Short answer
On many furnaces, 4 blinks means the high-limit switch opened because the furnace overheated, usually from a clogged filter or blocked vents. Confirm it on the chart inside your furnace door, replace the filter and open the vents. If the code comes back, stop resetting it and call a tech.
That red or green light blinking through the window in your furnace door is a code from the control board. Four blinks is one of the most common codes I see, and on a lot of furnaces it means the same thing: the furnace got too hot and the high-limit safety switch shut the burners off.
Every brand and board numbers its codes differently, though, so the chart on the inside of your furnace door is the final word. Check it first, then use the steps below.
Safety first
- Smell gas (rotten eggs)? Don't touch switches or the thermostat. Get everyone out, then call your gas company or 911 from outside.
- Carbon monoxide alarm going off? Get everyone outside into fresh air and call 911. Don't go back in to check the furnace.
- Turn the power off at the furnace or air-handler switch before you open any panel.
- Never bypass a safety switch (door switch, float switch, pressure switch, limit switch). They shut the system off for a reason.
What to check
Count the blinks before you touch the power
Watch the light through the window in the lower door and count the flashes between pauses (for example, 4 flashes, pause, repeat). Take a video. Turning the furnace off and on often clears the code, so read it first.
Match it to the chart on your door
The chart is usually printed on the inside of the blower door or the front panel. On many Goodman, Amana, Rheem and Ruud furnaces, 4 flashes is listed as an open high-limit switch, but some boards use 4 for something else, so trust your chart.
Carrier and Bryant furnaces use two-digit codes (for example 3 flashes, pause, 3 flashes for code 33). In Carrier's service manual, code 33 is a limit circuit fault: the limit or flame rollout switch is open.
Replace the air filter
An open high limit almost always means the furnace isn't moving enough air, and the most common reason is a clogged filter. If you can't see light through it, replace it.
After a new filter, the furnace often runs normally on the next cycle.
Open the vents and clear the returns
Closed supply vents, or furniture and rugs over the return grilles, choke the airflow the same way a dirty filter does. Open them all.
Make sure the blower runs during heat
When the burners are on, you should hear the blower and feel steady air at the vents. If the burners light but the blower never comes on, the furnace will overheat every time. That's a blower motor or capacitor problem and needs a tech.
Don't keep resetting it
If it trips again after you've fixed the filter and vents, stop resetting it. Repeated overheating stresses the heat exchanger, the part that keeps exhaust out of your air. A flame rollout switch trip in particular can point to a venting or heat exchanger problem.
When to call a pro
- The 4-blink code comes back with a new filter and open vents.
- The burners light but the blower doesn't start.
- Your chart says rollout switch, or the furnace has a manual-reset button that keeps tripping.
- You smell gas or a CO alarm goes off: get out and call 911 from outside.
Still not working? Same-day repair.
Frost Fire Heating & Cooling: $59 diagnostic (waived with repair), same-day service, no weekend surcharge, 4.8-star rated. Licensed & Insured. Serving Raleigh, Durham, the Triangle, Clayton and surrounding areas.
Common questions
What does 4 blinks mean on a Goodman furnace?
On many Goodman and Amana furnaces, 4 flashes is listed as an open high-limit switch, which usually means overheating from restricted airflow. Confirm it on the chart inside your furnace door, since boards vary.
What does code 33 mean on a Carrier furnace?
In Carrier's service manual, status code 33 is a limit circuit fault: the limit or flame rollout switch is open. If it stays open more than 3 minutes, the furnace locks out with code 13.
Is a high-limit trip dangerous?
The switch is doing its job by shutting the burners off. The problem is the overheating behind it. Fix the airflow, and if it keeps happening, have it checked before it damages the heat exchanger.
How much is a service call?
Frost Fire's diagnostic is $59, and it's waived if you go ahead with the repair. Same price on weekends.
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