The short answer
Four red flashes on a Trane furnace in Northern Virginia means the limit circuit is open. Something got too hot, and a safety switch shut the burners off. On a furnace with a number display, it’s E04, which Trane lists as an open limit, rollout switch or reverse airflow switch. Most of the time it’s airflow: a clogged filter, closed vents or a blower wheel packed with dust. If a rollout switch tripped, that’s a different problem, and it needs a tech before anyone resets it.
Two kinds of switch, two very different problems
The main limit sits near the heat exchanger. When too little air moves across it, the heat has nowhere to go, the limit opens, and the burners shut off while the blower keeps running to cool things down. Once it cools, it resets on its own and the furnace tries again. That’s why a 4 flash furnace often runs in short bursts and never warms the house. Furnace short cycling covers that pattern.
The rollout switch sits on the burner box. It opens when flame rolls out of the burners instead of going into the heat exchanger, which happens when the flue or the heat exchanger is blocked or cracked. Trane’s S9X guide says its rollout switches have a red reset button, and on two-stage models the burner box limit is a manual reset too. If one of those tripped, the furnace stays off until someone presses it.
Don’t keep pressing it. A rollout that trips once can be a gust down the flue. One that trips again means flame is going where it shouldn’t, and that’s a fire and carbon monoxide risk.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
The 4 flash codes by board
Swipe to see all columns →
Swipe to see all columns →
| Your furnace | Code | What it means |
|---|---|---|
| One red light | 4 flashes | Open limit circuit |
| Red and green lights | 4 flashes | Open limit or rollout |
| Number display, like the S9X | E04 | Open limit, rollout or reverse airflow switch |
| Communicating thermostat | Alert 26, or 87 | Open limit; 87 is rollout |
The causes, most common first
The filter
A filter that hasn’t been changed since spring can choke a furnace on its own. Pull it and hold it up to a light. If you can’t see light through it, that’s your first fix. Thick 4 or 5 inch filters last longer, not forever. The furnace filter MERV guide covers which one to buy.
Closed or blocked vents
Closing vents in empty rooms backs air up into the furnace. Open them all, and make sure the return grilles aren’t behind a couch or a bookcase.
A dirty blower wheel
The blower wheel is the squirrel cage fan that pushes air through the house. Dust packs between its blades a little each year until it moves a fraction of the air it should. A new filter doesn’t fix that. The wheel has to come out and be cleaned. With our heating check, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade, fit a new filter, set your dampers for winter and tune the furnace, through December 31.
A failing blower motor
If the blower runs slow, hums, or doesn’t start, not enough air crosses the heat exchanger. A blower motor runs $670 to $2,400; variable-speed motors sit at the top of that range.
A dirty AC coil on top of the furnace
The air conditioning coil sits on top of the furnace, and the heat blows through it all winter. Matted with dust, it chokes airflow the same way a dirty filter does.
A blocked drain, now and then
On a high-efficiency furnace, Trane’s guide says a badly blocked condensate drain can show E04 in extreme cases. Trane furnace 3 flashes covers the drain.
What you can do
- Change the filter.
- Open every supply vent and clear the return grilles.
- Reset once: the furnace switch off for a few seconds, then on.
- If the red rollout button popped, leave it and call.
If it smells hot or electrical when it runs, read furnace burning smell. If you smell gas, don’t touch the furnace or any switch. Leave the house and call the gas company’s emergency line from outside.
What it costs
A diagnostic visit is $99, and you get the problem and the price in writing before any work starts. Plenty of 4 flash calls end with a filter and open vents. A blower motor runs $670 to $2,400. A cracked heat exchanger is usually a replacement conversation, and a free second opinion before you sign is worth it. If it does come to that, the estimator shows prices with the $1,500 off already in them. The full list of Trane furnace error codes covers every other blink.
Book it
Book the heating check, call (703) 997-9757, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. 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 4 flashes mean on a Trane furnace?
The limit circuit is open. The furnace got too hot and a safety switch shut the burners off. Start with the filter and the vents. If a rollout switch tripped, call a tech.
What is E04 on a Trane furnace?
An open limit, rollout switch or reverse airflow switch. Most often it’s airflow: a dirty filter, closed vents or a dusty blower wheel.
Where is the rollout switch reset on a Trane furnace?
On the burner box behind the lower door: a small switch with a red button. If it tripped twice, don’t press it again. Flame is going where it shouldn’t, and a tech needs to find out why.
Can a dirty filter make a Trane furnace blink 4 times?
Yes. A clogged filter starves the furnace of air, the heat exchanger runs too hot, and the limit opens. Change it first.
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.
Films: Trane’s own.
