Tyre Defect Indicator (TDI).

The Tyre Defect Indicator (TDI) alerts you immediately if pressure in a tyre drops, enhancing safety and helping prevent further tyre damage. This electronic system lights a warning symbol in the instrument panel (supplemented by a warning system in some models) in the event of tyre damage.

If the pressure in a tyre drops, the tyre’s radius shrinks and, as a result, the wheel rotation speed rises. TDI uses sensors to monitor the rate at which the wheels rotate and the Anti-lock Brake System, ABS, compares the data for individual wheels.
This means that TDI recognises pressure loss when one wheel is rotating at a different speed to the others. An even loss of pressure in two or more wheels (natural diffusion) is not registered, so regular checkups are the best way of keeping your tyres in good condition. TDI needs to be initialised in order to function correctly.