Tire pressure sensors are part of the TPMS (Tire Pressure Monitoring System), also known as TPWS (Tire Pressure Warning System), which has been around for a few years now, but starting in 2008, ALL new cars are required to have a “direct” type TPMS system, with pressure transducer mounted inside the wheel wirelessly transmitting tire pressure data to a control unit as the car is driving down the road, in order to alert the driver if any tire loses pressure while driving.
* Update 2020: cars are not (and were not) required to use a direct system. They were required to achieve a certain level of accuracy, which some manufacturers (like Mazda and Honda) are now doing with modern high-resolution wheel speed sensors. This reduces cost of manufacture, maintenance, and repair, but the systems are pretty finicky and not as accurate as direct TPMS. I can’t say I’m a fan.
It’s time to learn about TPMS systems
Any shop doing tire work, even if it’s just tire rotation, will need to be familiar with these systems before they start rolling in the door. Learning about TPMS on the job will likely lead to dissatisfied customers and potentially costly damage to your customer’s car. I started this article as a memo to Art’s employees back in 2005 but figured I may as well make a web page of it, since the information may be useful to our customers, or maybe even other shops.
TPMS history
First, a little history. Most of the early tire pressure monitoring systems used the anti-lock brake system’s wheel speed sensors to determine if a tire was low on pressure. If a tire is low, the effective circumference is reduced, so the wheel will spin more times than the other three at the same vehicle speed. The ABS control unit would notice this, and turn the TPMS light on. This was an inexpensive system because it used the components already installed on the car. The problem was that it was not very accurate. This type of TPMS is called “indirect” TPMS.
Direct TPMS
The currently mandated “direct” TPMS systems use a pressure transducer mounted inside the wheel to measure the pressure, and send that information wirelessly to one or more antennas on the body of the car. Almost all of the TPMS system manufacturers attach the TPMS sensor to the base of the valve stem, but there are a few that are held to the rim with a band.
Each TPMS sensor must be unique
One of the issues with sending tire pressure information via radio transmission is — How will the TPMS ECU be able to tell which tire pressure sensors it’s supposed to monitor? In other words, when all the cars driving on the road have TPMS, how will the TPMS ECU be able to differentiate between the tire pressure sensors on “its” car and the sensors on the car driving next to it?
Sensor ID
Changing the broadcast frequency might work (like tuning to a radio station), but there is not enough available spectrum to support a “station” for each car on the road. The solution is to give each sensor a unique ID number, which it will broadcast along with the pressure information. Each sensor ID will be “registered” in the TPMS ECU, and the TPMS ECU will only pay attention to signals that start with a registered ID. Those of you familiar with setting up Bluetooth devices are familiar with this process. It’s called pairing with Bluetooth instead of registration, but it’s more or less the same thing.
Registering TPMS sensors
So, if you need to replace a TPMS sensor, how do you tell the TPMS ECU about it? So far, the only solution I know of is using a factory scan tool to initiate the registration process. (A screen from the Honda HDS scan tool is on the right.) However, I’m sure that this will be changing very soon if it hasn’t already. We have factory scanners for Toyota/Lexus/Scion, Honda/Acura, Nissan/Infiniti, Mazda and Subaru, so we have access to the TPMS control units for all the vehicles we repair.
Danger to TPMS sensors during tire replacement
We’ve been fortunate enough to avoid breaking any TPMS sensors so far (more on how to avoid damaging sensors later), so most of what I know about registering a new TPMS sensor is from reading. TPMS registration works in much the same way as registering new immobilzer key. A simple “Simon Says” type operation with the scanner. I’ll pass along a tip I was given for Toyota: be sure to check the current sensor IDs with a scanner and write them down accurately before beginning the registration process.
All sensors must be registered when one is replaced
During the registration process, all sensor IDs are erased, and you’ll need to enter each sensor ID, even the ones for for the sensors that are not new. If you fail to do this, you’ll need to dismount the tire to read the ID number off the side of the sensor according to my dealer source. Although not an official Toyota procedure, a TPMS tool, like the one made by OTC and others (more on the tools later) can be used to retrieve the ID numbers without the tire dismount. It would be easier to just write the numbers down before starting.
TPMS tool
So, what do those little handheld TPMS tools do? I’m sure your tool vendor has been trying to get you to buy one for the last six months or longer, but depending on the makes you repair, you may not have seen the need. Soon you will.
