Error code reference

Lennox HVAC Error Code DIAG1 On, DIAG2 Flash

Watchguard mode: system locked out for one hour after five consecutive unsuccessful ignition attempts or repeated flame failures.

SourceLennox CompleteHeat HM30/AM30 - Service Manualp. 15, 41

Lennox codes Lennox hvac codes

Diagnostic procedure

The full DIAG1 On, DIAG2 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

First diagnostic step

Reset power to the system and observe diagnostic LEDs for any other error conditions.

SourceLennox CompleteHeat HM30/AM30 - Service Manualp. 15, 41

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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 hvac.

Models covered by the manuals that document this code

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The full DIAG1 On, DIAG2 Flash procedure

Where this code is documented

Models covered by the manuals that document this code

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