Issues with air handler fan

A homeowner on a building-science forum ran into trouble with a Mitsubishi SVZ ducted heat pump, the kind of system that has an indoor air handler connected to ductwork. A technician had gone into the unit's control settings and changed two function codes (settings that control how the fan behaves when the thermostat isn't actively heating or cooling) so the fan would stop running instead of continuing at low speed. After that change, the fan stopped responding entirely in ventilation mode, at both low and high speed, even though it had run continuously before.

The technician's explanation was that the system won't allow that fan-stop setting if continuous ventilation is turned off, suggesting the two settings conflict with each other on this model. Before the change, running the fan continuously in "auto" mode was apparently normal and expected behavior for this type of ducted unit.

This is a single homeowner's unresolved troubleshooting question, not a product recall or a documented defect, and no fix was posted. If your own home has a Mitsubishi ducted heat pump or similar system, it's worth knowing that fan behavior is controlled by configurable settings a technician can adjust, and that changing settings related to fan speed during the "off" cycle can affect whether your system provides background ventilation at all. If you notice your system's fan has stopped running the way it used to after a service visit, that may be worth asking about directly.