What is a TPMS Sensor & How Does It Work?

Tire pressure has a very real effect on vehicle handling and braking, as well as degrading fuel economy due to heightened rolling resistance. A tire that’s underinflated by as little as five PSI can cost one or two more miles per gallon, and an underinflated tire has sidewalls that flex too much, making handling sloppy and dangerous. In addition, the added rolling resistance and friction can cause tires to overheat and wear more quickly, making a blowout at highway speed a very real possibility. It’s estimated that as much as ⅓ of the cars, trucks, minivans, and SUVs on the highway are running on underinflated tires. By some estimates, tire failure due to underinflation leads to 40,000 accidents, 33,000 injuries, and 650 deaths per year.

What is TPMS?

A TPMS system senses air pressure in tires in real time, and sends an alert to the driver either in the form of a dashboard warning light, a pictogram, or a gauge. TPMS systems first debuted on some luxury and performance vehicles in the 1980s or early 1990s, before widespread adoption.

Beginning in the early 2000s, TPMS (Tire Pressure Monitoring Systems) became mandatory on new vehicles. A tire that’s low on air will trigger a warning light on the dashboard, giving drivers a heads-up for a potentially dangerous situation.

How does TPMS work?

There are a couple of different designs for TPMS systems, but they all have the same purpose: alerting a driver to a tire that’s low on air pressure, helping to avoid a potentially dangerous situation. Automakers and regulators knew that drivers don’t perform tire maintenance enough and, prior to TPMS, a driver would have to check the inflation of each tire individually using a tire gauge. Air molecules can migrate through the rubber of a tire, and a tire can lose as much as 2 or 3 psi of inflation in a month’s time through normal wear and use.

Types of TPMS

Roughly speaking, there are two different styles of TPMS systems:

Indirect TPMS is the older, simpler design and detects the rotational speed of each individual tire, on the principle that an underinflated tire will have a somewhat smaller diameter and will spin faster than the other tires on the vehicle. With some vehicles, this is tied in with antilock braking systems (ABS) and electronic stability control (ESC), both of which also monitor rotational speed of each wheel. Direct TPMS uses a sensor on each individual wheel, usually integral with the valve stem. These sensors send a radio signal to a unit that then sends the information to the vehicle’s computer. Direct TPMS also has the advantage of being able to monitor pressure when the vehicle is stationary.

The benefits of TPMS

The main benefit of TPMS is that it gives drivers important information on tire pressure, letting them know that a tire is low on air (and possibly avoiding a dangerous situation). As noted above, improperly-inflated tires can compromise handling, braking performance, and fuel economy, and can lead to tread separation and blowouts at highway speed.

Safety

Think for a moment about riding a bicycle with underinflated tires. The bicycle’s handling will be mushy, with poor braking performance and cornering that’s unpredictable and sloppy. Now think about how that would affect the handling and braking of a 4,000 lb vehicle! It’s important to remember that the casing of modern automotive tires won’t deform and start to “look flat” until it’s drastically underinflated, so it’s important to get an accurate reading of tire pressure.

Tire wear

An underinflated tire will wear at the edges and shoulders as its footprint on the highway is distorted by the vehicle’s weight (it’s worth noting here that this kind of wear will void a tire’s tread life warranty). Worse yet, though, an underinflated tire generates more heat, which can destroy the integrity of the tire itself, causing steel belts and fabric layers to wear prematurely and possibly leading to tread separation and total failure of the tire.

Fuel efficiency

Going back to our bicycle example, think about riding a bike with low tires – it’s not unlike trying to ride through sand or wet cement. It’s the same thing with your car’s tires, as the added rolling resistance and friction of underinflated tires cause the vehicle’s engine to work a lot harder to get down the road, hurting fuel economy.

The drawbacks of TPMS

Each design of TPMS has its own pros and cons.

An indirect TPMS system is simpler, but can only sense tire pressure when the vehicle’s in motion and can’t get a specific reading of the pressure in that tire. In addition, the sending units on each rim of a TPMS system should be reset after a routine tire rotation or wheel balancing, and indirect TPMS on older vehicles can’t alert the driver to which specific tire is low.

Direct TPMS systems are more complex with more parts, and can’t register a problem unless the tire is 25% low on inflation. Direct TPMS systems use a battery on each wheel sensor, which will eventually discharge and need to be replaced over time – in addition, some users report that valve stem corrosion can affect TPMS sensors. The sophistication of direct TPMS systems also means that resetting and synchronizing the sensors can be more complex, requiring special tools and training.

Some reports suggest that a tire sealant like Fix-A-Flat can damage a TPMS sending unit. Truthfully, though, if you’ve experienced a flat tire and had to use an aerosol sealant you should have that tire checked or replaced anyway, which is a good time to check on the sensor. The industry’s rule of thumb is that the lithium-ion batteries in TPMS sensors typically have a life cycle of five to ten years, and these batteries are not replaceable. Some have noted that cold-weather driving can be easier on the batteries of TPMS sensors, while lots of stop-and-go driving can shorten battery life.

False alarms

False alarms from TPMS systems can be fairly common, especially on an older vehicle that has sensors that haven’t been maintained well. A dead sensor can register a code with the vehicle’s computer, lighting up the TPMS indicator on the dashboard when the tires might be fully inflated. With either type of system, ambient air temperatures can lead to a false alarm; air is a gas, and gasses will expand with heat and contract in colder weather, leading to a low-pressure reading.

Difficulty checking tire pressure

Checking tire pressure can be inconvenient, and the accuracy of different styles of tire pressure gauges can vary. The gauge that’s built into the air hoses at gas stations can be pretty inaccurate; we’d recommend pencil-style or dial-type gauges, although the modern digital tire pressure gauge is probably the best.

How to maintain your TPMS

Fortunately for drivers, TPMS systems are pretty reliable and maintenance-free. Your tire shop, however, should check/service/replace TPMS systems when it’s time to:

• Rotate tires
• Balance wheels
• Install new tires

You should also consider having the sensors checked after a hard hit on a pothole or a curb (which could be enough to skew your vehicle’s wheel alignment anyway). With many vehicles’ TPMS systems, the warning light on the dashboard should flash to indicate damage or failure of a component, while a continuous light just means a low tire.

Some vehicles have a TPMS reset button feature on the dashboard. If the vehicle’s TPMS light stays illuminated after inflating tires to their correct pressure, the driver should:

Drive at 50 mph for about ten minutes, which should let the sensors reset themselves and the processor relearn the readings from the sensors. If the system doesn’t reset and the TPMS dashboard light is still illuminated, turn the ignition key to the “on”position but don’t start the engine. Locate the TPMS reset button and hold it until the indicator lamp blinks three times. Release the button, start the engine and let it run for about 20 minutes to complete the reset/relearn process.

Regularly check tire pressure

While TPMS systems can alert you to a low tire, there’s no substitute for actually checking tire pressure once a month. Don’t go by the pressure recommendation that’s stamped on the tire’s sidewall; refer to the manufacturer’s sticker that’s usually on the door frame. Check the tires when they’re cold, before driving, and bear in mind that there can be a fluctuation of one or two PSI for every ten degrees of ambient air temperature.

Keep your TPMS sensors clean

For most drivers, the TPMS sensor is out of sight, out of mind. They’re not easy to get to unless the wheels are removed, and sometimes you can’t even get to them unless the tire is off the rim. Cleaning the sensor should be part of any routine maintenance at a tire shop, though, including tire rotations or wheel balance procedures.

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