Goodman furnace LO: Low stage heat

LO means the furnace is running on low stage heat. It is normal and it is where a two-stage or modulating furnace spends most of its time.

Applies to: Goodman GMVC96 and GCVC96 and Amana AMVC96 two-stage gas furnaces with a two-character alphanumeric display. The modulating GMVM97, GCVM97 and AMVM97 boards have no LO code — they report heat demand as H followed by a percentage instead.

What it means

Low stage is the lower firing rate, and a well-matched furnace runs there for the large majority of the heating season. Seeing LO is confirmation the furnace is staging correctly rather than defaulting to full output. It is worth documenting mainly because it can be misread as underperformance: a furnace running at low output for hours while the house is comfortable is not struggling, it is doing the thing that makes a staged furnace worth having. The case that does warrant attention is a furnace stuck at LO in cold weather while the house fails to keep up, which points at a second-stage fault.

Most common causes

Mild to moderate conditions where low output meets demand. This is normal and expected.
The furnace staging correctly and not needing more output.
A second-stage pressure switch fault preventing the furnace from stepping up, which is the case worth investigating.
A thermostat not calling for second stage.

How to diagnose it

Note whether the house is comfortable. If it is, low stage running is correct.
Note whether the furnace ever steps up to high in genuinely cold weather.
If the house is not keeping up and the furnace stays at LO, look for a second-stage pressure switch fault.
Check the air filter and registers before assuming a staging problem.

How to fix it

Nothing to fix if the house is comfortable.
If the furnace never reaches high stage in cold weather, have the second-stage circuit checked — the pressure switch codes cover this.
Check the air filter and open closed registers if the house is not keeping up.
Do not assume low output means the furnace is failing. Long low-output runs are the design intent.

Notes

People often expect a furnace to blast heat and stop. A staged or modulating furnace deliberately does the opposite, and the resulting long gentle runs are frequently reported as the furnace not working properly.

All codes for this manufacturer

Goodman and Amana furnace fault codes — every code, with what differs between models.

Sources

Goodman Manufacturing — Diagnostic Code Identification System (model-specific fault code lookup)

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.