TPMS Tool Purpose #1:
All variants of these tools can “activate” a TPMS sensor. When the TPMS ECU is in “learn mode”, activating the sensors in a predetermined sequence tells the TPMS ECU where the wheels are. The sequence usually starts with the left front wheel and moves clockwise. Learning is necessary on some cars even after a simple tire rotation.
Other cars, such as the Toyotas (so far) do not need a TPMS tool to activate the sensors for any reason. On Hondas, the TPMS tool is only needed for diagnosis, and is not required for rotations or other tire maintenance.
The TPMS tool does only half the job. It will not put the TPMS ECU in the learn mode. Right now, it’s looking like a factory scan tool may be the only way to get some cars into the learn mode. For instance, all Honda/Acura (so far) require the factory scan tool (HDS) to initiate the learn mode. This may be more of an investment than many multi-make shops want to make.
TPMS Tool Purpose #2:
Some TPMS tools will also gather information from the TPMS sensor, such as ID number, tire pressure, and tire temperature. This can be very handy as a quick function-check before replacing tires on a car with a TPMS light on. You can quickly verify the TPMS sensors are working before dismounting the tires, and potentially get blamed for a non-working sensor. Also, might be very handy if you forget to write down the ID numbers before starting the registration process on a Toyota.
TPMS sensors and Fix-A-Flat
Fix-A-Flat and TPMS sensors don’t get along. If Fix-A-Flat is added to a tire, remove it the same day if possible because it will ruin the sensor. When I first heard Fix-A-Flat would damage sensors, I assumed that Fix-A-Flat would clog the sensor’s pressure port: a small hole in the sensor body to let air contact the transducer inside the sensor. However, now that I’ve seen a few fail, I know that Fix-A-Flat causes the sensor to stop broadcasting, and a plugged pressure port would not cause this. I ran across an explanation for this on one of my favorite YouTube channels: Applied Science. In this video Ben Krasnow does a postmortem on a failed TPMS sensor and provides an explanation of what happened to his TPMS sensor after adding Fix-A-Flat to the tire. Check out some of his other videos too. Like I said, one of my favorite channels.
So, what do you need to know before you change a tire on a TPMS equipped car?
1. How to avoid breaking the sensor.
2. The importance of torquing the valve core & sensor nut.
3. How to avoid interference that might block the signal.
4. What service parts you’ll need to have on hand.
5. How to select a tire that will work with the TPMS system.
How to avoid breaking the tire pressure sensor.
1. Remove the valve stem core to relieve the pressure.
2. Remove the valve stem nut & push the TPMS sensor into the tire so it falls to the bottom.
3. Break the beads.
4. Remove the first bead. STOP.
5. Reach into the tire and get the sensor out.
6. Remove the second bead.
7. Reinstall the new sensor with a new grommet, retaining nut, using a torque wrench to torque it to the proper spec.
8. Rotate the turntable so the the valve is 270 degrees from the head on the tire machine. In other words, if the mounting head is at 12:00 the sensor should be at 3:00, assuming your tire machine’s turntable rotates clockwise (most, if not all, do).
9. Begin mounting the tire. There is usually no trouble with the first bead. However, if the rim begins to move without the tire, as sometimes happens with low profile tires — STOP! Back up and try again. If the tire is not fully mounted by the time the sensor comes around to the head, the head may snap the sensor off. With some wheels, head to sensor contact is not possible, but if you are not sure, it’s better to avoid the possibility by making sure they never come close.
10. Seat the beads and install a valve core using the torque limiting valve core tool. If you lose the core that came with the TPMS sensor reseal kit, be sure the the replacement is nickel plated, not brass plated.
Is replacing a tire pressure sensor difficult?
Is this more work than doing a non-TPMS tire? Yes. Is it fair to charge more. I think so. Besides, some TPMS sensor reseal kits, notably for the Honda/Acura TRW sensors, are $25 each! That’s $100 for 4 tires compared to $4 for 4 tires worth of normal valve stems. There’s no way to absorb an increase in cost like that. I’m sure there will be many shops that simply reuse grommets or leave the sensors in place to come up with lower quotes, but I think that it will bite them in the end. Sooner or later, they’ll break enough sensors or have enough leak comebacks. The pressure is being monitored all the time; there’s no way a customer is not going to notice a leak!
Torquing the valve core & stem nut
I mentioned the valve core torque and torquing the stem retaining nut in the tire replacement procedure. TPMS manufacturers are making a lot of noise about this, and I’m inclined to believe it’s important. Every single nut and bolt on a car has a manufacturer’s recommended torque value. However, with the exception of head bolts, spark plugs, and lug nuts, mechanics, for the most part, ignore these and use a combination of common sense and experience to tighten nuts and bolts.
