[Jen Luckenbach]

My '08 AND my '09 outbacks TPMI came on recently on a VERY cold day (compared to the previous days).

The '09 went off by itself in a day or so. The '08 has stayed on.

The tires aren't low.

Can the very cold (about 10 F ) affect this sensor?

 

[plain OM]

The tires aren't low.

Did you measure them when the light came on? Some time later?

The pressure in the tire changes with temperature. When the outside temperature goes down, the tire pressure will as well. It might not be visible when looking at the tires, but it will be different when measured. The rate is approx. one psi for each 10 F change.

The TPMS system will trigger when the tire pressure drops below 26 psi. The normal pressure is somewhere around 30 psi. If the tires are set to 30 psi at, say, 60 F, and the temperature drops to 10, that's a 50 degree change, which would cause the tire pressure to drop 5 psi. The tires that had been set to 30 are now at 25, and the TPSM warning comes on.

Was the light on steady, or flashing? If the TPMS lights came on steady, it's a "low pressure" warning. If the light was flashing, it's indicating a problem in the sensing system or control module. This might happen if, for example, the batteries in the wheel sensors weaken sufficiently due to the cold. In this case the module would be expecting to receive a signal from the wheels sensor and doesn't receive it due to the weak battery. But again, it would be a flashing light, not steady.

 

[SE95]

If you want you can bump up the pressure a little bit (as long as you do not exceed max sidewall pressure). But this is not necessarily ideal. Typically the light will shut off after you drive for a bit and get some heat back in the tires.

 

[signage]

There has been a post on here stating to run 35lbs front and 32lbs rear. The cold temperature effects these in two way as Plain OM stated. These sensors have lithium ion batteries in them, and they also have a momentum switch, so after the tire rotate at 5mph the switch activates and the sensor starts working! These then stay activated until they sit idle for approx. 15 min.

 

[Dajeepman]

There has been a post on here stating to run 35lbs front and 32lbs rear. The cold temperature effects these in two way as Plain OM stated. These sensors have lithium ion batteries in them, and they also have a momentum switch, so after the tire rotate at 5mph the switch activates and the sensor starts working! These then stay activated until they sit idle for approx. 15 min.

Cool something i didn't know.. thanks for the info

 

[Startiger]

I have had this happen to me.

When I take my OB out on a visit to my in-laws, there is one part of the trip on the interstate (Garret County, MD) that is higher and colder than the surrounding area. When it is particularly cold out, the TPMS light would come on. Whenever I stop and check the tire pressure, they are all fine. By the time I am out of the mountains, it goes off.

I have yet to figure out the cause of it. Even after getting new tires and sensors, I still get that light sometimes. Just one of the goofy things about that car, I guess.

 

[SE95]

Air density also changes with altitude. While I have never heard of it, it would certainly make sense that driving into a higher altitude would cause a drop in tire pressure, and therefore turn on the TPMS telltale.

 

[Startiger]

Actually, going up in altitude should raise the relative pressure in the tires.

Tire pressure = gauge pressure + ambient pressure. Going up in altitude, the air inside the tire stays constant, so the gauge pressure will go up as the ambient pressure goes down.

 

[SE95]

Actually, going up in altitude should raise the relative pressure in the tires.

Tire pressure = gauge pressure + ambient pressure. Going up in altitude, the air inside the tire stays constant, so the gauge pressure will go up as the ambient pressure goes down.

Hmm. I do know that some cars will turn the light on for overinflation, which may be the case here. However, I am not sure if Subarus are one of them.

 

[Cobalt]

Actually, going up in altitude should raise the relative pressure in the tires.

Tire pressure = gauge pressure + ambient pressure. Going up in altitude, the air inside the tire stays constant, so the gauge pressure will go up as the ambient pressure goes down.

Yep
Take a bag of chips up above 8000 elevation and the bag looks like it was pumped up like a balloon