cal is shown as a horizontal dashed line as can be seen in fig2 the tire pressure signal decreases steadily until its deviation from the calibration value p cal exceeds a first threshold value u03b4p 0 for instance u03b4p 0 25 p cal at that time t 0 a prior art warning unit would have issued a warning signal to the driver according to the invention however the warning unit starts an internal counter that counts a minimum time period u03b4t min during this minimum time period u03b4t min the warning unit 10 monitors the pressure signal p whether its deviation from the calibration value p cal continues to exceed the first threshold value u03b4p 0 if during this minimum time period u03b4t min the deviation has always been greater than the first threshold value u03b4p 0 the warning unit 10 may issue a warning signal alternatively the warning unit 10 might still continue to collect further data in order to avoid any false alarming the length of the data collecting prolongation may be based on statistical parameters indicating the confidence level of the detected pressure deviation if however the deviation of the pressure signal falls below the first threshold value u03b4p 0 the counter is reset and will only be restarted if the deviation re.