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2011 Forester I Auto
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116 Posts
Discussion Starter · #1 · (Edited)
This came on today while driving home in the snow storm that is hitting NYC at the moment.

This is my first drive in snow deeper then an inch since the vehicle was purchased used last August, '11 Forester.

I did a quick walk around when I got home but nothing "looked" like a tire with a slow leak or anything.

So what about the snow conditions, tires treads packed with snow ? cause this ?.

EDIT: Tire pressure is mostly uniform, RR is about 2 PSI lower then the others.

I would think that a low pressure indication would function at any point it indicates "low". Not sure what the threshold is, or the "low" point. When I just now checked, the vehicles been sitting for an hour in 20 degree weather so I'm not surprised at 28-29, but maybe the 26 on RR hits the warning point.
 

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Registered
2011 Forester
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107 Posts
The pressures are not within something like 3 lbs of one another. Just add air.


Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.
Benjamin Franklin
 

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2009 Forester XT 4EAT
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2,265 Posts
If you search around the forum here there is a thread somewhere that they idiot light comes on when tire pressure drops below 26 psi. If any of them drop below 26 when they are cold (i.e. not driven for at least an hour or more prior to checking in winter) it will cause the TPMS to light up.
I have had to inflate my tires before after a significant temperature drop overnight.
Its not that unusual. Make sure you have a good, quality tire pressure gauge and you could always swap to Nitrogen instead of air as it does not lose pressure (not nearly as bad as regular air).
 

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SierraHotel
2010 Forester 2.5x 4EAT
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547 Posts
You probably have 25 or 26 psi in a tire. At that pressure it can be hard to spot visually, particularly on the rear tires. You need to buy a gauge if you don't have one.
 

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17 Forester Premium
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106 Posts
The light on my wife's 12 Forester came on a few days ago. I checked both of our identical cars, and all tires were at 26 Psi (It was about 15F here in NYC). In the fall I checked my tires and they were at 34 Psi when it was warm out.

I pumped it up to 28Psi and after a 15miles drive the light turned off. It's strange that my car didn't have that problem under the same pressure.
 

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Lifer
2015 Forester CVT
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526 Posts
Nitrogen myth

. . .you could always swap to Nitrogen instead of air as it does not lose pressure (not nearly as bad as regular air).
Air is approximately 80% nitrogen. Air and nitrogen both have the same rate of pressure change with temperature.

From university physics regarding thermal expansion:

Gas Pressure Increase with Temperature
In 1702, Amontons discovered a linear increase of P with T for air, and found P to increase about 33% from the freezing point of water to the boiling point of water.

That is to say, he discovered that if a container of air were to be sealed at 0°C, at ordinary atmospheric pressure of 15 pounds per square inch, and then heated to 100°C but kept at the same volume, the air would now exert a pressure of about 20 pounds per square inch on the sides of the container.

Remarkably, Amontons discovered, if the gas were initially at a pressure of thirty pounds per square inch at 0°C, on heating to 100°C the pressure would go to about 40 pounds per square inch—so the percentage increase in pressure was the same for any initial pressure: on heating through 100°C, the pressure would always increase by about 33%.

Furthermore, the result turned out to be the same for different gases!
 
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