Google Nest · Thermostat error code

E2 / E22 / E72 / E402No power wire detected

Single source Power off, standard tools

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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.

Related diagnostic guides

Guide · Thermostat

Thermostat blank screen / no power

A dead display is usually a missing 24 V supply, not a bad thermostat. Work from batteries and breakers to the furnace door switch, condensate float switch, board fuse and transformer. A board fuse that blows again points to a shorted thermostat or outdoor wire.

6 steps ~45 minmoderate

Guide · Thermostat

Installing a C-wire adapter (Nest Power Connector, ecobee PEK and similar)

An adapter mounts at the furnace/air-handler board and lets the existing wires carry both the calls and constant 24 V. Check compatibility first: dual-transformer, fan-coil and communicating systems are commonly excluded.

5 steps ~45 minmoderate

Install · Thermostat

Thermostat replacement (low-voltage)

Most central HVAC thermostats are 24 V. Kill power to the HVAC system, photograph and label the existing wires, and confirm compatibility (C wire for most smart thermostats). Line-voltage (120/240 V) thermostats for baseboard heat are a different product and need a matching line-voltage model.

5 steps ~45 mineasy

More Google Nest thermostat codes

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C wire required

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E3 / E23

No power on the Rc wire

Single source High-voltage / caution

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No power on the Rh wire

Cross-checked High-voltage / caution

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E5 / E25 / E75 / E155 / E255 / E405

No heating or cooling wire detected

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Too many wires

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E7 / E27 / E77 / E407

Y1 missing on a heat pump

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