Rheem furnace 3 blinks: Limit switch open
Three blinks means a limit switch 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: Rheem standard gas furnaces. Rheem states these codes do not apply to modulating, LXE, or two-stage models. Ruud and Weather King furnaces of the same generation share the platform.
What it means
The limit switch opens when the air passing over the heat exchanger is not carrying heat away fast enough. The control 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 switch is usually working correctly and reporting something true, which is why replacing it is almost never the answer and why the fault is worth resolving rather than resetting.
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 switch, which is possible but much 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 and unobstructed.
- 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, and that is a carbon monoxide risk rather than a comfort issue.
Notes
Worth taking seriously despite its usually trivial cause. The switch exists because sustained overheating cracks heat exchangers, which is the most dangerous fault a gas furnace can develop.
Related
All codes for this manufacturer
Rheem, Ruud and Weather King furnace blink codes — every code, with what differs between models.
Not sure this is your brand?
The same count means something else on other furnaces. What 3 flashes means on every brand puts them side by side.