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2009 Subaru Legacy Limited - TPMS Light

Hi all,

The TPMS light is pretty persistent on my dashboard over the last few weeks. I've been measuring the air on a cold tire before it's driven per the door jamb on the car:

Fronts 33 psi, Rears 32 psi.

All of the tires are within 1-2 lbs of the recommended limit, with the exception of the right rear tire (passenger side rear tire), which came it at a surprising 25 psi.

Several times, I have added air to the tire to bring it to the 32 psi, and it will last for 1-2 days, and the light will return.

I've inspected the tire with a flashlight, run my hands over the treads, and I can't find any debris or other issues that could indicate a leak (like hitting a nail, etc.) So, this post is more of a "what exactly does my TPMS sensor do on my car" question.

Does the TPMS have the ability to "release" the valve to let out excess air pressure? My thought is that if the sensor is faulty, and it thinks that there's too much air, if it's releasing some to get it in check. I've had a relative that had this issue on their `08 Nissan Sentra and it turned out that the sensor was faulty.

I'm not sure if the Subaru ones are as complex or complicated, and either way I will be bringing it up to the dealer when I bring it in for my 18,750 mi service in a week or two.

Any thoughts? Thanks.


  • fibber2fibber2 Mid Hudson Valley, NYPosts: 3,732
    As far as I know, the TPMS is simply a "measure and report" system, and not interactive or self adjusting in any way. While it is integral with the schrader valve, standard parts like a valve core and seals should be there preventing air from exiting. Most likely you simply have a leak. Could be a bead or rim problem, or the TPMS/valve itself, but it shouldn't be losing air by design.
  • ateixeiraateixeira Posts: 72,587
    I learned to spray soapy water on the tire and look for the bubbles to indicate a slow leak. Air it up first, though.

    Could be a nail that stayed in, and only leaks when the car is moving, or when the tire temps rise.
  • Can you explain the bead/rim and how it affects the tire pressure?

    Reason I ask is because either myself or my wife hit a pretty serious pothole in April of `09 and the sidewall bubbled. We had the tire replaced shortly after that and we were told the "rim was bent bad, but we will use a bead sealer on it" to fix the problem. I was told that if the sealer didn't work, we would have to purchase another rim, which we were given an estimate of around $1000. (To which, I replied, better fix it with the bead sealer!)

    Not another word was spoken about the rim and we've had no issues until the cold weather started up around November of 2009, in which case I started this routine of putting more air in the tire.

    I know for a fact I've had some rotations since then, but I'm not sure if it's the same one as the one that was replaced. I can see if it has a different DOT number and see which tire it's on. But otherwise, yeah, I was thinking a slow leak from a nail that's still imbedded in the tire.

    I'll go grab a flashlight and go take a peek at the tires now.

    So, my last question is, what exactly does bead sealer do, when is it used, and is it a permanent solution, or is it possible I need to get more of it reapplied assuming that the rim in question is the issue?

    I wonder if I could find a rim from a junker nearby, if it came down to that.

  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391
    The bead is essentially the contact between the rim and the tire. When air pressure is applied to the inside of the tire, it forces the tire against the lip on the rim and creates a seal. When the rim's lip is bent, it can result in uneven pressure on this sealing point and allow air to leak. A bead sealer acts as a gasket between the tire and the rim to help ensure a seal. It does not actually fix the bent rim, but it can prevent the tire from leaking at that point.

    It is entirely possible that the leak is coming from this rim and cold weather would certainly exacerbate it. When things get cold, they shrink.

    OEM rims are ridiculously overpriced. If you can find a rim off a wrecked example for a reasonable cost, that would be the way to go. Otherwise, you could likely get a full aftermarket set for much less than $1000. It is not a time-critical issue, though, as long as you are aware of it and continue to check the pressure in that tire.
  • fibber2fibber2 Mid Hudson Valley, NYPosts: 3,732
    I think it is likely that you have identified the cause...

    The bead seal is where the stiffened edge on the tire has to make an airtight seal against the inner lip of the metal wheel/rim (alloy or steel). In the old days, or on most bicycles, the air is held in by an inner tube. "Tubeless" technology relies on a rubber to metal seal to do the same. During mounting, a slippery water based glue helps promote the seal, and from then on, air pressure holds it all in place.

    If the wheel no longer has a perfect mating surface, a more potent glue is used. But if everything is too compromised, large changes in shape (think winter) could pull it very slightly apart. Or, it could have even developed a hairline fracture that is giving way over time. If it was visibly damaged in the pothole incident, you should be able to see and identify which one. If it is this rim, you need to replace it, NOW! Please realize that you are playing with fire. A sudden a blowout and crash will ruin your whole day.

