Google Nest · Thermostat error code

E24No power on the Rh wire

Cross-checked High-voltage / caution

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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

Thermostat shows WAIT or flashing 'Cool On'

Most thermostats hold the compressor off for a few minutes after it stops or after power returns, to prevent short cycling. That delay is normal. If the message lasts well past the delay or the system never starts, check mode, breakers, filter-related lockouts and wiring.

5 steps ~20 mineasy

Guide · Thermostat

System short cycles (turns on and off quickly)

Short cycling can be a thermostat placement or power-stealing problem, but often it is the equipment tripping on a safety (high limit, frozen coil, float switch). Watch one full cycle and see who ends it: the thermostat or the equipment.

5 steps ~60 minmoderate

More Google Nest thermostat codes

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C wire required

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E2 / E22 / E72 / E402

No power wire detected

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E3 / E23

No power on the Rc wire

Single source High-voltage / caution

Google Nest · Thermostat

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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