Digital thermostat mounted on a concrete wall
9 components covered

Field manual · Thermostat

ecobee3 lite smart thermostat (EB-STATE3LT-02 and Pro EB-STATE3LTP-02): quick sheet

Covers: ecobee3 lite retail (EB-STATE3LT-02) and contractor/Pro version (EB-STATE3LTP-02). Now listed as discontinued by several distributors but very common in the field. ecobee Premium/Enhanced have their own sheet.

Cross-checkedBrands: ecobee9 components

A touchscreen Wi-Fi thermostat without the built-in speaker/microphone of the bigger ecobees. It needs steady 24 V power (C wire, or the Power Extender Kit) and supports up to 2 conventional heat / 2 cool stages, or heat pumps with up to 2 compressor stages plus 2 aux stages. Later firmware added SmartSensor support. Service calls usually come from PEK installs, missing C wire and threshold settings on heat pumps.

Safety first

  • Turn HVAC power off before wiring
  • 24 V only: not for line-voltage baseboard heat
  • Do not lower the compressor minimum outdoor temperature below the heat pump's rated operating range

How it works

At first power-up the thermostat detects which terminals have wires and builds the equipment profile from that plus a few questions. The configuration can be changed later under Main Menu > Settings > Installation Settings.

Heat pump logic is controlled by Thresholds settings on the thermostat itself (not the app or web portal). Compressor Minimum Outdoor Temperature (default about 35°F) stops the compressor below that outdoor temperature; Aux Heat Max Outdoor Temperature blocks strip heat above a set outdoor temperature. Outdoor temperature comes from Wi-Fi weather.

Minimum compressor and heat on-times default to about 5 minutes (300 s) to prevent short cycling. Differentials (how far temperature drifts before a call) are adjustable.

Without a C wire, the Power Extender Kit (PEK) mounts at the furnace/air handler board and lets the thermostat get common through the existing wires; the thermostat then uses its PEK terminal.

Components & tests

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Digital thermostat mounted on a concrete wall by Alexander Mass, Pexels License

Diagnostic guides for this appliance

Guide · Thermostat

Furnace / air handler 3 A low-voltage fuse keeps blowing (finding the 24 V short)

A blown 24 V fuse is a symptom of a short in the thermostat wire, outdoor unit wire or a load (contactor coil, damper, humidifier). Isolate circuits one at a time instead of replacing fuses.

8 steps ~60 minmoderate
Guide · Thermostat

Zoned system: one zone not heating or cooling (damper and zone panel checks)

If only one zone fails, the cause is almost always that zone's damper, actuator, thermostat or wiring, not the furnace or AC. Simulate a call at the panel and check whether the damper actually moves.

8 steps ~45 minmoderate
Guide · Thermostat

Field pattern: thermostat replaced, now nothing works or the wrong thing runs (fuse, system type, O/B)

Almost every post-install failure comes from one of four things: a 24 V short during install that blew the board fuse, a missing C wire, the wrong equipment type in the installer menu, or the O/B (reversing valve) setting reversed. The wrong turn is assuming the new thermostat is defective or that the furnace failed the same day; check the install before ordering anything.

7 steps ~40 mineasy

Other thermostats manuals

Manual · Thermostat

Google Nest Thermostat (2020, mirror face, no ring): quick sheet

Google Nest Thermostat launched October 2020 (model G4CVZ, sold as GA01334-US and color variants). Not the Nest Learning Thermostat (3rd/4th gen) or the Nest Thermostat E, which have their own sheets/notes.

9 components5 common failuresGoogle Nest
Manual · Thermostat

Honeywell Home T5 / T5+ Smart Thermostat (RCHT8610WF, RCHT8612WF, RTH8800WF): quick sheet

Honeywell Home / Resideo T5 Smart Thermostat RCHT8610WF (no C-wire adapter in the box), T5+ RCHT8612WF (ships with the THP9045A C-wire adapter) and RTH8800WF retail variant. T6 Pro, T9 and T10 have their own sheets.

7 components5 common failuresHoneywell Home, Resideo
Manual · Thermostat

Honeywell Home FocusPRO N100 non-programmable thermostats (TH1110U4000, TH1210U4001, TH1320U4002): quick sheet

FocusPRO N100 series on the UWP wall plate: TH1110U4000 (1H/1C, heat pump without aux), TH1210U4001 (heat pump 1 compressor + aux, or 1H/1C conventional, dual fuel), TH1320U4002 (heat pump up to 2 compressor stages + aux, up to 2H/2C conventional, dual fuel). Successor family to the older FocusPRO 5000/6000 contractor thermostats.

6 components5 common failuresHoneywell Home, Resideo
ecobee3 lite smart thermostat (EB-STATE3LT-02 and Pro EB-STATE3LTP-02): quick sheet · OnCall Tech