TPMS Adventures: new lessons learned and insights gained

After 13 years on the road, no surprise: my 2011 Gen 3 TPMS sensors have been going out. Here are lessons I've learned:

Sensor Sourcing
* YES, third party 315MHz sensors can be entered into a Prius and work well. I purchased MarsFlex PMV-107J (listed as compatible for Prius 2006-15).

Install and Set The Easy Way
* Discount Tire (nationwide in the USA) will separately replace a TPMS you supply for 2 x $5 tire dismount/remount fee. So, US$10/tire. (Free while replacing the tire.)
* Discount Tire will also (at no charge) use their (BarTec) TPMS scanner to grab all four ID codes and push them into your vehicle. NICE! Here, you just go to the "Air Check" station and a rep will take care of you. It's actually quicker than getting tire pressures set up properly

Why Is It So Hard To Get Sensors To Be Recognized?
* Is it on #1? First, understand that modern TPMS turns off when there's no extra pressure (ie not installed) and/or the sensor has not moved in a while. This is why the battery can last so long. (Used to be a strong binder clip would be seen as pressure. No more.)
* Is it on #2? How else can a TPMS sensor be activated? The TPMS standard incorporates an "activation" signal, a simple 125kHz RF wave. Your Prius sends this signal when started. If monitoring with a good OBD2, you can watch the sensors go active! Appropriate TPMS scanners (like the Bartec and many more) can also do this. But just because it's active doesn't mean it is recognized by the car...
* What's The Code #1? If your car doesn't know the ID codes of the sensors, it thinks they don't exist. First discovery: not everyone is diligent about ensuring the printed ID hex code matches the ID burned into the device! One of my four brand new MarsFlex sensors has outside ID DA9D2FD but inside is actually E816459. Without a high end scanner you'll never figure that out on your own.
* What's The Code #2? My high end OBD2 reader -- OBDlink LX -- reads the wrong code from the Prius ECU for 3 out of 4 of my TPMS sensors! The numbers are off by varying amounts -6 to +7. At first I thought they were programmed wrong. Nope. Just bad OBD2 firmware/software. Toyota Techstream (with VXdiag for me) had it right. A source of confusion at least.
* Good connection? Even with Techstream, if using an old/slow computer or slow link, it may not read/write reliably and you'll have a terrible time getting the codes entered. Doing it manually really isn't hard with a reliable link: Techstream/Prius gives you 300 seconds to enter four codes. Not that hard.

How Accurate Is TPMS?
* The TPMS standard specifies pressure as 2.5 * (an 8 bit unsigned number) in kPa. So if you read the pressure in kPa it will jump 2.5 kPa at a time. In PSI, that's almost +/- 0.4 PSI
* The sensors aren't all that accurate either. I've not seen an actual spec, but various tests show variance of up to 3% of the reading. At Prius pressure of 38 PSI, that's another +/- 1.14 -- so now we're at +/- 1.5 PSI
* Then there's the impact of temperature. From freezing to hot, that's another few PSI.
* Speaking of Temp: TPMS reads temperature as (Celsius degrees) = ((unsigned 8 bit number) - 40)

TPMS Battery Life?
* My first set lasted 12+ years.
* Recent verrrry cold weather made the last two look dead (-40C temp reading, and no pressure)
* When it warmed up, one of the two stayed dead, the other revived.

What Else?
* Ignoring the fact that it gets the TPMS ID code wrong, the OBDlink app with Toyota option easily connects to TPMS and can dynamically display all TPMS data. Nice, easy. I'm sure other apps/OBD's do that as well.
* NOTE that you can't necessarily tell the difference between a non-sending TPMS and a working one, while the car is in operation. The ECU remembers the previous reading for a long time!

Should we care? TOTALLY up to you. Mendel gave up a long time ago on TPMS. Me? I drive in places that seem to generate slow leaks (nails, screws and more) so I appreciate knowing immediately when a tire is going bad. It can be VERY dangerous to drive on almost-flat tires.

 

Makes sense. The actual sequence: 1) the tool is held to each tire, and reads the ID on that TPMS sensor (usual sequence: FL, FR, RR, RL but it doesn't really matter); 2) Plug the tool into the OBD2 port and it will upload the four ID's into the car.

 

Right now, you can do it for~ $20 a tire: $40 for a set of four sensors, $10 per tire to have them installed, and no charge for sensing the ID's and pushing them into your car. Doesn't get much better than that.

This also means that you can swap Winter / Summer ID's at no charge as well, as long as every wheel has a sensor in it. :-D

 

There are three separate elements to vehicle TPMS:

1. A control unit ("TPMS ECU") with RF transmitter/receiver and wired connection to the vehicle internal communication
2. Sensors in each tire
3. Some kind of built in display (on Prius it's just an LED icon in the dash) that's driven by the ECU (#1 in this list)

Since TPMS is not required in Japan (nor Canada last I checked), at least some of the above may be missing.
#1 above may not be easy to add, but should be easy to detect with Techstream or a good OBD2 like OBDlink LX. (If those can't find the TPMS system at all, it's most likely missing. See below on that. )
#2 is easy to add as I described above
#3 has many options if missing, depending on what you prefer...

If you don't need hardware in the car itself, a good OBD2 app or Techstream can read the data whenever you like. No dash LED required.

If you want a display, you can do better than the good/bad LED indicator. There are displays available (and even entire independent TPMS systems)