Error code reference

Lennox Furnace Error Code DIAG#1: Flash / DIAG#2: Off

Pressure switch Watchguard: pressure switch opened during operation or condensate drain blocked.

SourceLennox GHR26 Series - Service Manual: Gas FurnacesPage 8 (Table 1), Page 11 (Table 6)

Diagnostic procedure

The full DIAG#1: Flash / DIAG#2: Off 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

First diagnostic step

Check that pressure switch is open without combustion air blower operating; check for blocked inlet/exhaust vent or condensate line.

SourceLennox GHR26 Series - Service Manual: Gas FurnacesPage 8 (Table 1), Page 11 (Table 6)

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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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Where this code is documented

1 manual in the MyPros+ library documents this code for Lennox furnace.

Models covered by the manuals that document this code

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The full DIAG#1: Flash / DIAG#2: Off procedure

Where this code is documented

Models covered by the manuals that document this code

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