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Google Nest thermostat code T01, T02, T03 and the other T codes: meaning and exact next steps

T01, T02, T03 and the other T codes means A system-test problem: the test after installation found that one part of the system did not respond. Google's list: T01 single-stage fan, T02 single-stage cooling, T03 two-stage cooling, T04 forced-air system, T05 radiant heating, T06 radiator heating, T07 baseboard electric heating, T08 humidifier, T09 dehumidifier, T11 two-stage fan, T12 three-stage fan. This answer applies only to Google Nest Learning Thermostats (1st through 4th generation), the Nest Thermostat E and the Nest Thermostat that show a lettered help code on the display or in the Nest or Google Home app. Google prints one meaning for the aliases it groups together, and this page keeps that grouping. Codes shown by other brands of thermostat are on their own pages.

On the covered Google Nest thermostat, T01, T02, T03 and the other T codes means a system-test problem: the test after installation found that one part of the system did not respond. Google's list: T01 single-stage fan, T02 single-stage cooling, T03 two-stage cooling, T04 forced-air system, T05 radiant heating, T06 radiator heating, T07 baseboard electric heating, T08 humidifier, T09 dehumidifier, T11 two-stage fan, T12 three-stage fan. First, run the test longer: it runs 35 minutes and many systems have a delay of 5 minutes or more before they start, and radiant, baseboard or radiator systems can take up to 30 minutes to warm.

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  1. What to do first

    Reversible

    Run the test longer: it runs 35 minutes and many systems have a delay of 5 minutes or more before they start, and radiant, baseboard or radiator systems can take up to 30 minutes to warm.

  2. Correct only the documented fault domain

    Reversible

    Google's steps: make sure the system and any separate fan switches are on, check the thermostat wiring against the app's diagram, then use the system-specific notes: for T06 check the radiator valves are fully open; for T07 most baseboard heat is 120 volt and incompatible with Nest thermostats, so reinstall the old thermostat; for a heat pump check the O/B setting; for T08 and T09 Google strongly recommends professional installation because humidifiers and dehumidifiers often need a relay.

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What does T01, T02, T03 and the other T codes mean on a Google Nest thermostat?+

A system-test problem: the test after installation found that one part of the system did not respond. Google's list: T01 single-stage fan, T02 single-stage cooling, T03 two-stage cooling, T04 forced-air system, T05 radiant heating, T06 radiator heating, T07 baseboard electric heating, T08 humidifier, T09 dehumidifier, T11 two-stage fan, T12 three-stage fan.

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