Furnace limit switch tripped

A limit trip means a temperature safety switch opened and stopped the burner. Low airflow is one possible reason, but on some controls the same code also covers rollout or blocked-vent switches. Check the filter and grilles with the furnace off, keep heat off if rollout is possible, and call a technician.

A limit switch shuts off the burner when the furnace gets too hot, and the blower can keep running to cool it. The switch may be working correctly. What a limit code covers depends on the control: on Heil G80CTL Series A, code 4 includes the main limit, draft safeguard, rollout and blocked-vent switches, and code 7 is the lockout that follows.

This page covers limit-related codes and overheat shutdowns. It offers no reset steps. Rollout and blocked-vent trips are treated as combustion or venting concerns for a technician.

Safe checks with the furnace off

Record the code and label
Write down the full model, serial, LED color and pattern through the viewing window, and the label's own wording. Calling it a limit code does not say which switch opened. Do not open the furnace or press any reset.
Filter with the furnace off
With the furnace off, check an easily reached external filter. Confirm it is the size the equipment specifies and replace it if dirty. Put covers back and never run without a filter. A new filter does not prove the trip is resolved.
Supply and return grilles
Look for closed supply registers or return grilles blocked by furniture, rugs or curtains, and clear anything that can be moved. Leave internal dampers, ducts and blower settings for the technician.
What happened before the trip
Note whether warm air turned cool while the blower kept running, how often it repeats, and any recent filter change or service. Look for visible soot or a damaged vent pipe without opening anything. Do not restart the furnace to see if it trips again.

What a limit trip can mean

Overheating from low airflow

A clogged filter, blocked grilles, weak blower operation or a restricted coil or duct can let the furnace overheat until the limit opens. In that case the switch is protecting the furnace, not failing.

Rollout or blocked vent on the same circuit

Some controls wire rollout, draft safeguard and blocked-vent switches into one circuit with the main limit, so one code can cover all of them. Rollout points to flame where it should not be, which is why heat should stay off.

Repeated trips become a lockout

Some controls count trips. On Heil G80CTL Series A, a circuit open longer than three minutes or ten consecutive high-heat limit trips produces code 7, a lockout that recovers on its own after three hours. That restart is not proof the cause is gone.

Switch or wiring fault

A limit switch or its connection can also fail, but that is only confirmed by testing after other causes are ruled out. A replacement must match the OEM temperature rating and probe design.

When to call a technician

Call a technician for any repeated limit trip, a lockout, or a code that could involve rollout or a blocked vent, and keep heat off meanwhile. They identify which switch opened, find the overheating, venting or rollout cause, and check temperature rise, static pressure, blower, ducts, coil and combustion. Any manual safety reset is theirs to perform.

Repair quotes

Common questions

Should I replace the limit switch?

Not on the basis of a trip. A limit that opens may be doing exactly what it was designed to do. The reason for the overheating, or the rollout or vent trip, has to be found first.

There is a reset button on the switch. Can I press it?

Some rollout and secondary limit devices have a manual reset. A trip on one of them needs investigation, so leave the reset to a technician who finds and corrects the cause first. Pressing it yourself skips that step.

Can I run the furnace without a filter to stop the trips?

No. Running without a filter, or bypassing the limit, is not a safe airflow fix. Install a filter of the specified size and, if trips continue, keep heat off and have a technician find the cause.

Sources

Reviewed 2026-10-07 · How we verify codes