Trane furnace 4 flashes: Open high limit device
Four flashes means a high limit device has opened because the furnace overheated. The cause is almost always restricted airflow, and the most common single reason is a dirty filter.
Applies to: Trane and American Standard upflow, downflow and horizontal gas-fired direct vent condensing furnaces covered by Installer's Guide 18-CD29D1-14-EN, using the integrated furnace control described there.
What it means
The high limit opens when air passing over the heat exchanger is not carrying heat away quickly enough. The control then shuts off the gas and runs the blower to cool things down. This is protective rather than incidental: a heat exchanger that repeatedly overheats will eventually crack, and a cracked heat exchanger can leak combustion products, including carbon monoxide, into the air circulating through the house. The limit device is usually working correctly and reporting something true, which is why replacing it is almost never the fix.
Most common causes
- A clogged air filter — by a wide margin the most common cause.
- Closed, blocked, or covered supply registers, or too many closed at once.
- A blocked return air grille.
- Undersized or restrictive ductwork, or a duct that has come apart or collapsed.
- A failing blower motor, or a blower wheel packed with dust.
- Blower speed set too low for the furnace's output.
- Overfiring — manifold pressure set too high for the airflow available.
- A genuinely failed limit device, which is possible but far less likely than any airflow cause.
How to diagnose it
- Check the air filter first. Grey, matted, or opaque to light means you have very likely found it.
- Walk the house and count closed or blocked supply registers.
- Check the return air grille is clear.
- Note whether the blower runs at all. If the furnace fires and the blower never starts, overheating is certain.
- Note whether it only happens on long runs — heating normally at first and tripping after twenty minutes is the marginal-airflow signature.
How to fix it
- Replace the air filter with the correct size and type and check it monthly during the heating season. A filter far more restrictive than the system was designed for causes this as reliably as a dirty one.
- Open all supply registers and clear the return grille.
- Blower motor work, blower wheel cleaning, duct repair, speed changes, and manifold pressure adjustment are technician tasks.
- If the code returns after a filter change, stop resetting it and have the furnace inspected. Repeated overheating is what damages heat exchangers.
Notes
Worth taking seriously despite its usually trivial cause. The limit exists because sustained overheating cracks heat exchangers, which is the most dangerous fault a gas furnace can develop.