Next Phase Heating is an independent HVAC contractor. We are not an authorized dealer for, or affiliated with, any equipment manufacturer. Brand names are used only to identify the equipment we service. Codes vary by model and model year — the legend on your own furnace is always the last word.
First — how to get the code without losing it
The diagnostic light is on the control board inside the blower compartment, and there is a switch on that door. This makes reading a Trane code slightly awkward and it is worth knowing the trick.
Opening the door cuts the power
The blower door switch shuts off the gas valve and the blower when the door comes off. The live code goes with it. This catches people constantly.
The board replays the last four faults on power up
Every time it powers up it flashes the most recent four stored faults, newest first. That is the way to read a code you could not see live.
A slow green flash means nothing is wrong
Slow flashing is normal with no call for heat. Fast flashing is normal with a call for heat. Neither is a fault, and both get reported as one.
Resetting a lockout is a rhythm, not a switch
Power off, on, off, and on again within thirty seconds, either at the furnace switch or the thermostat. Once is reasonable. Twice means you are avoiding the question.
What the flashes mean
Stop, and call someone
Two of these are unambiguous gas faults. The third is the one this whole page exists for.
4 flashes
The temperature limit circuit is open — and the rollout switch is part of that circuit
This reads like a benign overheat code and most of the time that is exactly what it is: a filter nobody changed. But Trane wires the burner rollout device into the same circuit, and the manual is explicit that in the event of a flame rollout the device opens the circuit and shuts off all flow of gas. So four flashes means either a dirty filter or a flame coming out of the burner box, and the code cannot tell you which. Change the filter once. If it returns, treat it as the serious version.
5 flashes
A flame is being sensed when there should not be one
Switch the furnace off at the service switch. Any smell of gas and you leave first and call the gas company.
7 flashes
Gas valve circuit error
The control cannot drive the valve properly. Not something to keep resetting.
Worth ten minutes before you book
One code here is genuinely often yours to clear, and the other is worth a walk outside.
4 flashes, first time
Almost always a filter, a closed-off register, or a blocked return
Change the filter, open every supply vent, clear the return, and give it half an hour. This fixes it more often than anything else on the page. But read the entry above before you dismiss a second one.
3 flashes
Draft pressure error — venting, the pressure switch, or the inducer
Trane lists all three causes under this one code. The part you can check is the venting: on a high-efficiency furnace the white plastic intake and exhaust come out through a wall or the roof, and nests, snow and debris all cause exactly this.
Needs a technician, but it is not an emergency
Ordinary component faults and one code that genuinely varies between models.
2 flashes
System lockout — retries or recycles exceeded
This is a consequence, not a cause. It tells you the furnace gave up, not why. The reason is usually sitting in the stored fault history, which is what the power-up replay is for.
6 flashes
Reversed 115 volt power, poor grounding, or the supply voltage is too low
A wiring or supply issue rather than a furnace one. Turns up most often after someone else has worked on it.
8 flashes
Low flame sense signal
The classic dirty flame sensor. Small part, quick job, and one of the most common furnace calls in the trade.
9 flashes
Depends which furnace you have, and we are not going to pick one for you
American Standard’s installer guide gives this as an igniter circuit or grounding problem. The service facts for other models in the same family give it as an open inducer limit. Those are genuinely different faults and the manufacturer’s own documents disagree. Read the label on your unit, or let us look.
Light on continuously
Replace the integrated furnace control
A steady red light on these boards is the board itself asking to be replaced. This is the opposite of Carrier, where steady on is the healthy state.
Light off completely
No power reaching the board
Check the furnace switch, the breaker, and whether the blower door is properly shut.
Four ways a Trane code gets read wrong
- Four flashes is the code that gets waved away. It says limit circuit, which sounds like a filter, and usually is. But rollout lives in that circuit on these furnaces, and nothing in the code distinguishes the two. If four flashes comes back after a filter change, it has earned a proper look.
- The first code you see on power up may be months old. The board replays its four stored faults every time it powers up. Someone who kills the power, restores it, and reads the first thing that flashes is often reading history rather than what is wrong now.
- A slowly blinking green light is the furnace saying it is fine. Any blinking light reads as trouble to most people. On these boards the green status light blinks slowly when there is no call for heat and quickly when there is. Both are normal.
- Steady red does not mean the same thing here as on other brands. On a Trane or American Standard board, a light that stays on is asking for a new control. On a Carrier it means everything is fine. Charts written for the wrong brand cause real confusion here.
There is no error code for a cracked heat exchanger
Worth stating clearly on this page in particular, because four flashes gets misused for exactly this purpose. No Trane or American Standard board can detect a cracked heat exchanger. No brand’s board can. There is no such code. Repeated limit trips and rollout faults can be downstream signs of an exchanger that has failed, and they are a legitimate reason to get the furnace properly inspected — but the inspection is what finds it, using a camera and a combustion analyser, not the flashing light. A replacement quote written on the strength of a code alone is worth a second opinion before you spend anything.
Four flashes that came back?
That is the one worth booking rather than resetting. Tell us it is a four and we will come prepared for both versions.
Book a Furnace Diagnostic60 seconds online · or call (206) 486-6373
How it works when you book
- 1
Read the code before you reset anything The moment you cycle the power the live code is gone. Count it twice and write it down.
- 2
Book online and tell us what it showed Takes about a minute. Put the code in the notes — it changes what the technician loads into the van.
- 3
You get the price before we start Diagnosed, explained, and quoted in writing. Nothing happens until you say go.
Our service areas
We run out of Covington and cover the greater Puget Sound area.
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Not on the list? We cover the greater Puget Sound area around our Covington base. Book it and a technician confirms within 1 business hour.
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Quick answers
Is American Standard the same furnace as a Trane?
Same manufacturer, same hardware, and the service literature covers both in one document — the model numbers differ only by their first letter. Everything on this page applies to both equally.
How do I read the code if opening the door turns the furnace off?
Use the power-up replay. Shut the power off, put the door back on properly, then restore power and watch the light through the sight opening. The board flashes the last four stored faults, newest first.
I reset it and it is running fine now. Am I done?
For a one-off ignition failure on a cold morning, quite possibly. For a four-flash, no. The whole reason that code exists is that the furnace found a condition it was not willing to run in. Resetting past it repeatedly is exactly the habit these switches are designed to interrupt.
Is the code legend printed inside the door?
There is a wiring diagram inside the blower door on these furnaces. We have not been able to confirm that the flash code legend itself is on the door for every model, so check yours rather than assuming — and if it is not there, the installation literature for your model number has it.
Do you service Trane if you did not install it?
Yes. The Trane and American Standard warranty asks for repairs by a licensed service provider using proper parts. It does not require a particular dealer, so using us does not affect your warranty.
Tell us how many times it flashed.
Book online in about a minute. Put the count in the notes, and say whether it came back after a filter change.
Book a Furnace Diagnostic60 seconds online · or call (206) 486-6373
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