TPMS Sensor: Tire Pressure Monitoring System, How Does It Work

A Tire Pressure Monitoring System (TPMS) sensor is fitted inside the wheel and is usually attached to the tire’s valve.

It operates as part of an early warning system when your tires lose pressure, giving you as much opportunity as possible to find a safe place to pull over.

This article will explore the different types of TPMS and why all new cars in the US have it.

Use these links to navigate through the article if you wish.

What Is A TPMS Sensor?
Direct vs. Indirect TPMS
TPMS Sensor Problems
TPMS Sensor Cost
TPMS Laws
Other Important Tire Information

What Is A TPMS Sensor?

Within the Tire Pressure Monitoring System (TPMS), there are only a couple of components. These are the ECU, the receiver and the TPMS sensor. These components fit nicely into the existing layouts of the vehicle.

A TPMS sensor is designed or implemented by the manufacturer within the tire valve and attached to the metal of the wheel itself. When a technician fits a tire onto the wheel, they must do so carefully, with the correct technique. Not doing so could easily result in the sensor snapping off and needing to be replaced.

The TPMS sensor’s job is to detect when the tire pressure reduces. It’s a sort of early warning system for punctures, tire wear, and other air leaks.

There are two types of TPMS: direct and indirect. Only direct TPMS requires a TPMS sensor. We will talk about those a little more in the next section because it’s important to see the difference.

Although all direct TPMS systems work marginally differently, most follow similar programs and patterns.

When you fill your tires up with air, use the owner’s manual to ensure they’re all inflated to the correct pressures.
In most systems, you’ll now have to “log” these pressures. Usually, you do this by navigating through the options on the dashboard.
If there’s a tire pressure light on, it may take a mile or two to turn off as the system registers the new pressures. In most systems, the light will turn off immediately.
From here, if any of the tire pressures change (reduce) by more than a certain amount – usually 25%-30% or 6-7 PSI – the tire pressure warning light will illuminate on the dashboard. When this light turns on, it lets you know that at least one of the tires is losing air.

When a tire loses air, it could be due to a puncture, a slow puncture, or a complete blowout. We will look into these in more detail in the last section of this article.

What’s The Difference Between Direct and Indirect TPMS?

As mentioned previously, there are two types of Tire Pressure Monitoring Systems: Direct and Indirect.

In this section, we will explore the difference between them.

Direct TPMS

Cars with Direct TPMS have TPMS sensors.

The TPMS sensor “Directly” measures the air pressure within the tire. The sensor communicates with the receiver via radio waves. The ECU then takes the information from the receiver and compares it to the preset and expected values. If there is substantial inconsistency in these figures, the warning light on the dashboard is triggered.

There are three types of Direct TPMS sensors: rubber stem, clamp-in, and banded.

See the video below for more information on these three different types.

Advantages of Direct TPMS

The main advantage of Direct TPMS is that the car can tell you which tire the problem lies with. It may, for example, tell you that the offside front tire is losing air pressure and needs to be examined. It can do this because the TPMS sensors directly measure air pressures within individual tires.
Direct TPMS is generally more accurate than Indirect TPMS.
Some sensors also give you live, accurate readings of tire pressures, potentially saving you the need to go round with a gauge and check them individually. Of course, that is assuming that all the TPMS sensors are working correctly – it’s often better to be safe than sorry and check the tire pressures manually anyway.

Disadvantages of Direct TPMS

There is a lot more tech involved with Direct TPMS than Indirect. As a result, there’s more to go wrong, and, from that, repair costs might be higher.
When a technician is changing the tire, they have to be more careful than on a regular tire. It’s relatively easy to accidentally rip the TPMS sensor out of place, meaning they’ll have to install a new one for you. You shouldn’t bear this cost; the garage should pay for it. However, it might mean that a simple tire change could occasionally take longer than expected.

Indirect TPMS

Cars with Indirect TPMS, however, don’t have sensors. They don’t measure the precise amount of air within the tire, hence being “Indirect”.

