What is the main visible indicator that a fire extinguisher needs servicing?

A needle in the red zone of the pressure gauge clearly indicates that a fire extinguisher is not properly pressurized and may not function effectively during a fire emergency. The pressure gauge serves as an immediate visual inspection tool, allowing users to quickly assess whether the extinguisher is in a ready state. If the needle is in the red zone, it suggests that the extinguisher could be either over-pressurized or under-pressurized, which can affect its ability to discharge properly.

While corrosion on the extinguisher body is a significant concern, it doesn’t provide immediate information about the extinguisher's current operational status like the pressure gauge does. A missing safety pin is also a bad sign; however, it's more about the operational readiness when attempting to use the extinguisher, rather than the overall need for servicing. The gauge reflects the essential readiness of the extinguisher to function correctly at the time the assessment is made.