Goodman furnace d: Dehumidification cooling with a single stage compressor

A lower case d means the system is cooling for humidity control rather than for temperature. The blower is deliberately running slower than normal.

Applies to: Goodman GMVM97, GCVM97 and Amana AMVM97 modulating gas furnaces with a two-character display, set up with a single stage compressor and a humidity-controlling thermostat.

What it means

Dehumidification mode is a real operating mode, not a fault, and it looks wrong to anyone who does not know it exists. When a humidity-controlling thermostat calls for moisture removal, the control slows the indoor blower. Slower air spends longer over the cold evaporator coil, gets colder, and condenses more water out of it — better moisture removal at the cost of less airflow. So the symptom is a system that seems to be running weakly on a humid day, which is exactly what it is meant to do. Lower case d is the single stage compressor version; d1 and d2 report low and high stage on a communicating two stage system.

Most common causes

A humidity-controlling thermostat is calling for dehumidification.
Humid weather, which is when this mode is designed to engage.
The system configured for a single stage compressor rather than a two stage one.

How to diagnose it

Note whether the weather is humid and whether the thermostat has a humidity setting. That usually explains it entirely.
Reduced airflow during this mode is intended, not a fault.
If the humidity setpoint has been set unrealistically low, the system will run in this mode far more than it should — that is a setting rather than a failure.
If airflow is low in every mode rather than only this one, then look at filters and ductwork.

How to fix it

Nothing, if the weather is humid and the thermostat is asking for dehumidification.
If the system runs this way constantly, check the humidity setpoint on the thermostat before calling anyone.
Reduced airflow in this mode is by design and should not be countered by raising blower speed.

Notes

This mode is a frequent source of complaints about weak airflow in summer. The furnace is not underperforming; it is trading airflow for moisture removal, deliberately.

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)

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