A Low Tyre Pressure Light Can Come On Without a Puncture

You start the car and the tyre-shaped warning symbol is lit on the dashboard. Your first thought is a nail or sharp stone and a hole in the rubber. That may be true, but the more interesting question is what the car measures when it decides to light that symbol.

What a steady symbol tells you

The claim holds. A steady symbol means at least one tyre is significantly low, according to the United States road safety agency NHTSA, and it does not say why. Cooler air, a slow leak, a puncture and a fault in the monitoring system can each bring on a warning.

The car compares each tyre with a limit

Transport Canada describes two ways to watch a tyre. One uses the wheel speed sensors of the anti-lock brakes, because an under-inflated tyre turns slightly faster. The other reads the pressure with a sensor in each wheel. Either way, a pressure or a change in pressure is held up against a limit.

For cars approved under UN Regulation 141, which the European Union published in its Official Journal, that limit has a number. The warning must come on within ten minutes of cumulative driving after the pressure in one tyre has fallen by 20 percent or to 150 kPa, whichever is higher. For a tyre running at 220 kPa, 20 percent is 44 kPa, so the limit is 176 kPa. Rules differ by country, and your handbook describes what your own car’s system can and cannot do.

Air pressure follows air temperature

The limit is a pressure, and a pressure moves with temperature. For a fixed amount of air in a fixed space, the ideal gas law says pressure changes in proportion to absolute temperature, as the OpenStax College Physics textbook explains. The same page notes that makers specify tyre pressure for cold tyres. Transport Canada also lists large outside temperature changes as one reason the warning can come on.

Here is the arithmetic for a tyre at 220 kPa on the gauge at 25°C. A gauge shows pressure above the atmosphere, so the gas law needs the reading plus about 101 kPa of atmosphere at sea level, which makes 321 kPa. The table assumes no leak, a constant tyre volume and air that matches the outside temperature.

The 15°C row agrees with the tyre maker Continental, whose own page gives about 11 kPa for each 10°C, converted from its figure. Half the swing gives about half the change, so where day and night differ by only a few degrees, temperature alone moves the pressure very little. A well-inflated tyre needs a fall of about 40°C to reach the 176 kPa limit this way. A tyre that was already a little low needs far less.

Why the light can come and go

NHTSA says the symbol may light for a short time on cold mornings and then go out. Its explanation is marginally low pressure that dips below the warning threshold overnight and rises again as driving or warmer air heats the tyres. A hole does not refill itself, so a light that goes out with no air added fits that pattern better. NHTSA still advises inspecting the tyres and checking the pressure, and says the lamp should not come on when tyres are properly inflated.

When air really is leaving

Continental lists punctures and debris such as nails or screws in the tread among the causes of pressure loss, along with recent tyre service. NHTSA adds that a tyre can lose pressure suddenly after a pothole or a bump against a kerb.

This is what makes the symbol hard to read. To the car, a nail hole and a cooler night look the same: a pressure that fell below a limit. The reason has to be found with a gauge and your own eyes.

When the system itself is the problem

A flashing light is a different message. NHTSA says that on United States vehicles from model year 2008, a malfunction shows as the symbol flashing for 60 to 90 seconds and then staying on, repeating at each start until the fault is fixed.

One manufacturer’s manual, the Mazda CX-5 handbook for the Australian market, lists exhausted sensor batteries, a wheel with no sensor, tyre chains and nearby radio interference among the cases that can cause a system error. Transport Canada adds that a malfunction message also appears when tyres are changed or rotated and the system is not reset. It says tyres made for cold conditions need the correct type of sensors for the system to work properly.

A tyre can be low without the light on

The same rules that make the light useful also limit it. It comes on when a tyre is significantly low. NHTSA says under-inflated tyres are hard to spot by eye and the system is not meant to replace regular checks. Transport Canada says that with some systems the tyre may already be close to failure when the warning appears. Continental explains why that matters: the sidewalls of a low tyre flex more, which builds heat and can lead to structural failure or a blowout.

What to do when the symbol appears

Measure every tyre with a gauge, including the ones that look fine, while the tyres are cold. The target is the vehicle maker’s figure, which NHTSA says is on a label at the driver’s door edge or post or in the owner’s manual, and Continental adds the inside of the fuel filler cap for some vehicles. If you have no gauge or compressor and must drive on, NHTSA says slightly low pressure is safer than a significantly under-inflated tyre, and you should recheck when the tyres are cold. Look over the tread and sidewalls for cuts, cracks, bulges or an embedded object. If a tyre needs air again soon after you set it to the right pressure, take it to a tyre professional, because NHTSA lists failing to hold pressure among the problems to bring to one.