtime is not set too low.

6. 6

   App 'Offline' — Port 443 Outbound Blocked or ecobee Server Outage

   ecobee thermostats communicate with ecobee servers via HTTPS on port 443. If a router firewall, parental control system, or business-class router policy blocks port 443 outbound for the thermostat's IP address, the ecobee will appear offline in the app even though it continues to control HVAC locally. Verify port 443 outbound is open for all devices on the network, or add the thermostat's MAC address to a trusted device group. Check ecobee's service status at status.ecobee.com for server-side outages — during outages, the thermostat continues to function locally but cloud features (remote access, history, alerts) are unavailable.

Quick DIY Checks

1. 1Test heat pump OB polarity by swapping the setting: on the ecobee, go to Main Menu > Settings > Installation Settings > Equipment > Heat Pump. Note the current O/B setting. If set to 'O (Energize on Cool)' change it to 'B (Energize on Heat)' — or vice versa. Then test both heating and cooling modes by setting the thermostat 3–4°F above (heat) and below (cool) the current room temperature and waiting 5 minutes for the compressor protection timer to clear. If heating now delivers warm air and cooling delivers cool air, the polarity was incorrect. Set it to the correct position and leave it. If the behavior was already correct, change the setting back to its original value.
2. 2Check 24VAC at the C terminal to verify PEK is working or C-wire is present: remove the ecobee from the wallplate by pulling straight out. Set a multimeter to VAC (AC Volts). Place the red probe on the wire inserted in the R terminal and the black probe on the wire in the C terminal. Expected reading: 24–28VAC. A reading of 0V means the C-wire or PEK is not delivering power to the C terminal. If 0V: check PEK wiring at the furnace board or verify the C-wire connection at the air handler control board C terminal. A reading of 12–18V (partial voltage) on multi-stage systems indicates the PEK is present but conflicting with staging wires — this confirms the PEK must be removed and replaced with a true C-wire.
3. 3Verify Equipment Interface wiring labels match your system and clear stuck 'Delayed' messages: on the ecobee, go to Main Menu > Settings > Installation Settings > Equipment > Wires. The ecobee shows a diagram of detected wire terminals. Verify: Y1 = single-stage cooling compressor, Y2 = second-stage cooling (if present), W1 = gas heating valve or heat pump aux strips, W2 = second-stage heating (if present), G = fan, O/B = reversing valve (heat pump only), C = common. If any terminal shows 'Not Detected' but you know a wire is connected there, the wire connection at the wallplate terminal may be loose — remove the ecobee, press each wire firmly into its terminal until the white locking tab clicks, and reattach. For stuck 'Delayed' messages: Main Menu > Settings > Installation Settings > Thresholds > show the current MOT/MOFT values and the remaining countdown.

1. 4Replace SmartSensor CR2032 battery and re-pair: slide open the SmartSensor back cover using the tab on the top edge. Remove the old CR2032 battery. Install a fresh CR2032 with the + side facing up (facing the text on the battery holder). Press the reset button (small pinhole on the back) with a paper clip — the LED should flash green 3 times, indicating the battery is good and the sensor is ready to pair. On the ecobee: Main Menu > Sensors > Add Sensor. The sensor name will appear within 1–2 minutes if it is within 45 feet. If the sensor does not appear: move it to within 10 feet of the ecobee and retry. Walls, mirrors, and metal objects reduce 900 MHz range significantly.
2. 5Check and adjust Aux Heat lockout temperature: go to Main Menu > Settings > Installation Settings > Thresholds > Aux Heat Above Outdoor Temperature. If this is set to 35°F and outdoor temperatures have dropped below 35°F, aux heat is locked out. Temporarily increase this value to 50°F and test whether aux heat now engages when the system is in heating mode — you should hear the air handler strips energize (a distinct hum) and feel noticeably warmer supply air. If aux heat engages, the lockout temperature was too low for your climate. Adjust to 40–45°F for most climates below freezing. Note: some HVAC installers deliberately set this to protect against over-reliance on strip heat — check with your installer if in doubt.
3. 6Diagnose app offline issue and verify port 443: open a browser on a device connected to the same home network as the thermostat. Navigate to status.ecobee.com — if any systems show degraded or outage, the offline status is server-side and will resolve automatically. If ecobee's status is all-green: on the router admin page (typically 192.168.1.1 or 192.168.0.1), verify no outbound firewall rules block port 443 for the thermostat's IP address. If you use a pfSense, Firewalla, or Eero with 'Family Profiles,' temporarily disable content filtering and check whether the ecobee comes back online. Factory reset the ecobee's Wi-Fi connection: Main Menu > Settings > Reset > Reset Network Settings, then reconnect to your 2.4GHz network.

Repair vs Replace

Recommended Tools & Parts

• 18/5 Thermostat Wire (50 ft)

  18-gauge 5-conductor thermostat wire for running a new C-wire from the furnace to the ecobee wallplate, bypassing the PEK on multi-stage or heat pump systems. 5 conductors cover R, Y, G, W, and C terminals.

  $20–$35

  Buy on Amazon →

• CR2032 Lithium Coin Cell (10-Pack)

  Replacement battery for ecobee SmartSensors and SmartSensor for Doors and Windows. Each sensor uses one CR2032. Typical life 12–18 months in the SmartSensor. Keep spares on hand — a dead sensor battery is the #1 cause of 'Not Connected' sensor errors.

  $6–$12

  Buy on Amazon →

• ecobee SmartSensor (2-Pack)

  Two-pack of ecobee SmartSensors for room temperature and occupancy sensing. Pairs with ecobee SmartThermostat Premium, Enhanced, ecobee3 lite, and ecobee4. Includes mounting base and CR2032 battery. Range: 45 ft through 1 interior wall.

  $75–$95

  Buy on Amazon →

• Digital Multimeter

  Required for verifying 24VAC at R and C terminals, testing partial voltage from a conflicting PEK, and continuity-testing the furnace control board fuse. A basic $18–$35 multimeter handles all thermostat diagnostics.

  $18–$35

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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