The original type of TPMS seen on vehicles was Indirect.

Instead of having sensors, Indirect TPMS utilizes the pre-existing ABS and traction control systems (you can learn more in our guide on what does traction control do). These systems use the Wheel Speed Sensor – one for each wheel – to operate effectively. They combine this information with other data.

Cars fitted with Indirect TPMS compare wheel speeds across the axle. When the sensors indicate the one wheel isn’t turning as quickly as the other, it’s often due to the one tire having less air than the other. As the tire becomes flat, the frictional force with the road increases, meaning it doesn’t turn as quickly.

When this happens, the tire warning light comes up on the instrument cluster.

Advantages of Indirect TPMS

Indirect TPMS is an excellent, low-cost option, hence why many manufacturers still implement it today. Since it uses pre-existing components, it often doesn’t cost much to develop and, therefore, doesn’t cost much to you as the customer.
Since there are fewer components involved than with Direct TPMS, you’ll save money on potential repair costs too.
You’ll still get a warning light when – potentially – one of your tires begins to lose air pressure. (See the disadvantages list.)

Disadvantages of Indirect TPMS

Indirect TPMS doesn’t tell you which tire is causing the problem.
As often as not, Indirect TPMS can be wrong. Driving conditions, weather conditions, and general technological glitches can all cause the warning light to come on when, in fact, your tires are all fine. Some people might freak out in these situations. Check all your tires manually when the light comes on. Just remember – if your tire gauge says all the tires are okay, they’re all okay.

What Are Some Common TPMS Sensor Problems?

When a TPMS sensor stops working, it won’t measure the tire’s pressure accurately, if at all.

Therefore, the first sign you’ll get of a faulty sensor is when it stops doing this.

Since the sensor is a relatively small piece of kit, there isn’t too much that can go wrong with one. However, some common problems include:

The TPMS sensor battery dying – remember, each sensor connects remotely to the ECU. It wouldn’t be practical to lead a wire into the wheel itself, for obvious reasons. Therefore, the sensors rely on battery power to function. These will, ultimately, eventually run dry. Since the batteries are sealed into the sensor casing, you usually need to replace the whole sensor. You should expect the internal battery on a TPMS sensor to last about 100,000 miles or 5 to 7 years.
Wear on the valve stem – just like a standard tire valve, the stem protrudes from the wheel and tire. The sensor itself is snug as a bug inside the tire, but the valve stem is open to the elements, including moisture, dirt, and salt from the roads. Over time, the stems can become brittle, and metal stems can begin to corrode. Due to being a weakness in the tire, it’ll need to be replaced. As the valve is essentially part of the sensor system, you will need to have the whole TPMS sensor replaced.
An ECU or receiver system malfunction – in this case, the light might be stuck on or keep on coming on, even when the pressures and sensors work just fine. In this situation, it’ll be pretty hard to diagnose the problem without taking the car to a mechanic and having the tires removed. If you have an OBD II code reader, the vehicle may tell you its problem itself. Any issues with the ECU are likely to need special attention from an electrical engineer.

How Much Does A TPMS Sensor Cost?

Although there are many DIY-style articles and videos out there explaining how to change your own TPMS sensor, I wouldn’t recommend it.

TPMS sensors aren’t too expensive – expect to pay around $50 for a new sensor on most cars and up to $100 on luxury models.

Getting your TPMS sensors serviced should only cost about $10 per wheel.

Overall, you probably won’t save much money by doing the work yourself, and you’re much more likely to make a mistake. I would, therefore, recommend taking your car to a trusted mechanic and paying them a reasonable fee.

Do I Need TPMS?

You wouldn’t need TPMS unless your car was outfitted initially with it.

It’s not legal to remove the TPMS sensors. So… don’t.

Read more about the legal history in the next section, “What Cars Use TPMS Sensors?”.

What Cars Use TPMS Sensors?

If you drive an American car built after September 2007, it should have TPMS. Odds are, it’ll have Direct TPMS (and therefore TPMS sensors).

This is actually required by law.

