in cold temps. I recently inflated the tires to 32 psi (ambient temps in the 60's) and reset the warning light. It was fine for a week and there are NO pressure leaks in any tire. Today, at 38 degrees, the light came on (steady). I checked all four tires and they are all the same pressure-- 28 psi. Rather than waste a trip to the dealer, I called them and the service writer said that it's better to set the tires to 34 psi to avoid the light coming on in very cold weather (which naturally drops the psi). I always thought the rotational speed difference is what caused the light to come on, and if all 4 tires have the same pressure, the light would stay off. Perhaps I'm wrong. He said that there is an absolute pressure at which the light will come on, even if all 4 tires are evenly inflated. Can anyone shed some light (no pun intended) on how the low tire warning system determines a pressure issue in the tires?

After ambient temps come up a bit, later today, I WILL inflate the tires to 34 PSI as the writer suggested, reset the light, and keep an eye on things, but because this is my wife's commute vehicle I need to get a jump on any potential problems ASAP, so if anyone knows for sure if simply cold weather/evenly dropped tire pressures could cause this, I'd appreciate your comments.

The dealer actually sounds like he knows what he's talking about for a change. From what I understand it's the absolute pressure in each tire the sets off the light. Even if one of the tires drops below that threshold your light will come on. The computer doesn't know the exact PSI in each tire, it just knows that each tire's PSI is above that threshold, so no light will come on, but once it goes below, it will come on. Speed has nothing to do with tire pressure in this instance. Just keep the tires inflated to 34PSI and you should be fine. Let us know what results from this, good luck.

cool. thanks. As soon as the temp comes up to about 60 degrees (or expected high today is 65), I'll inflate to 34 and drive it around today. The really weird thing is we've never had this problem before and have had Siennas since 2001 and I rarely add air to the tires, nor kept them inflated to 34PSI. Generally, I try to keep them between 30-32. Anyways, I'll up the pressure today and we'll see what happens. The good news in all this is it seems there aren't any nails or screws causing a leak. LOL! Last week when the light came on, I immediately checked all tires for leaks with soapy water sprayed onto the treads as my wife SLOWLY moved the van to expose each part of the tire.

You should always check the air in your tires at least once a week. Low tire pressure leads to premature tire wear and lower MPG. They should be inflated to what's indicated by Toyota. There should be a sticker on the driver side door jamb or inside the glove compartment. I just air mine up to 34 PSI.

you are right--I DO need to check them more often. :)