If you’ve ever been in the situation where you wondered, “Why is my tire pressure light on when my tires are fine?”, you’re definitely not the first.
The TPMS (or Tire Pressure Monitoring System) uses sensors on the vehicle to determine when a tire has low pressure and alerts the driver with a warning light so the tires can be checked out. However, there are several situations where the low tire pressure light turns on when nothing is wrong.
How Does TPMS Work?
There are a couple of ways that the tire pressure is measured and sent to the ECU in order to signal the driver. The tire pressure is measured independently for each wheel, and some cars will even tell you each individual tire pressure right from your instrument cluster.
The direct system is used in the majority of cars. Each quadrant has a physical tire sensor or transmitter, often attached to the inside part of the valve stem (where the air is supposed to go in and out of the tire).
Indirect TPMS monitoring uses wheel speed sensors to give information about the rotation of the tires. The ECU analyzes it and sends a signal if one of the tires has low pressure, as it will rotate differently than the others.
What Happens When the TPMS Light Turns On?
When the car starts, the TPMS light will normally illuminate for a second or two. This indicator often looks like a tire viewed from the front with an exclamation point inside it. Sometimes it will simply say “TPMS”.
If a tire has low pressure, the dashboard TPMS light will illuminate. It may flash or stay on. If you see this while you’re driving, pull over as soon as it is safe to do so, and check the tire pressure on all wheels. Fill each tire to the pressure specified in the manual or the information panel on the inner door jamb.
You can inflate the tires using a portable air compressor, jump pack, or tire inflator. Make sure you check the tire pressure every 10 seconds or so to ensure you aren’t overinflating the tire. An overinflated tire can be just as dangerous as an underinflated one.
Once you are certain that all tire pressures are set correctly, start the car again and continue on your way. Some TPMS lights will turn off after a few miles, giving time for the ECU to figure out that everything is fine.
If, however, you drive for more than about ten minutes and the light is still on, the sensors may need to be reset or recalibrated. It’s important to figure out why the tire pressure light is still on before resetting it to avoid potential safety issues or vehicle damage.
Causes of TPMS Light
If the tire pressure light is still on after you’ve driven for several minutes and made certain the tires are all at the correct pressure, one or more of the following may be the culprit.
1) Malfunctioning TPMS Sensor (Direct Systems)
Physical sensors are prone to damage and also have a lifespan of about 5-10 years due to the onboard battery in each sensor. If a battery is at the end of its life or if there is age-related wear and tear to the gaskets, seals, valve caps, or core of the sensor, it can malfunction.
Another thing that can cause damage to the sensors is if a substance other than air or nitrogen is in the tire. Flat-repair sealants are one of these to avoid in tires with TPMS sensors, and tire shops often recommend replacing the sensor if a tire flat was repaired with Fix-A-Flat or something similar.
2) Faulty Wheel Speed Sensor, ABS, or ECU (Indirect Systems)
The wheel speed sensors are a part of the ABS (Antilock Braking System), so if either of these or the ECU putting the information together are damaged, an signal can be sent to warn the driver of a problem that doesn’t exist.
Read Also: Causes of a Traction Control Light Coming On
3) Ambient Temperatures Changes
Cold air is more dense, so measuring or filling the tires on a cold morning to the correct pressure means that they’re overfilled after a while of driving or later on in the day when it’s warmer.
Conversely, if you last checked the tires on a warm fall day and they were fine, a sudden cold spell can make the tire pressure too low (low enough to signal the ECU).
The tire pressure drops by about 1 psi for every 10 degrees F the temperature drops, since cold air takes up less space than warm. The pressure in the tires also tends to raise by about 3 psi while you drive due to the car warming up.
4) Recent Tire Rotation or Change
Since the tires are moved to a new location on the car when they are rotated or new tires are put on (which may not have TPMS sensors), the car often requires a “relearning” process.
In some cars, this can be done just by driving the car at a constant speed for 5 to 10 minutes, but others may need external recalibration to be done by a tool or technician. Consult your owners manual for information pertaining to your specific vehicle.
A common problem is people buying a used set of tires which may not have TPMS sensors. For example, a set of winter tires in addition to the all-seasons that came with the vehicle may not have any or the right sensors.
5) Low Pressure in Spare Tire
Not all vehicles have TPMS in the spare tire, but those that do can sneakily throw a tire pressure light when this tire is inevitably forgotten. This is not a bad thing, though; it’s always a good idea to keep the spare inflated properly in case you need it.
Resetting the TPMS
If the tire pressure light is still on after filling the tires, the light may need to be reset. The process for this varies greatly by car make and model. Sometimes you can find the exact procedure in the owner’s manual or by searching your vehicle online.
Some vehicles don’t give you the ability to reset your own TPMS sensors without a special tool. Subarus are some of these: each TPMS sensor is given an ID that needs to be registered with the ECU along with the tire position, temperature, and pressure information at the time.
If there are any changes to any of these (such as with tire rotation, changing tires, or a recently filled tire), the sensors need to be recalibrated with a special tool before the light will turn off. This tool can be found at many auto repair shops, tire shops, and at the dealership.
In Hondas the tire pressure light won’t go off until the system is recalibrated, but newer Hondas integrate this process into the touchscreen so the user can easily reset the TPMS with the help of the manual.
Some newer Mazdas have a button you can hold to turn off the TPMS light and have the system relearn the tire pressures. Try not to abuse this feature though – you should still check your tire pressures every time the light comes on.
If you choose to go to a dealership or shop to have the TPMS reset, be aware that by law they may not be allowed to let the car drive away if the light is still on once they start work on the wheels. They will likely fix the issue so this shouldn’t be a problem, but otherwise your car may be held hostage until they can fix whatever is wrong.
Much of the time when the TPMS light is on, one or more tires do have low air pressure. This system has prevented many accidents by alerting the driver to a flat tire before the vehicle lost control.
It’s important not to rely on TPMS to maintain your tires, though. Most TPMS systems only show a warning when a tire is already significantly underinflated. Driving around with a tire that is underinflated at all will reduce fuel economy and tire life.
Tires are some of the most critical components on your vehicle, since they’re the only part of the car that actually make contact with the road. Check pressures at least monthly using a good tire pressure gauge. Tire pressure gauges are sold for just a few dollars at any auto parts store and many department stores.
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