The short answer
A Trane furnace in Northern Virginia tells you what’s wrong with a light on its control board. Older models blink one red light a set number of times, two-stage models add a green light, and newer ones like the S9X show a code such as E2.1 or E04 on a small number display. Count the blinks or read the code, match it in the tables here, and you’ll know whether it’s a reset, a filter or a service call. The codes that stop the heat most often are the lockout (2 flashes), the pressure switch (3) and the limit (4), and each one has its own guide.
Read the code before you pull the door
The control board sits behind the lower door. On most Trane furnaces that door presses a safety switch, so pulling it cuts power to the board and the light goes out. If your door has a small window, count through it first. If it doesn’t, pull the door and count right away. Two-stage boards replay the last four faults, newest first, every time power comes back on, so a code you missed isn’t lost. The code chart for your exact board is usually glued inside the door, and that chart is the final word.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
One red light: older single-stage furnaces
From Trane’s installer guide for its single-stage 80% furnaces:
Swipe to see all columns →
Swipe to see all columns →
| Light | What it means | First thing to do |
|---|---|---|
| Slow flash | Normal, no call for heat | Nothing |
| Fast flash | Normal, calling for heat | Nothing |
| On steady | Board failure; Trane says replace the board | Call |
| Off | No power to the board | Breaker, furnace switch, door closed |
| 2 flashes | Lockout: too many tries to light or stay lit | One reset (guide) |
| 3 flashes | Draft pressure: vent, pressure switch or inducer | Pipes outside (guide) |
| 4 flashes | Limit circuit open, running too hot | The filter (guide) |
| 5 flashes | Flame sensed when there should be none | Switch it off and call |
| 6 flashes | Reversed polarity, poor ground or low voltage | Call |
| 7 flashes | Gas valve circuit | Call (guide) |
| 8 flashes | Weak flame signal | Usually the flame sensor |
| 9 flashes | Igniter relay fault | Call |
Red and green lights: two-stage furnaces
A two-stage Trane with a variable-speed blower has a green light and a red one, per Trane’s two-stage installer guide. Green blinking slowly is normal with no call for heat; green blinking fast is normal while it heats. Red flashing once every 20 seconds is normal too. Green and red both on steady means an open fuse or a failure inside the board. Red flash counts mean nearly what the single-light table says, with two changes: 4 flashes adds the rollout switch to the limit, and 9 flashes is the inducer limit, not the igniter.
A number display: the S9X and newer
Newer Trane furnaces like the S9X show a code on the board’s display, alternating with the mode it’s in. From Trane’s S9X installer guide:
Swipe to see all columns →
Swipe to see all columns →
| Code | What it means |
|---|---|
| E01 | Internal board failure |
| E2.1 | Failed to light 3 times; locks out for an hour (guide) |
| E2.2 | Lost its flame, or the pressure switch opened 10 times; locks out for an hour |
| E2.3 | Gas valve didn’t open when it should have, 10 times |
| E3.1 | First-stage pressure switch stuck closed (guide) |
| E3.2 | First-stage pressure switch didn’t close |
| E3.3 | Second-stage pressure switch stuck closed |
| E04 | Open limit, rollout switch or reverse airflow switch (guide) |
| E05 | Flame where there shouldn’t be one |
| E6.1, E6.2 | Reversed polarity; bad ground |
| E6.3 | Igniter relay failed or igniter open |
| E7.1 | Gas valve on when it should be off (guide) |
| E08 | Weak flame signal, still running |
| E09 | Inducer limit or condensate switch open, often a blocked drain |
| E11 | Gas valve circuit, or a relay stuck closed (guide) |
| E12 | Open fuse |
The Err menu shows the active code, and L6F shows the last six. Once it’s fixed, holding the Option button for 5 seconds clears the history.
Modulating and Link furnaces
Trane’s modulating furnaces send the same faults to a communicating thermostat as alert numbers, per the modulating furnace guide: a lockout reads as alert 20 or 22, the pressure switch as alert 1, an open limit as 26, rollout as 87, false flame as 34, weak flame as 4, and lost communication as 90 or 91. Link furnaces show them as ERR 022.01 and the like. The Trane error code 126 guide covers that alert, and Trane thermostat and heat pump error codes covers the rest, like 79 and 157.00. A blank thermostat screen is in Trane thermostat blank screen, a thermostat stuck on Wait in Trane thermostat wait, and a heat pump blowing cool air in Trane heat pump not heating.
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.
American Standard uses the same codes
American Standard is the same company as Trane, and the same generation of furnace uses the same boards and the same codes under either name. Read the chart inside the door to be sure. If you’re weighing one, is American Standard a good brand has our take, and American Standard furnace codes sorts the same codes by symptom.
What you can check yourself
- The filter. A clogged one is behind a lot of 4 flash and E04 codes (filter guide).
- The power switch beside the furnace, and its breaker.
- The handle on the gas pipe beside the furnace. It should run in line with the pipe.
- The white plastic pipes outside, clear of snow, ice and leaves.
- One reset: the furnace switch off for a few seconds, then back on.
If the same code comes back, stop. Resetting it over and over doesn’t fix anything, and with a rollout or a flame code it can be dangerous. 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 the fixes cost
A diagnostic visit is $99, and you get the problem and the price in writing before any work starts. Most heating repairs land between $95 to $1,500: cleaning a flame sensor runs $95 to $180, an igniter $240 to $410, a control board $550 to $1,180. If someone quoted you a new furnace off one code, a free second opinion costs you nothing. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. More on Trane service is on our Trane repair page.
Other brands
Same idea, different charts: Carrier furnace codes, Goodman furnace codes and a Lennox flashing red light. No code at all and no heat? Start with furnace won’t turn on.
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
Why is my Trane furnace blinking red?
On a two-stage board, one red flash every 20 seconds is normal. A set count of red flashes and then a pause is a fault code: 2 is a lockout, 3 is the pressure switch or venting, 4 is an open limit. Count the flashes and match them to the chart inside the furnace door.
How do I reset a Trane furnace?
Turn the power switch beside the furnace off for a few seconds and back on, or turn the thermostat off and back to heat. That clears a lockout. If the same code comes back, the reset isn’t the fix and the furnace needs a tech.
Where is the error code on a Trane furnace?
On the control board behind the lower door. Older models blink a red light, two-stage models have a red and a green light, and newer ones like the S9X show a code on a small number display.
Does American Standard use the same error codes as Trane?
Yes. It’s the same company, and the same generation of furnace uses the same boards and codes. The chart inside the door is the final word for your model.
What does E04 mean on a Trane furnace?
An open limit, rollout or reverse airflow switch. The furnace ran too hot, or flame came out of the burner box. Start with the filter. If a rollout switch tripped, call a tech before resetting it.
