Error code reference

Lennox Furnace Error Code 5+6

Low secondary (24VAC) voltage (Board 100870).

SourceLennox G61MPV - Service Manual: High-Efficiency Gas Furnacep. 32

Diagnostic procedure

The full 5+6 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 transformer output; check for shorts on 24V circuit.

SourceLennox G61MPV - Service Manual: High-Efficiency Gas Furnacep. 32

Unlock with pass

More steps, wiring diagram references, and verified part numbers

Includes expected resistance and voltage values, connector pinouts, and supersession notes for replacement parts.

Unlock 5+6 procedure $9.95

Or subscribe: $29/month for all codes plus Max diagnostics, with 70 Max messages a day. Plus applicable tax.

Where this code is documented

5 manuals in the MyPros+ library document this code for Lennox furnace.

Models covered by the manuals that document this code

More models are covered than are listed here.

75,000+ service documents · 20,000+ error codes · 100+ brands

Get Started

$29/month · Cancel anytime · Or $9.95 3-day pass · Plus applicable tax

The full 5+6 procedure

Where this code is documented

Models covered by the manuals that document this code

Related Lennox furnace error codes