Low Tire Pressure Light but Tires are Fine? Unraveling the Paradox!

Cars have become so reliable nowadays that we are indifferent to their complexity. We think that any issue is easily corrected and that every system inside of them will always work just fine.

But the vehicle you drive is anything but simple. In fact, road-going cars are some of the most complex machines, with numerous mechanical and electrical systems that ensure high-speed transportation that is comfortable, fun, safe, and reliable.

Our cars even communicate everything with us. For instance, the Tire Pressure Monitoring System (TPMS) can tell you when one of the tires is underinflated so you can take measures. No more gauges and dirty hands, right?

Well, just like any complex structure, the TPMS system isn't perfect. First of all, it doesn't replace a proper gauge, as it doesn't show the exact pressure reading (except in some expensive premium vehicles). Moreover, it can show false readings from time to time due to various issues.

As a result, one of the most frequent questions I got in my shop was: why is the low tire pressure light illuminated, yet the tires are fine? Let's investigate that false alarm and make sure it never happens again!

Understanding the Tire Pressure Monitoring System (TPMS)

Our vehicles, in the modern era, have evolved into complex, computer-controlled machines. They have been meticulously engineered to incorporate a multitude of sensor-based systems to monitor, assess, and control numerous operational aspects. The aim of these sensors is to improve efficiency and protect the environment, but also improve the safety of our cars.

One such vital system is the Tire Pressure Monitoring System (TPMS). It's a technology that tries to solve one of the biggest issues that plague every car – underinflated tires. Let's dive in and learn more about this system.

1. What is TPMS?

The TPMS is an automated mechanism that observes the air pressure inside the pneumatic tires on a variety of vehicles. It started as a feature in luxury vehicles, but today it is standard on every new car sold in most countries globally.

The TPMS oversees the pressure inside every wheel using sensors and then alerts the driver if the pressure falls below a certain threshold. Most TPMSs alert the driver via a low tire pressure light, but some more advanced systems do that via a digital gauge or a pictogram.

The Tire Pressure Monitoring System plays a crucial role in modern cars, as it not only makes them safer but also more efficient and better to drive. Moreover, it ensures even wear across the tread, meaning it saves you money on tire changes.

Thanks to the system, even novice drivers will know when one of the tires has lost pressure and can then act accordingly.

2. How Does TPMS Work?

Before we dive into the low tire pressure light on but tires are fine, it is essential to understand the operational dynamics of the TPMS. Depending on the vehicle make and model, there are two predominant TPMS technologies: direct and indirect.

• Direct Tire Pressure Monitoring Systems

The direct system is much more prevalent in vehicles produced in the last decade. It uses battery-powered sensors built into each wheel that constantly monitor the tire pressure. These sensors send that information to the ECU, which, if necessary, notifies the driver of the low pressure.

However, these systems aren't built to show small discrepancies in tire pressure. In most cars, they only engage the low tire pressure light if the pressure in one or more tires drops 25% below the recommended level. This is done to avoid false alerts from changes in outside temperatures, as pressure increases with an increase in temperature.

• Indirect Tire Pressure Monitoring Systems

Indirect TPMS doesn't use any sensors and instead relies on the vehicle's antilock braking system's wheel speed sensors. Namely, if the tire pressure falls below a certain baseline, the tire will roll at a different speed than other tires because it will have a smaller diameter. When the system detects such changes in rotational velocity, it will notify the driver with the TPMS light.

Both systems serve the same purpose, though the direct TPMS is more accurate. Still, the complexity of the direct TPMS and inaccuracy of indirect TPMS can cause anomalies, like the one where the low tire pressure light stays on despite the tires seemingly being adequately inflated. It is an issue that happens very frequently and leaves drivers confused. Fortunately, the fix is usually quick and straightforward!

Reasons Behind the Low Tire Pressure Light Activation

Despite the advanced technologies in cars today, they aren't impervious to errors, and that's particularly true for systems driven by computers and sensors. That is especially true with the Tire Pressure Monitoring System, which is very sensitive and error-prone.

Luckily, such an occurrence doesn't necessarily imply a serious problem. Let's investigate some of the common reasons that can lead to this scenario.

