Error code reference

Lennox Furnace Error Code LED1 ALTERNATING SLOW FLASH / LED2 ALTERNATING SLOW FLASH

Watchguard - burners failed to ignite, OR limit open more than 3 minutes (32M88 only), OR lost flame sense 5 times in one heating cycle, OR prove switch opened 5 times in one heating cycle.

SourceLennox G43UF Series Furnace - Full Service Manualp. 11

Diagnostic procedure

The full LED1 ALTERNATING SLOW FLASH / LED2 ALTERNATING SLOW FLASH procedure

Step 1 is visible so you can confirm you are on the right page. The remaining steps, the ones that tell you whether to actually replace a part, unlock with any pass.

MyPros+ · DiagnosticsStep 1 preview
1
First diagnostic step
Check gas supply, gas valve operation, ignitor, flame sensor, and vent system.

SourceLennox G43UF Series Furnace - Full Service Manualp. 11

More steps, wiring diagram references, and verified part numbers
Includes expected resistance and voltage values, connector pinouts, and supersession notes for replacement parts.
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