Tire Pressure Monitor Sensor Lights Keeps Coming On

When the tire pressure monitoring system (TPMS) light on your dash comes on, it usually means that the air pressure in one or more of your tires has dropped below the expected level. The light can also be triggered erroneously by a bad sensor, and it can also come on, and go back off, seemingly at random.

If you have a TPMS light, it's important to remember that it isn't a replacement for regular maintenance. While a TPMS light coming on can be a great warning ahead of an impending emergency, there's no replacement for physically checking your tires with a tire pressure gauge and topping them up as needed.

What Does the TPMS Light Coming on Really Mean?

When you have a car that has a TPMS, what that means is every tire has a wireless sensor inside it. Each sensor transmits data to the computer, and the computer turns on the TPMS light if any of the sensors show a pressure value that is higher or lower than the safe operating range.

While the best response to a TPMS light coming on is to check the tire pressure with a manual gauge, the light can actually convey some pretty important information if you know what to look for.

TPMS Light Comes on When Driving

• Light behavior: Comes on and stays on.
• What it means: The air pressure is low in at least one tire.
• What you should do: Check the tire pressure with a manual gauge as soon as you can.
• Can you still drive: While you can drive with the TPMS light on, keep in mind that one or more of your tires may be very low on air pressure. Your vehicle may not handle as you expect it to, and driving on a flat tire can damage it.

TPMS Light Comes on and Goes Off

• Light Behavior: Illuminates and then turns off seemingly at random.
• What it Means: The tire pressure is at least one tire is probably very close to the minimum or maximum rated inflation. As the air contracts, due to cold weather, or heats up, the sensor is triggered.
• What you should do: Check the tire pressure and adjust it.
• Can you still drive: The air pressure is probably close to where it should be, so it's usually safe to drive. Keep in mind that the vehicle may not handle the way you expect it to.

TPMS Light Flashes Before Coming On

• Light Behavior: Flashes for a minute or so each time you start the engine and then stays on.
• What it Means: Your TPMS has probably malfunctioned and you can't count on it.
• What you should do: Take your car to a qualified technician as soon as you can. Check your tire pressure manually in the meantime.
• Can you still drive: If you check the air pressure in your tires, and it's fine, then you are safe to drive. Just don't count on the TPMS to warn you of a problem.

Tire Pressure and Changing Temperatures

In most cases, your tires will be full of air that’s identical to the ambient air in the atmosphere. The only real exception is if they are filled with nitrogen, but the same rules of thermodynamics apply to both elemental nitrogen and the mixture of nitrogen, carbon dioxide, oxygen, and other elements that make up the air we breathe and pump into tires.

According to the ideal gas law, if the temperature of a given volume of gas is reduced, the pressure is also reduced. Since the tires on a car are more or less a closed system, that essentially just means that when the temperature of the air in a tire goes down, the pressure of the air in the tire also goes down.

The opposite is also true, in that the pressure of the air in a tire will go up if the temperature of the air goes up. The gas expands as it heats up, has nowhere to go as it is trapped in the tire, and the pressure rises.

The exact amount that tire pressure rises or falls will depend on a number of factors, but a general rule of thumb is that you can expect a tire to lose about 1 PSI per 10 degrees Fahrenheit in ambient air temperature reduction and conversely gain 1 PSI per 10 degrees Fahrenheit as the environment warms up.

Cold Winter Weather and Tire Pressure Monitor Systems

In situations where a TPMS problem only shows up in the winter, it’s a fair bet that cold temperatures might have something to do with it, especially in areas where the winters are exceptionally cold. For instance, if the tires of a vehicle were filled to specification when the ambient temperature was 80 degrees, and nothing was done as the winter rolled in and the outside temperatures dropped to below freezing, that alone could account for a 5 PSI swing in tire pressure.

If you’re experiencing an issue where the TPMS light comes on in the morning, but it goes off later in the day, or the tire pressure looks fine with a gauge after you’ve been driving awhile, a similar issue could be at work.

When you drive a car, the friction causes the tires to heat up, which also causes the air inside the tires to heat up. This is one of the reasons that manufacturers recommend filling tires when they are cold, instead of when they are hot from being driven. So there’s a very real chance that your tires could be under specification in the morning, and then appear fine later in the day when a mechanic checks them.

Checking Tire Pressure vs. Relying on the TPMS Light

If you check the tires in the morning, before you’ve driven your car at all, and the pressure isn’t low, but the light still flickers when you drive, then you probably have a bad TPMS sensor. It isn’t terribly common, but it does happen, and some products like injectable fix-a-flat mixtures can hasten the demise of a TPMS sensor under certain circumstances.

On the other hand, if you find that the pressure is low when the tires are stone-cold, then that’s the problem. Filling the tires to the cold specification, when they are actually cold, will almost certainly get rid of the issue of a TPMS light coming on repeatedly in the cold winter weather.

Incidentally, this is also the reason that it’s a good idea to check and adjust tire pressure throughout the year. The idea of putting “fall air” or “spring air” in tires may seem like a joke, but accounting for pressure swings due to the ambient temperature as the seasons' change can head off issues with tire pressure monitor lights.