The TPMS (tyre pressure monitoring system) is a system fitted to many modern vehicles allowing the constant monitoring of the current tyre pressure. The tyres pressure can fluctuate considerably in weather and driving conditions. Most tyres contain normal air and this expands with heat and contracts in cold. So, if you're driving a long distance from a cold Scottish town at 5am to warm and sunny Orpington by 1pm, for example, you could see a 5psi increase in your tyre pressure. This would be outside the parameters of a TPMS and would trigger, and on the way home the tyre pressure would even out.
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