Same-day shipping on orders before 3PM CST · 10,893 OEM Parts In Stock · Carrier · Bryant · ICP · Heil · Tempstar
Track Order | 1-800-NHP-HVAC | Trade Accounts

Menu

Home Shop All Account Cart
MEDIUM URGENCY

Lennox 3 Flashes — Open Pressure Switch With Inducer On

3 flashes

Pressure switch open during inducer operation — vent blockage or bad switch.

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

What This Code Means

3 flashes means the pressure switch did not close when the inducer started. The inducer should create enough draft to close the switch.

Likely Causes (Ranked)

Most common

Blocked exhaust vent

What: Snow, ice, debris, or disconnected pipe blocks venting.

How to check: Inspect outdoor exhaust outlet.

Fix: Clear blockage.

Shop parts
Common

Failed inducer motor

What: Inducer can't create enough draft.

How to check: Listen at startup; should run smoothly.

Fix: Replace inducer.

Shop parts
Common

Bad pressure switch

What: Switch won't close on normal pressure.

How to check: Tech test with manometer.

Fix: Replace switch.

Shop parts

Diagnostic Steps

  1. Check outdoor exhaust outlet
  2. Listen for inducer
  3. Inspect pressure tubing
  4. Replace switch if needed

Frequently Asked Questions

What's the most common cause of Lennox 3 flashes?
Blocked exhaust vent, especially in winter.
DIY?
Vent clearing yes. Inducer is more advanced.
Same on Armstrong?
Yes — Lennox family shares codes.

Related Lennox Codes

Lennox Error Code 2 FlashesLennox Error Code 6 Flashes

Related Symptoms

Furnace Not IgnitingFurnace Blowing Cold Air

Need More Help?

Try our interactive symptom diagnostic tool or browse Lennox parts.

Home Shop Search Account Cart