Abnormal pressure loss
The molecules of oxgen are so small that they can break through the tyre membrane. As a consequence, there is a natural pressure loss of roughly 1.5 psi each month. That is why you have to check tyre pressure regularly.
If you tyres loses more than 1.5 psi a month, it is a sign that air leakage is abnomal
Valve malfunction is a common cause for abnormal pressure loss. They are normally airtight in order to keep the pressure in. However, they deteriorate over time because of the adverse effect of heat cycles .Eventually, they start malfunctioning
That is why tyre manufacturers and mechanics that take pride in their work will recommend changing the valves when you replace tyres
Never mind if you use the Tire Pressure Monitoring System ( TPMS) or not, your car mechanic can change the valve at a low cost, regardless.
The systems of direct pressure control that warn the drivers of abnormal pressure loss in real time on the dashboard are compulsory for all cars destined for the European market since November 2012.
These regulation were passed in order to ensure correct pressure is used. You can go more in depth by reading our article on the contact patch and tyre pressure.