Google Nest · Thermostat error code

E5 / E25 / E75 / E155 / E255 / E405No heating or cooling 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

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Thermostat calls but heat or cooling does not start

Split the problem in two: is the thermostat sending 24 V on the call wire, and is the equipment responding to it? A jumper test at the board answers that in minutes.

5 steps ~40 minmoderate

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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

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Heat pump blows cold on heat (or warm on cool): O/B setting

The O/B terminal drives the reversing valve. Most brands energize it in cooling (O); Rheem and Ruud typically energize it in heating (B). A wrong O/B setting reverses the modes. Check the equipment diagram, not wire color.

4 steps ~20 mineasy

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

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No power on the Rh wire

Cross-checked High-voltage / caution

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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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