L, S and T Thermostat Terminals: Can You Skip Them?

L (or L/A) lights a "check heat pump" indicator on some older thermostats. S/S1/S2 connect an outdoor or remote sensor. T shows up on some older thermostats for an outdoor sensor or anticipator. None of these normally turn equipment on, and Google lists S and L as unnecessary for Nest.

Cap each unused wire individually (electrical tape or a small wire nut) so bare ends can’t touch. The Honeywell T9 can use L/A if you want the service indicator.

Where it connects on each smart thermostat

Heat pump with L

Wires: R, C, Y, G, O, AUX, L · open in the tool

Thermostat	L goes to	Whole setup
Nest Learning (4th gen)	Cap it	Works: direct swap
ecobee Premium	Cap it	Works: direct swap
ecobee Enhanced	Cap it	Works: direct swap
Honeywell T9	L/A	Works: direct swap
Amazon Smart Thermostat	Cap it	Works: direct swap

L, S and T Thermostat Terminals: Can You Skip Them?

Where it connects on each smart thermostat

Heat pump with L

Other wire letters