What effect does a closed cabin outflow valve have on cabin pressure?

When the cabin outflow valve is closed, it prevents the escape of air from the cabin to the outside environment. This restriction allows for an increase in cabin pressure as the pressurization system continues to supply air to the cabin without a means for it to vent. As more air is added and unable to exit, the pressure within the cabin is effectively raised.

The other potential outcomes do not align with the function of a closed outflow valve. For instance, if cabin pressure were to remain stable, it would imply that the inflow of air is perfectly balanced by an equivalent outflow, which is not possible with a closed valve. Similarly, a decrease in pressure would indicate that air is escaping or being expelled from the cabin, which cannot happen when the outflow valve is shut. Fluctuating pressures could suggest varying inflow and outflow rates, but in the case of a closed outflow valve, the system would not allow for such variances, thus confirming an increase in cabin pressure instead.