MEDIUM URGENCY

Lennox 4 Flashes — Primary Limit Switch Open

4 flashes

Primary limit switch open — overheating. Usually restricted airflow.

Brand: Lennox (Also covers Armstrong Air, Aire-Flo, Ducane) | Equipment: Gas Furnace

What This Code Means

4 flashes on a Lennox ML/EL furnace means the primary limit switch has tripped due to overheating in the heat exchanger.

Likely Causes (Ranked)

Most common (~70%)

Dirty air filter

What: Restricted airflow lets heat build up.

How to check: Inspect filter; replace if dirty.

Fix: Install clean filter.

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Common

Closed registers

What: Static pressure spike from too many closed vents.

How to check: Open all vents.

Fix: Clear vents and returns.

Common

Failed blower

What: Weak blower can't carry heat away.

How to check: Feel airflow at registers.

Fix: Test capacitor then replace motor.

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Diagnostic Steps

Cut power 30s
Replace filter
Open all vents
Watch for code return
If returns, check blower

Frequently Asked Questions

Why does Lennox 4 flashes keep coming?

Furnace is overheating. Filter resolves most cases.

Same on Armstrong/Ducane?

Yes.

DIY?

Filter and vents yes. Blower is more advanced.

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Need More Help?

Try our interactive symptom diagnostic tool or browse Lennox parts.

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