Pull the lower door off a Trane furnace and look through the small sight window on the control board. The status light repeats a pattern: a set number of quick blinks, a longer pause, then the same count again. Count the blinks in one group, not the total over several seconds — miscounting by one is the single most common reason a homeowner ends up reading the wrong row of any code chart.
Trane and American Standard furnaces are the same equipment wearing two badges, so an American Standard Freedom or Silver series furnace reads exactly the same way. Variable-speed models paired with a ComfortLink II thermostat add a second path: the wall control shows the fault in words, and on some furnaces a two-character display on the board itself replaces the blink counting entirely. When the thermostat gives you plain English, trust that description over any count you thought you saw.
One caution before the table. Trane has built furnaces on several control platforms, and the blink-to-fault legend is not identical across all of them. The table below matches the legend used on the current integrated controls, but the diagnostic label glued inside your own blower compartment is the authority: if that label disagrees with this page, follow the label. A rollout switch is also worth knowing about separately — it is a manual-reset button, and if it has tripped, something pushed flame or hot gas out of the burner box, which is never a reset-and-carry-on situation. If you would rather have someone read it out with you, our Trane repair page covers what we stock, and ZK Mechanical answers the phone at any hour.
Trane furnace codes, decoded
| Code | What it means | Safe? | Try first | When to call |
|---|---|---|---|---|
Steady on | Normal. The control has power and no stored fault. | keep running | Nothing needed. If the house is still cold, the fault is elsewhere. | Only if the furnace is not producing heat despite a healthy light. |
LED off | No power reaching the control, a blown board fuse, or a failed board. | turn it off | Check the furnace switch, the breaker and that the lower door is seated. | If the switch and breaker are on and the light stays dark. |
2 flashes | System lockout. The furnace tried to light the required number of times and gave up. | turn it off | One reset at the furnace switch is fair. A dirty flame sensor is the usual cause. | If it locks out a second time. Repeated trials dump unburned gas into the box. |
3 flashes | Pressure switch circuit wrong for the current state — open when it should be closed, or closed at rest. | turn it off | Look outside for a blocked intake or exhaust and clear snow or ice from the terminals. | If the vents are clear. A stuck switch can also mean a failing inducer or a plugged drain. |
4 flashes | A limit switch is open. The furnace got hotter than the limit allows and shut the burner down. | turn it off | Replace a dirty filter and open every supply register you can find. | If the code returns with a clean filter. Overheating is an airflow problem until proven otherwise. |
5 flashes | Flame detected when the gas valve should be closed. | call now | Nothing. Shut the furnace off at the switch. | Immediately. This means gas may be burning when the control did not ask for it. |
6 flashes | Line voltage polarity reversed, or the furnace is not properly grounded. | turn it off | Nothing safe. This is wiring at the receptacle or the panel, not the furnace. | Yes. Flame sensing needs a correct ground to work at all. |
7 flashes | Fault in the gas valve circuit — the control cannot drive the valve as expected. | call now | Leave it off and do not attempt to jump anything at the valve. | Immediately. Gas valve wiring is licensed work under any circumstance. |
8 flashes | Flame signal present but weak. The furnace is running near the threshold where it will start dropping out. | keep running | Book a tune-up. A flame sensor coated in oxide reads low long before it fails outright. | Soon, on your schedule — this is the fault that becomes a no-heat call in January. |
Rapid continuous flash | Not a countable fault. Depending on the control platform this is either a power-up self-check or a flag that line polarity is wrong, and only the legend on your own unit separates the two. | turn it off | Do not try to read it as a flash count. Check the legend inside the blower compartment. | If the flashing never settles into a countable pattern, or the furnace will not start a heat cycle. |
Rollout switch tripped | Not a flash code at all — a manual-reset button on the burner box that has opened because flame or hot gas came forward out of the burner compartment. | call now | Leave the button alone and shut the furnace off at the service switch. | Immediately, and keep the furnace off until the cause has been found. |
The codes that usually mean a real repair
Two flashes that keep coming back
An ignition lockout that returns after one reset is almost always the flame sensor, the igniter or the gas pressure — in that order of likelihood. Cleaning or replacing a sensor sits at the low end of the $150–$450 band we quote common fixes in, and the truck carries them for every Trane platform we service. What matters is not resetting past it: each attempt purges raw gas into the heat exchanger before the trial, and repeated cycles are how a nuisance fault turns into a rollout.
Four flashes on a cold morning
An open limit in the coldest week of the year is rarely the limit switch itself. It is the furnace running hotter than the airflow can carry away — a filter that has not been changed since the fall, closed registers in unused bedrooms, or a return that was never sized for a finished basement. We measure temperature rise and static pressure before quoting any part, because replacing a limit that did its job correctly fixes nothing.
Eight flashes before the failure
Weak flame signal is the most useful code Trane reports, because it appears while the furnace is still heating your house. It buys you a scheduled visit instead of an emergency one. A furnace tune-up cleans the sensor, checks combustion and confirms the reading has recovered, and it runs a fraction of what a February no-heat call costs.
Licensed technicians, trucks stocked with Trane parts, no overtime charges.
Common questions
Can I just reset my Trane furnace and keep going?
One reset is reasonable for an ignition lockout. A second identical lockout means stop. Cycling power clears the code from the display but does nothing to the fault that set it, and on flame or venting faults each retry is another purge of gas into the heat exchanger. See how to reset a furnace safely for the sequence we recommend.
Why does my ComfortLink thermostat show a different code than the board?
They are describing the same fault in two languages. The board counts blinks; the thermostat translates the fault into a sentence and often adds a severity level. Neither is wrong. When you call us, read out the thermostat wording if you have it — it removes the risk of a miscounted blink.
Does a Trane code chart work on an American Standard furnace?
Yes. The two brands are the same equipment with different badges and the diagnostic legends match. What does not transfer is a chart from another manufacturer — a four-flash on a Trane and a four-flash on a Goodman are not the same fault, which is why the label inside your own furnace is the one that counts.
The light is off entirely. Is the board dead?
Not necessarily. A dark LED usually means no power reaching the control: the furnace service switch, the breaker, a blown low-voltage fuse on the board, or a door switch that is not being pressed because the lower panel is not fully seated. Check the door first — it is free and it is a surprisingly common answer.
How much does a Trane control board cost to replace?
Board replacement falls in the $500–$1,200 range for most residential furnaces, parts and labour, and it is worth being sceptical when a board is diagnosed on a first visit. Boards fail, but they are also the default answer when nobody measured the flame signal or the pressure switch circuit. We publish what a control board costs and why we test before quoting one.