The short answer
Trane error codes on a touch-screen thermostat, like 126.00 or 79.00, are alerts from the whole system: the thermostat, the indoor unit and the heat pump or air conditioner outside. In Northern Virginia winters the common ones are 126.00 (part of the system stopped communicating), 79 (the outdoor unit’s low pressure switch tripped), 157.00 (the condensate float switch shut things down) and communication codes in the 90s. A red alert means you’ve lost heating or cooling; yellow means it’s running but holding back. Write the code down, check the breakers and the drain, power cycle once, and call if it comes back.
TruComfort, RunTru, or TruRun?
If you’ve heard “TruRun,” you most likely mean one of two real names. TruComfort is Trane’s variable-speed system on its XV heat pumps and air conditioners (XV20i, XV19, XV18, XV17): the outdoor unit ramps up and down and reports detailed alerts to the thermostat. RunTru by Trane is a separate value line with its own model numbers. The label on the outdoor unit tells you which you have.
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How to find the alert on a Trane thermostat
On an XL824, XL850 or XL1050, open the Service Menu and tap Diagnostics. Current Alerts shows what’s active; Alert History keeps the last 30 days. Tap any code for its description and likely causes (Trane XL824 installation guide). On a Link UX360, go to Menu, then System Info, then Alerts (Link UX360 installer guide).
The color tells you how serious it is. Red, critical, means heating or cooling stopped. Yellow, major, means it’s running with reduced function. Minor alerts only show in Diagnostics. The Trane Home app only pushes critical alerts, so a yellow one can sit unnoticed for days.
Most alerts clear on their own. If one sticks, turn the system’s breaker off for 20 to 30 seconds and back on. Skip “restore factory defaults”: it erases the installer’s setup.
Common Trane alert codes
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Swipe to see all columns →
| Code | What it means |
|---|---|
| 126.00 | A piece of equipment stopped communicating |
| 89.00 / 89.01 | Equipment the thermostat expects is missing |
| 91.00 / 91.01 | The blower or inducer stopped communicating |
| 91.02 | System communication error on the data wire |
| 91.03 | No system clock |
| 91.05 / 91.06 | Data wiring miswired or shorted, common right after an install |
| 90.x | Communication dropping in and out |
| 79.xx | Low pressure cutout on the outdoor unit; locks out on the third trip |
| 80.xx | High pressure cutout on the outdoor unit |
| 157.00 | Condensate float switch tripped: water backing up |
| 158 | Indoor coil frosting |
| 153 | Electric backup heat limit opened (the heat strips overheated) |
| 154 | Low airflow across the electric backup heat |
| 184.07 / 184.08 | Variable-speed protection lockout |
| 18 | Control fault on a piece of equipment |
| 92 | 24 volt fuse blown |
| 125 | Low 24 volt power |
What the common ones mean and what to do
126.00: equipment stopped communicating
What you notice: A red alert and no heat or cooling. The thermostat can’t reach the indoor unit, the outdoor unit or both.
What causes it: On a communicating Trane system, two of the thermostat wires carry data, not just power. A loose wire, a tripped breaker on one piece of equipment, a power surge or a failed board breaks the conversation. Right after a new install, a wiring mistake is the usual cause, and it often shows with 91.05 or 91.06.
What you can do: Check both breakers, indoor and outdoor, and the disconnect box by the outdoor unit. Then power cycle once.
When to call us: If it comes back. Tracing it takes a meter and the wiring diagram. The 126.00 walk-through goes step by step.
79: low pressure cutout
What you notice: The heat pump stops, the thermostat shows a 79 alert, and the backup heat may be carrying the house.
What causes it: A switch on the outdoor unit’s refrigerant line opened because pressure dropped too low. In winter that can be ice on the coil, a defrost that didn’t finish, a dirty coil or low refrigerant from a leak. On the third trip the outdoor unit holds a hard lockout until the power is cycled.
What you can do: Look at the outdoor unit. If it’s packed in ice, see heat pump covered in ice. Clear snow and leaves from around it, then power cycle once.
When to call us: If it trips again. Repeated low pressure trips most often mean a refrigerant leak, and running it that way is hard on the compressor.
80: high pressure cutout
What you notice: The outdoor unit stops, the thermostat may show WAIT, and then it tries again.
What causes it: Pressure got too high: in summer usually a dirty outdoor coil or a dead outdoor fan, in heating mode low indoor airflow from a clogged filter. A short lockout waits about five minutes; a hard lockout holds until the power is cycled.
What you can do: Change the filter, make sure the outdoor fan spins when the unit runs, and in cooling season rinse the outdoor coil gently with a garden hose.
When to call us: If it keeps tripping, or the outdoor fan isn’t turning.
157.00: condensate float switch
What you notice: The system shuts down and the thermostat shows 157.00.
What causes it: A float switch in the drain pan or drain line felt water backing up and stopped the system before it overflowed. Furnaces and air handlers drain all winter, not just in summer.