1. Changing Weather Conditions

The weather always plays tricks with your tires, and that includes the TPMS. Any change in ambient temperature will cause the pressure inside the tire to increase or decrease – that is simply how our universe works. Namely, the air inside the tire expands when heated and contracts when cooled, which also changes the pressure.

To put it into perspective, for every decrease of 10 degrees Fahrenheit (approximately 5.5 degrees Celsius), tires could potentially lose around 1 psi (approximately 0.07 bar) of pressure. So, a big temperature drop, which has happened a lot lately, can cause the TPMS light to illuminate.

But this can also happen overnight. For instance, if you drive during a hot summer day, park your car in the garage, and the ambient temperature has fallen dramatically during the night, it can also illuminate the low tire pressure light.

When this happens, driving for at least 5 miles will be enough to warm up the tires again, and the TPMS light should be off.

2. TPMS Malfunction

Despite the best engineering efforts, electronic systems, including TPMS, aren't immune to glitches and malfunctions. A persistent low tire pressure warning despite well-inflated tires could indicate an issue with the system itself.

In my experience in the shop, the most likely culprit is a bad sensor in one of the tires. These sensors usually last for 7-10 years. If they are past their lifespan, the TPMS light will illuminate because the ECU won't receive any values from that particular wheel.

Unfortunately, the low tire pressure light can't tell you which sensor is faulty, meaning you will either need to bring your vehicle to a professional technician or connect an OBD2 tool to find the problematic sensor.

Either way, TPMS sensors aren't user-replaceable, as you will need to remove the tire from the wheel and reprogram the whole system after placing the new sensor. For that, you will need a more expensive OBD2 tool or the automaker's special service software.

3. Other Possible Issues

While temperature fluctuations and TPMS malfunctions account for the most common scenarios, several other potential issues could trigger the TPMS warning light.

For instance, you might have a very slow puncture on one of the tires, which is usually hard to detect. Still, it can cause a 25% pressure drop, which would then illuminate the TPMS light. Other small leaks, like a leaking tire valve, can also cause the low tire pressure light to illuminate.

However, in some cases, recent tire changes can also cause errors in the system. Namely, the technician needs to correctly synchronize the new tires/wheels with the system, and if he/she fails to do so, it could also illuminate the TPMS light.

Even something as simple as a missing or loosely fitted gas cap can cause the system to trigger the warning light, as it might interpret it as a leak in the tire pressure.

Confirming Your Tire Pressure is Fine

Before you even come to the conclusion that the TPMS provides you with a false reading, you need to make sure that the tire pressure is as recommended by the automaker. Do this with a special tire pressure gauge, as the tire might look fine from the outside yet still lack proper pressure.

1. How to Check Tire Pressure

Checking tire pressure is a simple and straightforward task that, frankly, anyone can do, even people that never touched a screwdriver in their lives. Still, you need to follow some instructions to ensure correct readings.

The first one I will give you is to always check the tire pressure on cold tires that weren't driven for at least one hour, but three hours would be even better. Remember the fact that the tire pressure increases with temperature? Well, due to friction, your tires heat up while driving, which causes an increase in pressure. If you measure at that moment, the pressure might be fine, but it will decrease once the tires cool down.

After you make sure that the tires are cold, follow these simple steps:

• Check the recommended tire pressure for your vehicle: you can find the information on a sticker on the driver's door jamb in most vehicles, though it can also be attached inside the glovebox. If you can't find the sticker, check the owner's manual, where you will surely find the manufacturer's recommended pressure.
• Remove the valve cap: you will need to do this in order to attach the pressure gauge.
• Attach the tire pressure gauge to the valve: make sure that the pressure gauge is correctly attached without any hiss. The gauge should have a locking mechanism to keep it locked to the valve, but if that thing doesn't work well, press with your hand and keep it firmly in place.
• Compare the reading with the manufacturer's recommendations: you should get a reading in either psi, bar, or kPa, depending on the region (though most gauges have all possible metrics built-in). Then, check if the reading on your tire pressure gauge matches the recommended tire pressure for your vehicle. If it's low, that could be why your TPMS light is on. If the pressure is as recommended, then the problem could be a malfunctioning TPMS sensor.

