The Old Car Files: TPMS Sensors

 Things have been relatively quiet in the Wisdumb garage recently, though the old Prius seems to have a lot more scratches and dings than it used to.  Hmm. That's a mystery that is stumping me.  Oh, and there is a newly-licensed driver in the house since my last installment, who primarily drives the Prius.  And since there is a newly licensed driver, this was a good excuse to add another pony to the stable, so there is now a 2025 Nissan Leaf SL to use for around-town driving.  The incentives to buy that car were crazy good, so buying it new was cheaper than most of the used ones we were looking at.  Someday I may do a writeup on that car.  It serves its purpose, but has a lot of compromises.  So, I guess I mean that things were quiet in that nothing broke and needed fixing.  Except for a sailboat and trailer that was given to me. Before I get any further off track, today's discussion is about TPMS sensors and the tire places that use them to rip you off.

The little tire sensor exclamation point has been lighting up the Prius dash for a month.  

This can mean either that a tire has low pressure or a TPMS sensor is not connecting.  A manual check of all of the tires showed they were properly inflated, so this was likely the latter.  Given the car is a 2009 and the sensors have never been changed, it is plausible that a battery in one of them had died.  

If you are not an idiot, and if you are reading this blog, I am sure you are not, keeping up with your tire pressures is pretty easy.  You should be able to tell one is dangerously low just by the way the car feels.  You should also be able to see a tire that is low on air.  You do look at your car every time you see it, right?  This bad TPMS sensor, therefore, did not rank highly on my need-to-fix scale.  However, the treads are getting low on the tires, and it is about time to replace them.  After a little research into TPMS replacement by Mrs. Wisdumb, I had to get involved.  The quotes from tire places were pretty ridiculous, considering you can buy a 4 pack of new TPMS sensors pre-programmed for a Toyata Prius for about $30 on Amazon.  For example, one unnamed tire chain quoted $35 (per sensor!) for replacement when we were getting new tires and $70 per sensor to replace them without getting new tires.

I dove into the Internet to start researching TPMS replacement.  What I learned is, you either need a programmable sensor that you program for the car you are installing it on, or a pre-programmed sensor already configured to work with your vehicle.  After that, you need a tool or method to get the vehicle to learn what sensors are attached (and which wheel they are on for vehicles more modern than the Prius).  If you get a programmable sensor, your tool also needs to be able to program that sensor.  From what I found, programmable sensors are much more expensive than buying one pre-programmed for your specific vehicle, so I focused on that method.

The most popular tool I found in my research was the Autel TS508WF.  This lists for $279, but there were new ones for sale on E-Bay for $124.  This device can handle gathering data from the sensors and the relearning for your car.  It can also program Autel generic TPMS modules if you choose to go that route.  So, I could pay $154 for new sensors plus a programmer and be able to deal with TPMS sensor replacement on all my cars now and in the future, or pay $140 one time to have the tire shop replace the sensors and do the relearning.  I think you know which way I'm leaning.  The one problem is, I don't have a machine to get tires on/off the rim.  I asked the tire place if they could just throw in my new sensors when they had the tires off for replacement.  They said "sure".  For $20 per wheel.  That is flat out, blatant highway robbery! 😖😠😡😭  With the tire already off the wheel, it takes less than a minute to unscrew the old TPMS and screw in the new one.  I'm already paying for the hard part, taking the tire off, as part of the tire installation cost.  They are using this as an opportunity to screw me over  on TPMS replacement because I can't do this myself.

I was not defeated, however.  Through further research, I discovered some people have used a jack and the weight of the car to push the tire sidewall down, break the bead from the rim and create enough space to get at the TPMS sensor and replace it.  This seemed fraught with risk- would it work for these particular tires?  Would I be able to get the bead to reseat so I could inflate the tire?  I decided to risk it.  If I did them one at a time, I could always just put the spare on, take it to the tire place and pay them their extortion money to replace the TPMS when they replaced the tires.

Once I had the sensors and the Autel tool, I got to work.  I jacked up the Prius with my floor jack, removed the wheel and let all the air out of it.  I positioned the scissor jack from the prius between the tire and the jack point on the side of the car with the valve stem around the jack area.  I used a block of wood under the jack to distribute the pressure around the tire and to try to prevent damaging the side wall.  This is probably hard to picture from the description, so here is a link to a video showing how someone else did it.  On the first tire, it took me a bit of work to get the bead to break.  I had to move the jack to multiple areas and use a screwdriver to help break the bead from the sidewall.  Once the bead was broken, I still couldn't get the TPMS out.  I needed to use the jack to compress the sidewall further in the area of the sensor to create enough space to get the sensor out.

Removing the sensor and putting in the new one was very easy.  Even easier than shown in the video as I didn't have any problems getting the old sensors out and didn't have to cut them out.  Also, very importantly, all of the sensors I got used a nut to secure them.  

There are some stems that are press-in that have a rubber grommet that needs to be compressed and pushed through the hole in the rim.  These seem much harder to install properly and seem like they could be more prone to leaks, so I advise to stay away from them.

Getting the bead to re-seat on the rim took a bit more work and trial and error.  Unlike in the video, when I added air to the tire, it just leaked out gaps between the tire and rim.  It didn't form a tight enough seal to hold any air.  I eventually learned that hitting the tire with a dead blow hammer on the tread while putting air in the tire would occasionally create a good enough seal that the pressure could start to build and eventually pop the bead back to its fully seated position.  Spraying soapy water in the area also helped to lube things up and facilitated sliding the bead over the rim.  It took about 25psi in the tire for the bead to pop back on, and it did so with a loud pop that will scare you if you aren't prepared for it.

When all was said and done and the tools were packed up, it had taken me 2.5 hours to replace all four TPMS sensors.  The first one took the longest, and I got better at doing the next ones.  If I valued my time, that would be a $32 / hour labor rate  (I traded 2.5 hours of work to save $80).  Not too shabby- more than double minimum wage in MA!  The tire place would have taken 4 minutes to replace 4 sensors when they were doing my tires.  So, $80 / 4 minutes = $1200 per hour for TPMS installation.  I don't even know if the world's most expensive lawyers charge that much!

The final part of the fix was to get the Prius to read the new sensors.  The Autel tool has some guides to step you through the process.  I got hung up the first couple times as it never asked me to connect to the OBD port of the car.  It turned out it was waiting for me to scan the spare tire TPMS (which doesn't exist in this car) and wouldn't continue without it.  I later figured out that if you tried to connect to it and it failed, it would just leave it as an X and then move on with programming the car through the OBD port.  The programming went well.  The Prius doesn't report tire pressures anywhere on a screen.  Maybe it includes them in an OBD readout.  It really just uses them as a go / no go trigger set to a certain pressure threshold to trigger the pressure warning light shown above.  After programming, the light went off, so I believe everything worked as expected and there were no diagnostic trouble codes reported.

I'm now looking forward to using the tool on my newer cars that actually report pressures on an information screen.  Before now, if I were to rotate the tires, the reports would have not shown up in the correct location anymore unless I paid a mechanic to relearn the sensor locations to the car.  With this tool I'll be able to do that myself.