In the year 2000, President Bill Clinton signed the initial version of the TREAD act. This was an act whereby all new vehicles, with a GVWR of 10,000 lbs or less, from 2004 would be required to have some kind of tire pressure warning system.

In the years that followed, the wording was endlessly debated until, on April 8, 2005, the government finally established the law. The NHTSA declared that all new American-made or imported vehicles from MY (Model Year) 2008 should be fitted with TPMS, most of which should be Direct TPMS.

If you like reading, here is the full 172-page document. Happy eye strain.

The new law was to be phased in as follows.

For MY 2006 (beginning September 1, 2005), there had to be 20% compliance.
For MY 2007 (beginning September 1, 2006), there had to be 70% compliance.
For MY 2008 (beginning September 1, 2007), there had to be 100% compliance.

When the law first came into effect, it caused a big stir. Auto shops and tire stores needed to adapt quickly to the new regulations. Replacing a tire could suddenly involve much more than just taking the old one off and putting the new one on. Now, the sensors had to be calibrated and synced to the car’s ECU every time too.

Working with and around TPMS sensors is now the norm.

As you can see from above, any new American-made or imported car from MY 2008 and onward has TPMS. It’s also possible that some earlier cars have it too.

What Else Should I Know About Tires?

What’s The Difference Between A Puncture, A Slow Puncture, And A Blowout?

A puncture is the standard “hole-in-your-tire” type problem. You may have unwittingly driven over a nail or a piece of glass, causing a “puncture” in the tire’s rubber. This hole or cut allows air to escape, and so the tire goes flat.

When you get a puncture, it’s not always immediately apparent from a driver’s perspective. The air could escape slowly or quickly – the faster this happens, the more likely the driver is to notice (assuming the car doesn’t have TPMS). Once enough air has come out of the tire, you’ll feel some drag from that side of the vehicle and heavy steering, especially if it’s one of the fronts.

Either way, the tire(s) will eventually become flat, either while you’re driving or after you’ve parked up. At this point, you may be able to repair the puncture, depending on its location. If you can’t, you’ll need to put a new tire on before driving anywhere.

A slow puncture is essentially the same thing as a puncture. However, the air only comes out slowly – so slowly that it might be virtually impossible to detect without checking your tire pressures over a few hours or days.

It usually comes about when you get either a tiny puncture or a puncture where the foreign body sticks in the tire. A common culprit would be a screw. When you drive over a screw, it can quite easily penetrate the rubber of the tire. The screw’s thread enables it to stick in there. As a result, only a minimal amount of air may leak at a time.

People with slow punctures might notice a flat tire one morning, pump it up, and not see any other problems that day. However, the next morning, it might be flat again.

Like regular punctures, you should repair slow punctures with either a puncture repair kit or a replacement tire.

Finally, tire blowouts are when excessive tire wear or sudden trauma causes the tire to deflate almost instantly. Given that a tire contains highly-pressurized air, a blowout often causes quite a bang and may sound like a little explosion.

Blowouts can be caused by potholes, hitting curbs, uneven or excessive tire wear, or many other similar things. They are the most dangerous as they usually happen while driving. Should a tire blow out while driving, you can easily lose control of the car and it may also flip and roll.

For this reason, it’s imperative to look after your tires and steering.

If you experience a blowout, you’ll need a new tire – no question. And possibly a new car.

What Are Run-Flat – Or Run-On-Flat – Tires?

Different tire companies give these tires slightly different names, but they’re all following the same concept. “Run-Flat” tires (as I’ll be referring to them for now) are designed to hold their structural integrity for a time, even when they’re flat.

Having them fitted means you don’t have to worry about stopping at the side of the road to change your wheel for the spare.

It sounds like a great deal, right?

As always, there are catches.

