LED1 SLOW flash / LED2 ON Primary or secondary limit open

What the control is telling you

The control sees an open limit circuit. The inducer will not start on a heat call, but cooling and continuous fan still work. If the limit does not close within 3 minutes the control goes into a 1-hour watchguard lockout.

Applies to: G43UF-24B-045 through G43UF-60D-135 (SureLight boards 32M88, 78M47, 100973-01)

Likely causes

• Furnace overheated (restricted airflow, dirty filter, blocked supply or return)
• Primary or secondary limit switch open or failed
• Loose or wrong wiring at the limit switch(es)

Test and repair steps

1. Turn off power at the disconnect before touching wiring or components.
2. Check continuity across the limit switch(es). They reset on their own once cool.
3. Look for restrictions on the blower inlet (including the filter) and on the outlet air.
4. Find out why the limit tripped before putting the furnace back in service.
5. Check limit wiring for miswiring or loose terminals.

Settings for these units

• Recommended filter size by cabinet width · EL296UHV
• Recommended filter size by cabinet width · Lennox SL280UHV
• Filter sizes by cabinet width · Lennox ML180UHE
• Filter sizes (filter rack kits) · Lennox EL195UHNE
• Filter sizes by cabinet width · Lennox SL297NV

Models that show this code

10 models in Lennox G43UF

Other codes on the same units

• LED1 OFF / LED2 OFF No power to the control
• LED1 OFF / LED2 ON Control board failure or control wired incorrectly
• LED1 OFF / LED2 SLOW flash Prove switch (pressure switch) open
• LED1 ON / LED2 OFF Control board failure or control wired incorrectly
• LED1 ON / LED2 ON Control board failure or control wired incorrectly
• LED1 ON / LED2 SLOW flash Rollout switch open
• LED1 SLOW flash / LED2 FAST flash Low flame signal
• LED1 SLOW flash / LED2 OFF Flame sensed with gas valve not energized