OT - TPMS

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Thought this, clipped from Subaru ng, would be of interest.

"It's up to you to define the range in variance of pressure from the recommended pressure. Whether something tells you all the time or you keep rechecking yourself using a digital pressure guage, the instruments can't help whether you're obsessed or not.

Checking every 2 to 4 weeks is obsessive unless you know you have a slow leak. If you're losing several PSI every month, maybe it's time to start replacing the tire valves. They're cheap. Tire pressures will vary by about 2% for every change of 10 degrees F. Driving ups the temperature due to the heat produced by the changing flex in the tire for the contact patch. Recommended tire pressure is listed on the decal on the driver's door. Do not use the maximum pressure listed on the sidewall of the tire. However, an underinflated tire will flex more which means it is fail sooner. When measured cold, refill before there is a loss of 5%. 2 PSI doesn't sound like much except a tire has a decent volume of air inside of it (and why those piddly mini-piston air compressors take so damn long to up the pressure in a tire, even for just a couple PSI).

TPMS (Tire Pressure Monitoring System) sensors were required after 2008 due to the TREAD Act (Transportation Recall Enhancement, Accountability, and Documentation act) enacted in 2000. TPMS isn't required to activate an alert icon in the dash until tire pressure is *below* 25% of the manufacturer's recommended tire pressure (so they won't alert when consumers overinflate their tires). To me, that's too much leeway. Since tire pressure, when inflating, is measured with the tire "cold", and because tire pressure increases when the tires are used (driving), a quarter less of pressure seems a bit high. When using the tires, the pressure will go up, so the 75% minimum threshold is farther away.

There are 2 types of TPMS: indirect and direct. Indirect doesn't measure tire pressure. Instead it measures the anti-lock brake system's wheel RPM to detect if one, or more, tires are rotating faster than the others. The underinflated tire has a slightly less circumference making it spin faster to maintain the same RPM as for the other tires. Indirect TPMS has a large margin of error. I did not research which type of TPMS is used in your car.

You said Touring for the model, but not if for the 2.5i or 3.6R engine (and parts might differ). All I did was a search on "tpms" at oreillyauto.com, put in 2018, Subaru, Touring, and selected 3.6R, and later changed to 2.5i. The TPMS they showed there are the next type described: direct.

Direct TPMS actually measure the air pressure, and are accurate to within 1 PSI. A wireless remote gauge is mounted to the inside of the wheel or to the inside of the tire valve. That means there's a battery in the gauge. You can still replace a leaky tire valve stem since that is in the stem outside of the gauge component. The CR2032 coin cell batteries are obvious a chemical device, so they go weak over time. Expected lifespan is 5 to 10 years (that's quite a range difference) or

100K miles.

I doubt you'll find shops that will replace the battery, plus they'd be charging for the labor time. They'll just replace the entire wireless remote gauge assembly. The part will cost $65, or more, at the parts shops, and a hell of lot more at the dealerships. You need to replace all of them at the same time, plus there is the labor charge for tire removal, gauge install, tire install, and balancing. TPMS replacement can run from $450 to $2000, and you're doing that every 5-10 years.

Consumers often don't realize there is a maintenance cost to TPMS, just like owners of those hybrid cars don't realize all those gas savings are eaten up by having to replace the LION battery every 5 years adding $1000 to $6000 to the 5-year cost schedule for their oh-so-efficient hybrid car. For Toyota Prius, battery replace costs $2400, and that's before adding in the labor cost. A Subaru XV Crosstrek hybrid battery will cost $4500 (dealer cost), and that's just for the part, no labor. Consumers rarely calculate the TCO (Total Cost of Ownership) over, say,

5 years when choosing a car. Then later they get the nasty surprise. Don't know (well, haven't searched) if anyone has done a survey on the cost of LION car batteries, even with recycling, to see if consumers are actually saving any money and the effect on the environment.

Another problem with direct TPMS is that you can register only one set of TPMS sensors at one time with the car. Some folks run 2 different sets of tires (either swapping the tires, or having them on wheels so the swap is just replacing the tire+wheel instead of going to a shop to put different tires on same wheels and get a rebalance). You might have one set for summer and another with snow treads for winter. When you take off the set with the registered TPMS, the dash light comes on to bug you. Having to install another set of TPMS sensors in all 4 alternate wheels+tires can get pricey, and adds to the cost of those winter set of tires.

That's one way by disabling the TPMS gauge light. His technique is pretty much like putting black tape over the light. There probably is a guide on how to reset the TPMS to register a new set in your winter tires. The following shows how on a Forester:

Here's one showing a guy using a TPMS sensor programmer tool (about $150, or much more):

He uses the tool not only to program a replacement sensor, but get the ID (from the old one). This guy scanned the old sensors (so their batteries had to be okay), and programmed the new sensors to clone them to look like the old ones. No having to register the new sensors with the car.

You need the tool (the Matco here costs $250) to scan the TPMS sensors and have the car relearn the IDs for the TPMS sensors. After the sensor scan, you use the ODB port to program the ECU to recognize the sensors. You'll have to do this reprogramming every time you switch from your summer to your winter tires, and visa versa. A pain, and you'll need to by a TPMS tool. Maybe there's a way to do it with the buttons in the car, like on the steering wheel, but I didn't bother looking that up."