Thermostats

Low (cold) temperature alert

Indoor temperature fell below the alert threshold. On a call for heat, this points to a heating failure.

High (hot) temperature alert

Indoor temperature rose above the alert threshold, often a cooling failure in summer.

Sensor shut down the compressor

A connected sensor (for example a condensate or dry-contact input) triggered a compressor shutdown.

Furnace or boiler heating problem

Heat was called but the space is not warming as expected.

Heat pump heating problem

The heat pump is not delivering heat as expected.

Cooling problem

Cooling was called but the space is not cooling as expected.

Auxiliary heat running too long

Aux heat ran longer than the configured limit, suggesting the heat pump is not keeping up.

Aux heat used at a high outdoor temperature

Aux heat ran when it was mild outside, which usually means a setting or heat pump problem.

Remote sensor not communicating

A wireless room sensor stopped reporting.

Internal power-supply protector tripped

A protective device inside the thermostat's power circuit has tripped. Removing the C wire can hide the message, but the manufacturer asks that support be contacted.

Not connected (often a power/C-wire issue)

A solid X means the thermostat cannot join the network or reach the Sensi cloud; a flashing X with good batteries usually points to a C-wire or compatibility issue rather than the router.

C wire required

The installed wiring cannot power the thermostat without a common (C) wire.

No power wire detected

The base does not detect any R, Rc or Rh wire, so the thermostat has no 24 V source.

No power on the Rc wire

Earlier-generation versions of E73: the Rc terminal has no 24 V.

No power on the Rh wire

First-generation version of the no-power-on-Rh fault.

No heating or cooling wire detected

Neither a W1 nor a Y1 wire is detected, so the thermostat has nothing to call.

Too many wires

More wires are connected than the configuration allows; one of O/B, W1, Y1 or AUX must come off.

Y1 missing on a heat pump

The wiring looks like a heat pump but no Y1 is detected; the compressor needs Y1.

No power on the C wire

A C wire is connected but carries no voltage.

Lost power through the Y1 (cooling) wire

The thermostat was drawing power through Y1 and lost it, often because the outdoor unit's low-pressure safety opened in cold weather or the system needs service.

O/B wire without a heat pump

An O/B wire is connected but the system is not configured or detected as a heat pump.

No power on the W1 (heat) wire

The W1 terminal does not see the expected voltage from the heating equipment.

No power on the Rh wire

Same fault as E74 on an earlier unit: no 24 V reaching the Rh terminal.

Overcurrent on W1

The thermostat's internal protection has tripped ten or more times on the W1 circuit, so it locks out heat calls.

Overcurrent on Y1

Repeated overcurrent trips on the cooling/compressor call wire.

Overcurrent on G (fan)

Repeated overcurrent trips on the fan wire.

Overcurrent on O/B

Repeated overcurrent trips on the reversing-valve wire.

Overcurrent on W2/AUX

Repeated overcurrent trips on the second-stage or auxiliary heat wire.

Overcurrent on Y2

Repeated overcurrent trips on the second-stage cooling wire.

Overcurrent on the star (*) terminal

Repeated overcurrent trips on the configurable star terminal.

No power on the Rc wire

No 24 V on the Rc (cooling power) terminal. Google lists it among the hot-weather power faults that often trace back to a tripped condensate safety switch or a frozen coil.

No power on the Rh wire

The thermostat sees no 24 V on the Rh terminal, so it cannot run heating or cooling. The cause is almost always upstream of the thermostat: wiring, breaker, board fuse or a safety switch opening the low-voltage circuit.

No power on the R wire

The R terminal has lost its 24 V supply. Google groups it with the hot-weather power faults.

C or Y1 required

The unit needs a C wire or a Y1 path to power itself.

No system power

The thermostat sees no power from the HVAC system on any wire. The E version cannot be dismissed until fixed.

No power on Y1

Non-critical notice that the Y1 wire is not supplying power.

C wire or Power Connector recommended

The thermostat is running on borrowed power and recommends a dedicated C wire or adapter. Non-critical, but often the root of intermittent problems.

Low power on the C wire

A C wire is present but supplies too little power.

Low power or low battery (Y-wire / cold-weather related)

The thermostat's internal battery is low or Y1 power was lost, often from the AC's cold-weather safety or a system needing maintenance.

Overvoltage or overcurrent on HVAC wires

The thermostat measured abnormal voltage or current on one of the equipment wires.

Wi-Fi drops repeatedly

The thermostat cannot stay connected, often because the router's power-saving behavior is not compatible.

Wi-Fi connection refused

Router security settings are blocking the thermostat.

No IP address from router

The thermostat joined the network but the router did not hand it an IP address.

Wi-Fi OK, no internet

The thermostat is on the local network but cannot reach the internet.

Cannot connect to Wi-Fi (possible hardware fault)

The thermostat cannot connect to Wi-Fi at all; if basic steps fail, the Wi-Fi hardware may be faulty and the unit may need replacement.

Heat pump needs service

The thermostat flagged a heat pump problem that needs a service visit.

Wi-Fi radio fault

The thermostat's built-in Wi-Fi radio has failed.

Internal memory error

The thermostat detected a fault in its internal memory.

Date and time not set

Clock is not set, so schedules cannot run.

Built-in temperature sensor fault

The thermostat's own temperature sensor has failed.

AC power restored

Informational: power came back after an outage. No action needed.

Indoor sensor error

A wired remote indoor temperature sensor is disconnected, miswired or failed.

Outdoor sensor error

A wired outdoor temperature sensor is disconnected, miswired or failed.

Air filter reminder

Filter replacement reminder based on the runtime or calendar setting.

No internet connection

One source describes it as no internet connection; another lists it as a Wi-Fi radio fault. Treat as a connectivity alert and confirm in the model's manual.

No Wi-Fi signal

The thermostat lost the Wi-Fi signal.

Wi-Fi not set up

The thermostat has not been connected to a Wi-Fi network yet.