I have the tyre pressure monitor on the car. A few weeks ago I blew the tyres up to the higher of the two pressures on the door sticker (42psi in my case), and reset the monitor on the MMI. I've not used the car a huge amount since then for various reasons, nor have I checked the pressures. Finally did them today and was surprised to discover they had all dropped to between 34 and 38psi, with no warning from the monitor. In both cases the pressures were checked with my own compressor which I believe is accurate. Now, I admit I am at fault for not checking the tyres more frequently, but that said I would have expected the monitor to have picked up a drop in pressure of this magnitude. Does anyone know any more about the technical aspects of these systems; how accurate are they? I get the impression their only use is to give you a bit more warning of a tyre that's about to blow out; still useful, but not quite what I was hoping for.