Errant Low Tire Pressure Warning Light?

[Rocket99]

This morning the low tire pressure light came on right away. It was a cold morning, around 7 degs F. I visually checked the tires and they all looked fine. I drove to work, about 12 miles highway, and then checked the pressure. All were at around 35 psi.

Question: does the low pressure warning light come on sometimes due to the reduced tire pressure on a cold morning? If so, do I just need to reset it?

Finally, when they say "check tire pressure cold", what exactly does "cold" mean? What temp?

[Itsablurr]

"Cold" means at ambient, undriven temp. Tires heat up to varying degrees depending on driving.

Ensure the psi in all tires are correct, and re-set/calibrate the pressure monitoring system. Instructions should be in your manual.

[Rocket99]

"Cold" means at ambient, undriven temp. Tires heat up to varying degrees depending on driving.

Yes, I understand, but does it matter whether "ambient" is 10 degs, 50 degs or 90 degs?

Do you think I got the warning light today because the cold ambient temps lowered air pressure enough to trigger the light?

[GunnerNell]

This might depend on the system your car uses to assess the tire pressures. My '06 monitored the revolutions of the wheels to see if one tire might be rolling more or less than the others, due to low pressure. I think my '09 uses a pressure monitor. Either way, I think a sudden drop in outside temps could have caused the light.

I also think that your tires may have equalized themselves during your drive to work, even if they were unequal at first. That is, the lower-pressure tire may well have heated up more then the others, causing that pressure to rise more than the others. I'd check them again after the car sits for 2 or 3 hours (maybe less in this cold weather). Driving as little as a mile can warm up the tires to where you get a false reading.

Of course, you could also have a bad sending unit. And although I do not know about your specific model, I'm assuming you don't have a spare, which is often where a pressure problem originates due to simple lack of attention. (If you do have a spare, with a monitor, I'd look there first.)

[Rocket99]

This might depend on the system your car uses to assess the tire pressures. My '06 monitored the revolutions of the wheels to see if one tire might be rolling more or less than the others, due to low pressure. I think my '09 uses a pressure monitor. Either way, I think a sudden drop in outside temps could have caused the light.

Does the light come on from a pressure difference of more than X psi, or just when pressure drops below a certain threshold, or both?

[mryakan]

Does the light come on from a pressure difference of more than X psi, or just when pressure drops below a certain threshold, or both?

Usually it comes on if it drops ~20% from the last time you reset it. It may also come on if one tire is significantly less than the others.

[GunnerNell]

On the '06, it was a pressure differential among the tires. Otherwise, the computer would not have been able to detect the difference in rolling circumferences. I suspect with the pressure senders, it's both a change and a difference; in practical terms, that could work out to be the same.

I'm more familiar with my wife's Lexus,* which will throw the warning light both ways. And the manual tells me to reset the system when I pump them up more than a few pounds.

BTW, OP, I mentioned the spare** because that Lexus had the tire warning light on for quite a while, due to a leak in the spare. I had equalized all five tire pressures to spec and reset the system, which kept the light off for a while. I also scratched the spare off of my mental list of potential problems because it didn't have any obvious leaks, and I knew it couldn't have picked up a nail or anything since it wasn't "on" the car. I just figured that the system had a fault in it somehow. Turns out there was a slow leak around the bead on the spare tire. Grrrrr.

*I'm still having trouble getting the correct manual for my '09.
** I assume your 2010 car has run-flats, and therefore, no spare, but I'm not familiar with the X cars -- do you have one?

[Rocket99]

I assume your 2010 car has run-flats, and therefore, no spare, but I'm not familiar with the X cars -- do you have one?

I have the xDrive, and the car comes with RFTs.

Thank you for the info.

[DaveChall85]

Hi,

I had the front brake pads changed only 123D a couple of days ago and now i am having a RED tyre pressure warning! having checked the pressures they are all good (and at the same level) BUT my wife tells me that the brakes are squeaking.

Could it be that the brakes are rubbing and causing 1 or both of the front wheels to roll slower than the others therefore triggering the tyre pressure warning as highlighted by GunnerNell??

Thanks for any advice