The damn thing had been on for a couple of months. Shining yellow on the dash, saying I was running with a low tire.
Dutifully I checked my tires; all up to recommended pressure. Hmmm. I added less than a pound to each. Then rechecked the light. Still on. WTF? I ended up going around and around the car, checking again, adding air, removing air, adding it back. Light stayed on.
Gave up. Kept driving. Tires looked great, rode great. But the damn warning light stayed on.
Finally took it in for routine service two days ago. Left my mechanic with the task of figuring it out. And figure it out he did. Now I roll without the warning light on.
I did have a low tire. It was the spare tire in the trunk that was low.
On our 7-year-old Honda Accord and 8-year-old Honda Fit the spare tire (one of those mini-tires in both cases) is not on the TPMS system. Obviously your car is different.
I’m glad I insisted that my mechanic test my tire for leaks, because they’re so honest that they told me there was nothing I had to pay for in my car! Now I might have gone on my merry way 20 years ago when there were free air places everywhere but there are few air places any more and they charge $1.50 and I didn’t feel like paying this every week or so. So they tested for leaks again and found the leak and patched it.
A less honest mechanic would have sold me a set of replacement tires because I am on my starter tires at 35K miles and the dime test is inconclusive, but all it needed was a cheap patch.
I was stunned, frankly. I never thought of the spare at all. In my head, there was something built into the car that monitored tire pressure, not in the tire.
Seems obvious in retrospect. I talked to a few work friends about this, and they all agreed they never would have thought to check the spare either. Proves the rule about great minds and fools, no?
there are two types of TPMS out there, indirect, and direct.
indirect TPMS uses the ABS sensors to monitor wheel speeds, and if it sees that one tire is spinning measurably faster than the others (because lowering the air pressure reduces it’s rolling diameter) it assumes that tire is low and sets the “Low Tire Pressure” light.
direct TPMS uses sensors mounted inside the wheel, sometimes integrated with the valve stem, to directly measure tire pressure. they’re usually battery powered and report tire pressure via radio. if your car can actually display the tire pressures on the cluster/screen, you have a direct system.
Given that my low tire was not spinning at all, I must assume mine is direct. It does not report actual tire pressure, just indicates that a tire is low.
Indeed, and the indirect type is relatively new. It’s also quite a bit more inconvenient because you either have to manually reset it or else it takes a fair amount of time to judge your tire is back to normal. I used to work closely with companies that developed this software.
At the advent of the last two fall seasons, when the a.m. air temp goes below comfy, maybe 50’s, my 2010 Mazda 5’s left or right front tires gave me the squiggly omega idiot light until I added air to them all, not determining which one had been low. Then you have to drive it around so the system resets the dash light (per the Mazda shop).
I sometimes drive a Chevy pickup at work and its dash message tells me which of the 4 visible tires is low! Fleet truck doesn’t carry a spare, even if capable, for fear of its theft.
The service guys were amused when I called to ask why the Jeep was telling me to check some horse hoof. They said practically no one knows what it means.
The icon on my Ranger is an obvious tire icon. Mine was on for about a month, despite topping off my tires. When I had a rental at Christmas, it actually showed the pressure. Que me figuring out my little compressor reads 5# low. Doh!
Our Rogue uses the direct method. When winter hit and the temperatures dropped, I started getting warnings about the left rear tire being low. Usually it fixed itself by the time I got to work. Only needed a few miles to heat the tires enough to kick the pressure back over the threshold. But I still had them check and fill all tires yesterday when I had the oil changed.
I rented a car a couple years ago (don’t recall the make/model) where the low tire warning came on. None of the tires looked low but I was close enough to a rental location that I decided to let them deal with it. According the people there, one tire was over-inflated to nearly 40 psi which set off the low tire light. They took air out of it back to 32 psi and all was fine the rest of the rental.
