My car just went in to VGS as they think the problem is the electric box inside the wheel, apparently 05-56ish 18'' monza's had a unit inside them if the car monitors the PSI like mine.
That's rubbish as UK cars (assuming you are in the UK) didn't come with TPMS installed. The Yanks got it as standard.
Oh... Gona call them in the morning and ask about that! Cheers!
Again, Bacillus is correct. The TPMS system fitted to our UK cars uses the ABS system to detect a tyre deflating if it thinks the rolling circumference of the tyre is changing from the parameters last 'set' by holing the button until the bleep. There are no sensors inside the wheel.