    I would expect the cost of an OEM alloy wheel probably runs about $300 - $450. You might be able to find one cheaper on-line. Steel wheels are under $100. A quote of $1000 sounds crazy!
  • fibber2fibber2 Mid Hudson Valley, NYPosts: 3,732
    Hey Wes! I got interrupted, and when I finished and posted, you got there first. Only thing I disagree with is the urgency. Not knowing why it is losing pressure would prompt faster action on my part. Then again he does have TPMS to tell him when disaster is imminent.
  • Thanks for the info. I'll check the number on the tires tomorrow in the morning and see if it's the one that was replaced. We were told that the rim was bent - we didn't see it. The only visible markings we saw on the rim itself was the distinctive pothole marking from when the tire "rubs" against the rim, leaving a black mark.

    I'm a pretty big fan of the OEM rims and I'd prefer to keep them all consistent, though I did see on some websites that I can get them individually for around 125-175.

    On the off chance that this is a different wheel we're talking about, I'm slightly irked at the fact that this is now the second Yokohama I've had an issue with in the last year and a half I've owned the car.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391
    Yeah, I caught that, and it is a good point. I was not considering the possibility of a compromised rim (in other words, the aluminum alloy is cracked). That could definitely give out in a catastrophic manner especially if the leaking is a result of the fatigue growing worse and not a result of winter shrinkage compromising the bead.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391
    edited February 2010
    It may be that the quote you received was for a set of four, not a single rim. I would think the price you listed of $125-175 is much more accurate for a single rim. If you go with OEM, there would be no reason to replace the whole set.

    Of course, if you hit a pothole, wouldn't two tires/wheels be affected (front and back)? Maybe the problem now is from the tire on that side that was not replaced the first time around.
  • I suppose anything's possible. We live in Northern NJ so it's pothole central up here, and the constant snow accumulation & plowing certainly isn't helping them go away.

    My wife and I were pretty stunned when we were told it's typical pothole damage to the tire & rim, because neither of us (she is the primary drive, though I do take it from time to time) recalled any "jolting" potholes like they were describing.

    I'll keep that in mind when I check the tires tomorrow - since rotations follow a standard path, I'll see if it was "behind" the front passenger side that was actually damaged. I'll go dig out my paperwork and find out how many rotations I've done. Pretty sure I've done 2 so far - 7500 and 15000. So, it all depends on where they mounted the replacement.
  • fibber2fibber2 Mid Hudson Valley, NYPosts: 3,732
    It's good that we're 'double teaming' this, as Wes brought up an excellent point to look at the other tire on the same side as that might be the current leaker.

    OEM wheels are way overpriced. I bought respectable looking alloy rims from TireRack for both our Outback and Honda Odyssey with snow tires (I live a 100 or so miles north of you). A set of 4 was cheaper than a single OEM rim for either vehicle.

    Remember a couple of rules regarding Subaru AWD systems:
    1) Rotate front to back - do not cross (I never understood this one, but it's what they say).
    2) Tire circumference must be close to the same all around to not put stress on the center differential. See your owners manual for details. They recommend replacing tires in pairs if there is a substantial difference in thread depth.
  • I'm almost positive that my tires are not directional and follow a standard rotation pattern. Either way, I've done all my rotations at my Subaru dealer nearby, so if that was done in error, it's their problem.

    I'm familiar with the circumference issue. It seems like the spare tire is 16" whereas the other tires are 17". I remember reading the owner's manual when we first got the car and it included instructions on disabling the AWD via a fuse.

    Alright, out to the car now to see the which was replaced.
  • Service Papers in hand and pictures of the tires...

    12/15/08 - 3750 mi service
    04/14/09 - 7500 mi service (Tire Rotation)
    04/16/09 - Passenger Rear Tire replaced (sidewall bubble)
    07/24/09 - 11250 mi service
    11/10/09 - 15000 mi service (Tire Rotation)

    So, looking at the pictures:

    Passenger Front: fdh4 nxf4907
    Passenger Rear: fdh4 nxf4907*
    Driver Rear: fdh4 nxf1908
    Driver Front: fdh4 nxf4907

    So, we can see that the Driver Rear tire is the one that was replaced as this DOT number matches my documentation from the shop where I replaced the tire. Given the 2 rotations, it appears that the replaced tire has followed the typical rotations per the Subaru owner's manual below:


    Whether the dealer actually follows this or not, I don't know. I know that my previous car, a Civic, had a different rotation path even with non-directional tires.

