When the tyre pressure monitor is firmly hand screwed onto the valve stem of a cold tyre that has the correct tyre pressure, the see through dome at the top turns green.

When the tyre pressure drops 3 to 6psi, the yellow indicator starts to become visible, and it is now recommended that you top up this tyre with air.

When the tyre pressure drops more than 5 psi the red indicator starts to become visible. At 6 to 10 psi drop in pressure, the red is very visible, and this indicates a seriously under-inflated tyre which should be re-inflated immediately. 

When fitting to the valve for the first time, please bear in mind that the moving parts inside the monitors will be tight with newness, so a short drive or ride will be necessary to loosen them up.