Wire letter guide
Y, Y1 and Y2 Thermostat Wires: Cooling and Compressor Stages
Y / Y1 starts the outdoor compressor: cooling on an AC, or both heating and cooling on a heat pump. Y2 means a two-stage compressor, which runs on low most of the time and steps up in extreme heat. The Amazon Smart Thermostat has a single Y terminal, so it can’t run a second stage.
Y wire troubleshooting
- Indoor fan runs but the outdoor unit doesn’t start. Measure from Y to C at the furnace board with cooling called. About 24 V means the signal is leaving the furnace; check the outdoor unit’s breaker, the disconnect by the unit, and the thin wire pair that runs to it.
- AC runs and won’t turn off. Y may be touching R, or the outdoor contactor is stuck. Switch the thermostat off: if the outdoor unit keeps running, it is a contactor problem, not the thermostat.
- Short cycling. Most smart thermostats have minimum on and off times to protect the compressor. Leave compressor protection turned on.
Y1 and Y2: how to know you really have two stages
A wire on Y2 at the thermostat doesn’t always mean the outdoor unit is two-stage: sometimes a spare wire was simply landed there. Check the outdoor unit’s model label or manual for “two-stage” or “2-stage”, and follow the Y2 wire to the furnace board to see if it continues to the outdoor unit. If the outdoor unit is single-stage, connect only Y (Y1) and cap the Y2 wire.
Where the Y wire goes outside
The cable from the furnace to the outdoor unit usually has only two conductors in use: Y (the call) and C (common). They are spliced to the thermostat cable at the furnace board, often with different colors. If the outdoor unit never starts, that splice and the outdoor cable (which can be damaged by lawn equipment) are worth checking.
Y on a heat pump
On a heat pump, Y runs the compressor in both heating and cooling, and the O/B wire decides which. So a heat pump with a dead Y signal gives neither heat nor cooling from the outdoor unit, and the backup heat will try to carry the house.