    So, it seems like between the two rotations, the passenger rear went to the driver front, then the driver rear where it is now.

    This means that the tire with the persistent TPMS light was on the Driver Rear at the time the other had the sidewall bubble - on the other side of the car. So, to me, this rules out mauling a curb or the same pothole being the culprit. Again, a lot of this "theory" depends on whether or not they were rotated the way the owner's manual said, but so far, all my rotations have been done by the dealer. And, we ruled out that it's definitely not my new tire that's leaking, which is a relief.

    I didn't see a nail or anything irregular in the treads themselves. Either way I'll have the dealer look at it when I take it in for the 18,750 mi service next week.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391
    Either way I'll have the dealer look at it when I take it in for the 18,750 mi service next week.

    Sounds like a good plan at this point.
  • fibber2fibber2 Mid Hudson Valley, NYPosts: 3,732
    Now there's an interesting fun fact. Subaru has changed it's long standing tire rotation policy and now recommends cross-rotation??? In my previous post, I was advising you to follow what appears to be an outdated set of instructions.

    From my owners manual:
    Tire wear varies from wheel to wheel. To maximize the life of each tire
    and ensure that the tires wear uniformly, it is best to rotate the tires every
    7,500 miles (12,500 km). Rotating the tires involves switching the front
    and rear tires on the right-hand side of the vehicle and similarly switching
    the front and rear tires on the left-hand side of the vehicle. (Each tire
    must be kept on its original side of the vehicle.)
  • pilot1226pilot1226 Posts: 165
    Well, I had the car serviced today. They took the tire off and put it in a tire bath and basically came to the conclusion that there's no air escaping from it. So, I guess that's good news. No nail or any other irregular markings to suggest tire damage.

    The dealer suggested that if I continue to have the issue that I take the car to a tire shop because they have some more advanced equipment.


    Not sure if I asked this above, but is it possible for the Subaru TPMS system to "vent" when the pressure is too high? Reason I ask is because I'm wondering if the recent heat changes in my neck of the woods could contribute to the light coming on. (From weather in the 50's to the 20's).

    Otherwise, I'll have to keep an eye on it. The light came on today while driving it to the dealer for service, but before that, it was good for about 2 weeks without filling it.
  • fibber2fibber2 Mid Hudson Valley, NYPosts: 3,732
    edited March 2010
    Not that I have ever heard. It is simply a monitor system, and not reactive.

    I still think there is a leak somewhere. The problem with a tire bath is that it is a 'no-load' test. Your tire might only leak out a small amount of air during certain dynamic operations, such as hitting a bump or pothole, cornering loads that push against the bead, when hot and the internal pressure exceeds a certain threshold, etc.

    It could be a rim problem, where the tire meets the rim, or the TPMS/valve unit itself.
  • pilot1226pilot1226 Posts: 165
    Okay, that makes sense. So, if I bring it to the shop where I had the one tire replaced (a Yokohama retailer nearby), what kind of test(s) should I be asking for, and what's a reasonable price for it?
  • snowbeltersnowbelter Posts: 286
    I had a slow leak (1 lb/wk) on my 07 and took it to a tire place. They used a water bath and had trouble finding a leak, but finally found a leak around the tiny gasket used to attach the TPMS unit to the rim. They showed me where it was corroded, but not from rust.

    They told me that the material the gasket was made of was reactive to the alloy wheels which caused corrosion and then a leak. They replaced the gasket with one of a different material. They said there was a Bulletin out on the problem.

    I don't remember whether the new gasket was silicone or whether silicone was the one that was reactive to the wheels.

    I later mentioned this to my Subaru dealer, and they were not aware of the problem.

    We've also had slow leaks around the rim due to corrosion from the salt used on the roads here. I don't think your car is old enough for your leak to be due to this problem.
  • pilot1226pilot1226 Posts: 165
    Another winter is here and sure enough this same issue persists. Only in the colder months - when the temperature outside is around or below freezing (32F), the TPMS light will come on affecting the front two tires. This is persistant, despite rotations. I think there may be some type of problem with the Yokohama tire itself?

    When I fill it to 33PSI cold, as the door jamb suggests, it lasts for about 2 days, then the light returns. It seems the front tires will "stable off" around 27PSI and won't lose any more air at this point.

    The rear tires have been a solid 30PSI and don't require any air.

    But, considering all the quarters I've put into air pumps at the local gas stations, I might as well have bought new tires by now.

    Do I have to have the lower-profile tires with the Legacy, or is there a way to get a conventional tire profile for when I get these things replaced?
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