What causes Trane HVAC System 106.00?

A clogged condensate drain line backing water into the drain pan, which lifts the condensate overflow float switch (the most common external switch on these systems). A full or overflowing secondary or auxiliary drain pan with a float switch in it. A smoke detector or other safety device wired to the external switch terminals has tripped. A loose or broken wire between the external switch and the SWa/SWb (external switch) terminals, which looks the same as an open switch. A failed float switch that stays open even when the pan is dry. The external switch input is enabled in the configuration but nothing (or the wrong type of contact) is connected to it; Trane says these inputs need normally closed contacts.

How do I fix Trane HVAC System 106.00?

Note whether the thermostat shows 106.00 (indoor unit) or 106.01 (outdoor unit). That tells you where the switch is. Turn the system off at the thermostat, then turn off power to the furnace or air handler before you open any panels. Find the float switch. It's usually threaded into the condensate drain line near the coil, or sitting in the secondary pan under an attic unit. Look for standing water in the pan or in the switch body. If there's water, clear the drain. Use a wet/dry vacuum on the outside end of the drain line, or flush the line from the access tee near the unit, until water runs freely. Empty and dry the overflow pan. Check that the float has dropped back down and the switch has reset, then restore power. The alert clears when the switch contacts close again. If the pan is dry, look at the two wires from the switch for a loose or broken connection. If the system has a smoke detector wired to the external switch terminals, check whether it has tripped. If 106 started right after new equipment or a new accessory was installed, the installer should confirm the external switch configuration in the thermostat or Diagnostics app, since Trane's alert guide lists that check first.

When should I call a pro for Trane HVAC System 106.00?

The drain keeps clogging, or water is leaking from the unit or through a ceiling. 106.00 stays on with a dry pan and a closed switch, which points to wiring, a bad float switch or the switch configuration. 106.01 points to a switch at the outdoor unit and you can't find what's connected there. A smoke detector wired into the system has tripped and you don't know why. The system was just installed or reconfigured and the external switch settings need checking.

Code: 106.00❄️ Trane HVAC System

Trane / American Standard Error 106.00: External Switch Has Been Activated (Meaning, Causes, Fixes)

Q: What does Trane HVAC System 106.00 mean?

Err 106.00 on a Trane or American Standard communicating system means an external safety switch wired to the indoor unit has opened. On most homes that switch is a condensate overflow float switch, so the usual cause is a clogged condensate drain backing water into the pan. Check the float switch and drain pan at the furnace or air handler, clear the drain, and the alert should clear once the switch closes again. 106.01 is the same alert from a switch at the outdoor unit. Trane's System Alert Guide lists ERR.106.00 as 'Indoor External Switch Has Been Activated': the External Switch configuration has been enabled and the external switch contacts are open at the indoor unit. ERR.106.01 is the matching 'Outdoor External Switch Has Been Activated' alert for a switch wired to the outdoor unit. Trane's air handler installation manuals explain what those switches are: the external switch terminals supply 24 VAC and are meant to connect to normally closed contacts on an outside safety device, typically a condensate overflow switch or a smoke detector, and the switch is configured as one of those in the thermostat or the Trane/American Standard Diagnostics app. When the device trips, its contacts open and the system reports 106. Because a float switch only opens when water rises in the pan or drain, a 106.00 most often means the condensate drain is blocked. Expect cooling to stop until the switch closes again. The alert doesn't mean the furnace or air handler itself has failed.

Reviewed 2026-10-25 · Based on 2 manufacturer & service sources

What the code means

Trane's System Alert Guide lists ERR.106.00 as 'Indoor External Switch Has Been Activated': the External Switch configuration has been enabled and the external switch contacts are open at the indoor unit. ERR.106.01 is the matching 'Outdoor External Switch Has Been Activated' alert for a switch wired to the outdoor unit. Trane's air handler installation manuals explain what those switches are: the external switch terminals supply 24 VAC and are meant to connect to normally closed contacts on an outside safety device, typically a condensate overflow switch or a smoke detector, and the switch is configured as one of those in the thermostat or the Trane/American Standard Diagnostics app. When the device trips, its contacts open and the system reports 106. Because a float switch only opens when water rises in the pan or drain, a 106.00 most often means the condensate drain is blocked. Expect cooling to stop until the switch closes again. The alert doesn't mean the furnace or air handler itself has failed.

Likely causes

A clogged condensate drain line backing water into the drain pan, which lifts the condensate overflow float switch (the most common external switch on these systems)
A full or overflowing secondary or auxiliary drain pan with a float switch in it
A smoke detector or other safety device wired to the external switch terminals has tripped
A loose or broken wire between the external switch and the SWa/SWb (external switch) terminals, which looks the same as an open switch
A failed float switch that stays open even when the pan is dry
The external switch input is enabled in the configuration but nothing (or the wrong type of contact) is connected to it; Trane says these inputs need normally closed contacts

Safe DIY checks

⚠ Before you start

Turn off power at the furnace/HVAC service switch and the breaker before opening panels.

Note whether the thermostat shows 106.00 (indoor unit) or 106.01 (outdoor unit). That tells you where the switch is.
Turn the system off at the thermostat, then turn off power to the furnace or air handler before you open any panels.
Find the float switch. It's usually threaded into the condensate drain line near the coil, or sitting in the secondary pan under an attic unit. Look for standing water in the pan or in the switch body.
If there's water, clear the drain. Use a wet/dry vacuum on the outside end of the drain line, or flush the line from the access tee near the unit, until water runs freely. Empty and dry the overflow pan.
Check that the float has dropped back down and the switch has reset, then restore power. The alert clears when the switch contacts close again.
If the pan is dry, look at the two wires from the switch for a loose or broken connection. If the system has a smoke detector wired to the external switch terminals, check whether it has tripped.
If 106 started right after new equipment or a new accessory was installed, the installer should confirm the external switch configuration in the thermostat or Diagnostics app, since Trane's alert guide lists that check first.

🛠 When to call a pro

The drain keeps clogging, or water is leaking from the unit or through a ceiling.
106.00 stays on with a dry pan and a closed switch, which points to wiring, a bad float switch or the switch configuration.
106.01 points to a switch at the outdoor unit and you can't find what's connected there.
A smoke detector wired into the system has tripped and you don't know why.
The system was just installed or reconfigured and the external switch settings need checking.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Wet/dry vacuum

For pulling a clog out of the condensate drain line from the outside end.

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⚙

Condensate overflow float switch

Only if the switch stays open with a dry pan. It must have normally closed contacts to work with Trane's external switch input.

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⚙

Digital multimeter

For checking that the float switch closes and that 24 VAC is present at the external switch terminals.

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Sources

Last reviewed 2026-10-25. Always confirm against the manual for your exact model number.

Trane / American Standard Error 106.00: External Switch Has Been Activated (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Wet/dry vacuum

Condensate overflow float switch

Digital multimeter

Related codes

Sources