[AlanHo]

The temperature was minus 4^oC this morning after 3 days of heavy rain, flooded roads and now snow. The car was last used yesterday evening without a tyre pressure warning and was parked overnight under a 3 inch blanket of snow which was the very devil to remove this morning . It was no longer fluffy snow - instead solid aerated ice.

When I started it, I got a low tyre pressure warning on just one tyre - 24 psi when the other 3 were reading 36 psi.

I inflated it to 36 psi, went shopping, and the pressure on all 4 tyres crept up to 38 psi after driving 5 miles - which is normal behaviour.

I returned home 4 hours ago and all 4 tyres are still reading 36 psi.

It would appear that the errant tyre does not have a puncture so I am not sure what caused the considerable loss of air.

My only thought, probably crazy, is that water got inside the valve cap - it froze overnight, the trapped ice expanded, and pushed against the valve spindle. Thus releasing air

Is this really plausible?

[Lorian]

I'd be checking for a nail/screw.

[AlanHo]

I'd be checking for a nail/screw.

It is now 23.10 and I have just got back after taking the car for a short run to check the tyre pressures. With my Niro the tyre pressure display after starting the engine is blank and stays so until the car has done a quarter mile or so, then it displays the pressure in each wheel.

All 4 wheels are the same - at 36 psi.

So the plot thickens - why one wheel lost 12 psi last night remains a mystery.

[Surferdude]

I'd be checking for a nail/screw.

It is now 23.10 and I have just got back after taking the car for a short run to check the tyre pressures. With my Niro the tyre pressure display after starting the engine is blank and stays so until the car has done a quarter mile or so, then it displays the pressure in each wheel.

All 4 wheels are the same - at 36 psi.

So the plot thickens - why one wheel lost 12 psi last night remains a mystery.

Can't be a nail if the pressure is still holding.
Your ice in the valve core theory would appear sound. But if the valve caps were right, I can't see how water got in.