With the temps at a all time low in the U.S. this week, my TPMS warning light has come on in my 2012 RX350.
It will stay very cold for most of the week, so is it ok to drive with my warning light on.

I don't want to check my psi in the tires with a tire gauge because I was told a valve stem can freeze open in extreme cold letting the tire to go completely flat.

 

There is a range of PSI with a bottom limit, after which the sensor will signal low air.
Sounds like the tire(s) may have been at the bottom limit before it got cold.. Get it to a dealer, have them set the air to factory specs. & then flash the computer. Might/might not be a charge depending on the dealer. I'll bet any Toyota dealer could do this also.

We saw -40 here over Xmas, no issues with our TPMS.

 

Our 2010 RX did the same thing two days ago. I just reset the TPMS and was good to go. I assume it triggered the TPMS light due to the cold air in the tire...once it warmed up, and I reset the light, everything was good to go.

 

Driving wil heat up your tires. If you're worried about the stems freezing open, drive to the nearest tire shop and check them there. If they freeze open there (unlikely as tire stems are used in cold weather climates worldwide), at least you have the fix readily available.

As was stated, you were probably at the low end of the scale and the cold air caused the air in your tire to contract and give the warning. Warning light may even go away by the time you reach the tire shop depending on how much the tires heat up.

 

If you have nitrogen (N) in your tires there is no risk of freezing the valve as N gas has zero moisture content. With regular air there is a chance of an amount of moisture in the air which could possibly cause a frozen valve. Mostly the moisture seems to come from the air tank condensation if they don't use air dryers.

If you are using N, just drop by a Lexus dealer and they usually will happily top off the N for you. I'd not want to drive the car too long with low pressure as it can cause excessive wear or worse, loss of control.

Your tires will lose or gain 1 psi for every 10 degrees of temperature change whether you use plain air or N. If you set your tires last when it was in the 70's the tires could now be running 6-9 psi low which could lead to loss of control on snowy or icy condition.

 

The light itself doesn't do anything. I have it on all the time during winter because my winter rim/tire set doesn't have TPMS. At least do a walk around and kick the tires. If the car feels wobbly while driving, it may be an indication of low air pressure. I bumped the air pressure in mine, anticipating the freeze going through.
Having a garage where temps are higher than outside is very helpful. As alredy mentioned, take it to a shop, better yet, the dealer, and have them adjust the tire pressure in a climate-controlled environment if you're afraid of freezing the valves. If you do have a garage, you could try using a hair dryer to warm up the valve stems before checking the pressure. Just don't attempt using a blow torch.

 

Haven't seen a frozen valve stem yet. Living here almost all my life.
-40 can seem like a warm day..............at times.

Must have lots of humidity in the air that filled the tires ?

 

Also who knows what's happening when the TPMS light flashes on and off for 5 seconds?

 

I would not worry about the valve freezing open. Can it happen? - I suppose so. I worked as mechanic in NJ for more than a decade (before wising up and moving to FL). The colder it was outside the more time I spent in it (getting vehicles started, unstuck, towing, repairs on truck air-brake systems, etc.). I never heard of this problem before.
I just Googled it - and it appears to be nonsense. http://articles.chicagotribune.com/2...-filters-brake

Of course I'm sure one could find other articles supporting that claim.

IMO, check your air and add some when it is needed regardless of temperature.