I’d rather not take any chances. A valve core torque tool is only $12, which is, lemme break out my calculator here, mmm, zero dollars more than a standard core tool. A 0-150 inch-pound torque wrench is a bit more, but most mechanics already own one. We bought on just for the tire room to encourage its use. I figure if it’s the closest tool to the tire machine, it’ll be what gets used.
3. Electrical interference
The TPMS sensor signal can reportedly be blocked by electrical interference. I’ve seen advice that non-stock valve caps can cause problems. I’ve also read that cell phones should not be used while registering or activating tire pressure sensors. I don’t really have enough experience to comment on whether interference is a real problem or not, but I suspect that it’s not going to be a problem. There is so much electrical interference in all urban areas all the time, I can’t imagine the engineers didn’t consider it while designing TPMS systems. Here in the shop we have over 20 Bluetooth devices, 6 wireless access points, several 3G cell phones, a TIG welder, a bunch of 3 phase equipment, and lots of fluorescent lights. If a cell phone could really bring the whole thing crashing down, we’ll be doomed for sure.
What parts do you need to stock
We have TPMS sensor service kits on hand, since it’s difficult to tell what kind of sensor the car has until the tire has been dismounted, and that’s not the best time to begin ordering parts. The stem nuts all look pretty similar (at least for the cars we repair) and that is all that is visible from the outside the rim.
The rubber sealing grommets and washers are all different each sensor type. For some sensors, like TRW, you’ll need a whole metal valve stem to reseal. Others, like Schrader and Pacific, just come with a grommet, washer, Schrader valve, cap, and stem nut. Some stem nuts have a built-in sealing surface, others do not. Some washers have a special shape, others are pretty standard. We have 4 kits that look identical from 2 feet away. You have to look really closely to see the differences. Unfortunately, there are not really any great application guides yet, but I’m sure there will be eventually. Meantime, we glued one of each type of kit to a piece of particle board for identification.
5. Selecting a tire that will work with the TPMS system.
Some cars *may* have a hard coded upper and lower tire pressure limit, which could potentially be an issue if you change tire models. For instance, the car may have originally come with a 185/65R15 86H, and a recommended pressure of 30 PSI. On this OE installed tire, 30 PSI would be adequate to maintain the 86 load rating. However, if you replaced the tire with a different model (same size & load rating), 30 PSI may not be an adequate pressure.
Tire manufacturers usually list the minimum pressure to meet the stated load rating in their catalogs. If a tire with a load rating of 86 at 35 PSI is installed on a car that requires a tire with an 86 load rating and a tire placard that says 30 PSI, the tire still must be inflated to at least 35 PSI to be safe, wear properly, and achieve good fuel economy.
I have heard that some cars have hard coded upper and lower limits tire pressure limits (but haven’t actually seen one yet). On a car like this, it would be very important to verify any replacement tire selected would not only meet the vehicle manufacturer’s required load rating, but would also do so at the pressure listed on the placard.
Can you disable the TPMS light?
No. It is illegal for us to disable the TPMS light per FMVSS 138. We’re not willing to pay fines or go to jail to reduce your annoyance. Are you required to fix the TPMS light? No, you’re not. Could we install 24″ non-TPMS rims with 35 series tires on your Corolla. Absolutely not, but not because it’s illegal, it would just look really silly. The law says your warning system must not be disabled, but apparently a working warning light is considered “working”. Goofy rims without TPMS are legal, so long as an auto repair shop does not disable your TPMS light. Can you disable your own TPMS light? It’s fine with me. I won’t tell anybody.
What should you do when the TPMS light is on?
There are two types of TPMS lights for Honda & Acura cars.
One type indicates low tire pressure and requires nothing more than resetting the tire pressure. You should ask your customer if they have already tried this before signing them in. If they haven’t, explain how the system works & send them on their way. You can set their tire pressures for them but bear in mind that if they just drove in, you won’t be setting pressures cold. If the customer says they’ve already tried setting their tire pressure, and they sound like they have been doing it correctly, sign them in for inspection.
The other type of TPMS warning light indicates a problem with the TPMS system. There’s no need to check tire pressures. Go ahead and sign the customer in for diagnosis.
(above) Just fill the tires & drive for a while. If the light goes out, you’re done!