Run-Flats tend to be more expensive than their “normal” counterparts.
Even though you have Run-Flats fitted, you’ll still need to get the tire changed as soon as possible when it goes flat. Having these tires doesn’t mean you can drive around everywhere with no air in them. As soon as one of the tires develops a puncture, it still needs to go straight to the shop – the same as any other vehicle.
Run-Flats can “run when flat” because they have robust, rigid sidewalls. Having these makes them stronger, yes, but they’re also more challenging to fit and remove. This is because the mechanic and machine need to twist the tire onto the wheel. Because they’re more difficult to fit, there’s a greater likelihood of accidental damage to your wheel.

This type of tire should be used alongside TPMS sensors. If you don’t do this, there might be no way to tell if you’ve got a puncture until it’s too late, making the whole point of Run-Flat technology useless.

You tend to find Run-Flats on more-expensive, “businessman-style” cars, such as BMWs or Audis. However, they’re by no means limited to this sector.

Overall, they’re a tradeoff – instead of getting your hands dirty by the side of the road, you can go straight to the garage, saving you time and effort. As a result, you’ll pay more for the tire and increase the risk of damage to your alloy.

When Can Tires Be Repaired Rather Than Replaced?

You’ve probably heard of tires being repaired rather than having a new tire fitted. In many cases, a puncture repair might cost less than $30 – a bargain, especially compared to a new tire.

Puncture repairs are very reliable and are trusted by professionals.

There are, however, some circumstances where it’s not possible to repair your tire.

When the puncture is in the middle of the tire – where the tread is – someone can probably repair it. This depends on the extent of the damage and puncture type. However, if it’s towards the edges, where the tire starts to curve towards the wheel, it’s not possible.

The general rule is that if the puncture is less than a thumb’s length from the edge of the tire, it can’t be repaired. The “rule of thumb”, if you will. In this case, you’ll need a new tire. If the puncture is somewhere near the edge of the tire, it’s always better to be safe than sorry.

This rule applies because of how puncture repairs work. The technician removes the tire from the wheel and the foreign body in the tire, if applicable. They’ll then drill out the puncture to make a nice, even hole and smooth out the inside of the tire. From here, they’ll pull a puncture repair plug through the hole and seal it on the inside.

Rather than going through it in minute detail, I’ve attached a video from Eric The Car Guy on YouTube. I’d highly recommend watching this video all the way through. It will show you precisely what a professional mechanic does when repairing a puncture in your tire.

Note: the internet is also full of “DIY tire repair” articles and videos. My honest opinion? Just don’t even go there. Ever. Tires are just too essential, and too many things can go wrong if you don’t take proper care of them. Whenever you have any kind of tire problem, take your vehicle straight to the nearest tire shop. The only exception might be if you need a quick fix for the journey to the repair shop.

TPMS Warning Light Facts:

TPMS stands for Tire Pressure Monitoring System, which alerts drivers when tire pressure is too low and could create unsafe driving conditions.
Both overinflation and underinflation can cause premature tread wear and possible tire failure, and it’s important to maintain proper tire inflation.
The TPMS warning light is a horseshoe-shaped light with an exclamation point in the center.
The TPMS warning light comes on when the tire pressure gets too low or too high.
When the TPMS light comes on and stays on, at least one tire is at a low-pressure level and should be manually checked.
Fluctuating temperatures may cause the TPMS light to turn on and off, but tire pressure should still be manually checked and adjusted as necessary.
If the TPMS light flashes for approximately 60 to 90 seconds every time you start your car and then remains illuminated, it means the TPMS isn’t functioning properly and should be inspected.
TPMS does not replace regular tire pressure checks, as it may have limitations in accurately transmitting tire pressure data.
Drivers should check tire pressure once a month and before a long trip or carrying extra load, regardless of whether the TPMS warning light is illuminated.
The correct tire pressure specification can be found on the driver’s door jamb or in the owner’s manual, not on the sidewall of the tire.

TPMS Sensor Conclusion

I hope that, now that we’re at the end of this article, you have a better understanding of TPMS sensors and their role in a modern vehicle.

Unfortunately, these devices do develop problems now and again, just like all technology. However, it shouldn’t cost you an arm and a leg to get it fixed by a professional if they do.

Thank you for reading and check back soon for more interesting articles.

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