What you can do: Look for water in the pan under the indoor unit, or a condensate pump with a full tank. A clogged line can sometimes be cleared at its cleanout.
When to call us: If the pan keeps filling, or the pump has stopped running.
91 and 90 codes: communication
What you notice: Alerts that come and go, sometimes with the system short cycling.
What causes it: The data line between the thermostat and the equipment is dropping: 91.05 and 91.06 usually mean a miswire or short, 90 codes an intermittent signal.
What you can do: Check that the thermostat is seated firmly on its wall plate, and power cycle once.
When to call us: If they keep coming back. It’s usually a loose connection at one end of the wire.
158: indoor coil frost
What you notice: Cooling stops, sometimes with ice on the copper lines by the indoor unit.
What causes it: The indoor coil got cold enough to freeze, usually from low airflow (a dirty filter, closed vents) or low refrigerant.
What you can do: Change the filter, then run the fan only until the ice melts.
When to call us: If it frosts again with a clean filter.
Lights on a TruComfort outdoor unit
Variable-speed TruComfort outdoor units (model numbers starting 4TWV0 or 4TTV0) have a green and an amber light on the board inside the service panel. That’s high voltage, so it’s a tech’s job, and the thermostat shows the same problem as an alert anyway:
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Swipe to see all columns →
| Light | What it means |
|---|---|
| Green, one blink a second | Idle, no call |
| Green, two blinks a second | Running |
| Green, five blinks a second | Power-up delay right after the power comes on |
| Green, solid | Test mode |
| Green, one blink every four seconds | Hard lockout: stopped until someone cycles the power |
| Amber, fast blink | Lost communication with the thermostat |
The outdoor board also sends its own codes to the thermostat. Ones worth knowing: 156 is low refrigerant charge, 175 means it’s running in a reduced limp mode to protect itself, 67 and 68 are defrost sensor and defrost faults, 176 through 182 are compressor drive faults, and 88 is a ground fault. On an 88, turn the unit off at its breaker and call right away.
Two-stage and single-stage Trane heat pumps
Two-stage Trane outdoor units with 4TWZ0 on the label blink a red fault light by count: 1 is communication, 7 is low pressure (alert 79) and 17 is high pressure (alert 80). Single-stage 4TWR heat pumps blink a light on the defrost board: 1 or 2 is a failed sensor, 3 means the low pressure switch is open, 4 is a hard lockout, 5 a soft lockout that retries on its own, 6 defrosting too often. Cycling the power clears a lockout once; if it locks out again, there’s a pressure or refrigerant problem to find.
Furnace codes, American Standard and winter
If the code is on a Trane gas furnace, a display through the door or a blinking red light, the Trane furnace error code guide has every one. American Standard thermostats and heat pumps use these same alerts, and the American Standard error code guide walks through them by symptom. Heat pump struggling in the cold? Read how heat pumps handle a Virginia winter and aux heat versus emergency heat. Prices for common Trane repairs are on our Trane repair page, and the heat pump repair page covers every brand.
Book a Trane repair
Call (703) 997-9757 or book online. A diagnostic visit is $99, and the tech reads the code, finds the part that set it and gives you the price before any work starts. We answer day or night. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. If someone already quoted you a new system, our free second opinion looks at it first. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What does 126.00 mean on a Trane thermostat?
A piece of equipment stopped communicating with the thermostat. Check the indoor and outdoor breakers and the outdoor disconnect, then power cycle once. If it returns, a wire or board needs a technician.
What does error 79 mean on a Trane heat pump?
The low pressure switch on the outdoor unit opened. In winter that can be ice, a defrost problem, a dirty coil or low refrigerant. After the third trip it locks out until the power is cycled, and repeated trips usually mean a leak.
How do I see error codes on a Trane XL824?
Open the Service Menu and tap Diagnostics. Current Alerts shows what’s active, and Alert History keeps the last 30 days. Tap a code to see its description and likely causes.
Is there a Trane TruRun?
Not by that name. Most people mean TruComfort, Trane’s variable-speed system on its XV models, or RunTru by Trane, a separate value line. The label on the outdoor unit tells you which one you have.
How do I reset a Trane heat pump?
Set the thermostat to off, turn off the outdoor unit’s breaker or disconnect for about five minutes, turn it back on and set the thermostat back. Do it once. A unit that locks out again needs a technician to find the cause.
Do Trane thermostat alerts clear by themselves?
Most do, once the problem goes away, and they move to Alert History for 30 days. If the same code keeps coming back, leave that history for the technician; it shows the pattern.
The system we put in first
Built to take a beating. Put in by our crew.
Heat lamps, snow and rain on one outdoor unit: that’s Trane’s own film. We’re a Trane Comfort Specialist, on Trane’s own dealer locator, and when you pick a Trane, our crew puts it in.
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