2. Verifying TPMS Sensor Readings

Once you've confirmed that your tire pressure is within the recommended range and your TPMS light is still on, you may be dealing with a faulty sensor since most vehicles today use a direct TPMS.

The thing is, you can't really know which sensor is the culprit, as the low tire pressure light illuminates equally for each wheel. Some luxury cars will show you which wheel provides an incorrect reading, but they are far and few between.

To verify if your TPMS sensors are working properly, you will likely need to visit a professional mechanic or a tire shop. They have special tools that can read the TPMS sensor signals directly, allowing them to diagnose if the problem is a faulty sensor.

Furthermore, the technician can install the new sensor and then connect it to the ECU.
However, you can also use an OBD2 tool. Some modern OBD2 tools can also read the TPMS data, though not all of them, so be sure to check whether it has that feature before splurging the cash. With that said, the correct tool will show you which sensor is problematic.

Moreover, some OBD2 tools will even let you connect the new sensor. Namely, just installing the new sensor won't solve the issue – your will need to "tell" the ECU that there is a new sensor so they can establish communication.

How to Troubleshoot the Low Tire Pressure Light

You can troubleshoot the TPMS yourself by looking for a few potential issues, like resetting or recalibrating the TPMS and inspecting for sensor damage. However, if you are not well-versed with cars, I recommend seeking the help of a professional mechanic.

1. Resetting or Recalibrating the TPMS

Once you confirm that the tires are properly inflated (when cold!), you can reset the TPMS. In most cars, the system will reset/recalibrate after driving for a certain duration (usually a few miles at a higher speed).

However, others require manual recalibration via the vehicle's dashboard controls or infotainment system. Either way, I highly recommend checking the correct method inside the owner's manual.

2. Checking for TPMS Sensor Damage

Inspect the area around your tire's valve stem for any signs of damage. This is where the TPMS sensor is located in a direct TPMS, so damage to the valve might also mean a damaged sensor. Since TPMS sensors are very sensitive, minor impacts can damage them and cause the TPMS light to illuminate.

However, in my experience, the sensor is most often damaged by mechanics. This can happen to everyone, though a quality tire repair shop usually has more experienced technicians. It usually happens when the tire is changed, as then the sensor is exposed.

3. Seeking Professional Assistance

Though I personally am a DIY fan, I understand that most people don't find repairing their cars convenient. If you are one of those people, I highly recommend visiting a tire shop after you checked the tire pressure, recalibrated the TPMS and the tire pressure light stayed on. Tire shops have the specialized equipment necessary to diagnose and fix potential issues with your TPMS.

Preventive Measures to Avoid Future False Alarms

The tire pressure light usually illuminates if the wheels/tires don't receive the proper care. Namely, if you inspect your wheels and tires regularly, you can find potential issues beforehand. Moreover, technicians will tell you if the sensors are about to go off due to change, though you will need to maintain your vehicle regularly for that.

One of the most effective preventive measures is to perform regular maintenance and inspections of your vehicle's tires and TPMS. This includes regularly checking your tire pressure, even when the low tire pressure light is not on. The more familiar you are with the typical pressure levels in your tires, the more likely you are to notice any discrepancies that may signal a problem.

But checking the valves/TPMS sensors for damage and corrosion will also ensure that you don't get false readings from the system. Unfortunately, a proper inspection warrants the removal of the tire because you can't really see the sensor from the outside.

Tire shops will let you know whether one of the sensors is damaged if you have two sets of tires (summer/all-season and winter) and change them every year. However, if you are only using one set, the technician can only inspect the sensors once you replace the tires, which usually happens every 3-4 years.

Conclusion

The tire pressure light is only there to warn you of a possible pressure loss. Thus, if you check the pressure on all tires and everything is fine, it isn't an immediate safety concern. 

However, that doesn't mean you shouldn't tackle the issue as soon as possible. Sure, the light might not bother you, but a faulty TPMS will not alert you of any subsequent pressure loss, which can significantly obstruct the safety of your vehicle. Not to mention, it can also lead to tire/rim damage due to underinflation, which can be expensive to repair.

So, just like with every other issue in your car, tackling it on time is crucial to ensure a safe ride with low running costs.