(above) The TPMS system needs diagnosis.
Toyota just uses one light to indicate low tire pressure & system problems.
A steady light indicates low tire pressure. You should advise the customer to fill their tires to the desired pressure cold, then use the TPMS button to reset the system (more on that below).
A flashing light indicates a problem with the system, and the customer should be signed in without a tire TPMS lecture.
The Nissan TPMS warning light works the same as Toyota. A solid light means low tire pressure. A flashing light indicates a TPMS system fault.
Here are some examples of post rotation procedures. Obviously this is only a sample, you’ll need to search for each car’s procedure until you know all of them by heart.
2005 Acura MDX
Rotate tires, set pressures, you’re done.
The MDX has an initiator over each wheel to activate the tire pressure sensors, so it self-learns tire position. Pressure thresholds are set in the TPMS ECU using the HDS scanner. There is no need for pressure or position learning after a rotation.
You must set the pressure according to tire placard unless you change the set pressure in the TPMS ECU with the Honda scan tool. However, if this is done (to accommodate different tires), you should make a note near the placard so the customer does not end up with a tire pressure warning light every time they get an oil change.
2008 Honda Civic
Rotate tires, set pressures, you’re done.
The Civic does not monitor tire position. Pressure thresholds are set in the TPMS ECU.
You must set the pressure according to tire placard unless you change the set pressure in the TPMS ECU with the Honda scan tool. However, if this is done (to accommodate different tires), you should make a note near the placard so the customer does not end up with a tire pressure warning light every time they get an oil change.
2006 Toyota Prius
Rotate tires, set pressures, press & hold TPMS reset button w/ KOEO or READY until the TPMS warning light blinks 3 times at one second intervals.
If the reset procedure is not done, or not done properly, the TPMS light will come on after the customer has driven quite a distance from the shop (~20 miles or so).
When the batteries fail in the tire pressure sensors, the TPMS light will blink for a while after the car is started.
Pressures thresholds are set when the reset is performed. Tire position is not monitored.
2006 – 2007 Toyota Rav4
The only Toyota without a TPMS reset button so far.
Rotate tires, set pressure to placard, you’re done.
Pressure thresholds are factory set and not adjustable. I have not found a way to change the pressure threshold with the Techstream, so replacement tires will need to be carefully selected to meet the Rav4’s load requirements at 32 PSI.
2007 Toyota Corolla
Rotate tires, set pressures, press & hold TPMS reset button with the engine running until the TPMS warning light blinks 3 times at one second intervals. KOEO WILL NOT WORK!!
If the reset procedure is not done, or not done properly, the TPMS light will come on after the customer has driven quite a distance from the shop (~20 miles or so).
Pressures thresholds are set when the reset is performed. Tire position is not monitored.
2004 Nissan 350Z
Set pressure & you’re done. No rotation possible due to different sized front & rear tires. Pressure thresholds are set in BCM. No reset necessary after resetting tire pressure.
2008 Nissan Altima Hybrid
It appears that rotating & setting pressure will be all that’s required. Nissan does use an activation tool and learn mode, but it appears there is no tire position indicator for the driver, so learning after each rotation seems pointless. I may revise this later once we’ve seen a few come in.
A few examples of how to register a new TPMS sensor.
2008 Camry Hybrid
Use the MasterTech or the TechStream to display all registered TPMS sensor IDs
Write down all of the IDs (or print with the TechStream).
Install new TPMS sensor & fill tire
Deflate tire more than 6 PSI in 30 seconds
Use scanner register all sensors
2008 Honda Civic
It’s worth noting that the new Hondas seem to be moving away from their “initiator type” TPMS, that uses an initiator mounted over each wheel to track location. Most of the new models do not have the initiator, so before you register a new sensor, you’ll need to figure out which sensor is bad.
Finding & registering TPMS sensors seem to be the only official use of the TPMS tool for Honda products so far. To find a TPMS sensor, the HDS is connected and put in to the “TPMS Position Check” mode. The sensors are activated one at a time with the TPMS tool. When a sensor is activated, it’s ID is displayed on the HDS so you can determine its location. Once you’ve found & replaced the bad sensor, the HDS is put into the Sensor ID Learning mode, and all 4 sensors are registered (regardless of how many are new).
2005 Honda Pilot
The Pilot has initiators over each of the wheels. If a TPMS sensor is replaced, just hook up the HDS and perform the memorization procedure. The initiators are used to activate the tire pressure sensors, so no TPMS tool is needed.