Tyre Pressure Warning Light On? What It Is Actually Telling You

An amber symbol has appeared that looks a bit like a horseshoe with an exclamation mark in it. The car drives normally and none of the tyres look flat.

The tyre pressure warning light is one of the more useful things on a dashboard and one of the most misread. It is not only a puncture warning, and the circumstances in which it appeared usually tell you which of four things it is.

Two systems that behave differently

Knowing which your car has changes how you interpret the light, and it is worth finding out once.

A direct system has a sensor inside each wheel measuring actual pressure. It can usually tell you which tyre and often the figure, and it detects a slow loss on a single tyre reliably. The sensors contain batteries and eventually fail, which is a fault in its own right.

An indirect system has no sensors. It infers pressure from wheel speed, because a softer tyre has a slightly smaller rolling radius and turns marginally faster. It is cheaper and has nothing to fail, but it cannot tell you which tyre, cannot detect all four going down together, and has to be reset manually after you correct the pressures.

The four things it usually means

• A genuine loss of pressure in one tyre. The important one, and often a slow puncture rather than a dramatic one.
• Cold weather. Air contracts as it cools, so a drop of several degrees overnight can take a correctly set tyre below the threshold with nothing wrong at all.
• Pressures never reset after a top-up, on an indirect system, so the car is still comparing against the old reference.
• A failed sensor or a flat sensor battery on a direct system, which shows as a warning that will not clear however correct the pressures are.
• A wheel change, including fitting a space-saver spare, which an indirect system reads as a mismatch.

The seasonal one catches an enormous number of people every autumn. If all four came on together during the first cold snap, check the pressures before assuming anything is wrong.

What to do when it appears

1. Slow down and find somewhere safe to look at the tyres. Do not continue at speed assuming it is nothing.
2. Look at all four. A tyre can be well below pressure and still look normal, particularly a low-profile one.
3. Check the pressures with a gauge, cold, against the figure on the placard in the door frame or fuel flap.
4. Inflate to the correct figure, including the second column if you are carrying a load or passengers.
5. Reset the system if your car requires it, then drive and see whether the light stays off.

If the light returns within a day or two after correcting the pressures, you have a slow leak rather than a seasonal drop, and that needs looking at rather than topping up repeatedly.

Flashing rather than steady

Most cars use a distinction here that is worth knowing, and it is documented in the handbook.

A steady light generally indicates a pressure problem. A light that flashes for a period on start-up and then stays on generally indicates a fault in the monitoring system itself, such as a failed sensor, rather than a pressure issue.

That matters because the second one will not clear by inflating anything, and chasing it with a compressor is time wasted. It also means the system is no longer watching your tyres, which is a reason to fix it rather than live with it.

Why you should not just ignore it

It is tempting, because the car drives normally and the light is not red.

An underinflated tyre wears its shoulders quickly, costs fuel, gets hotter than it should and handles worse, particularly in an emergency. A slow puncture that is topped up rather than repaired eventually becomes a failure at speed, and the tyre that could have been repaired for very little often cannot be by then.

The system exists because most people cannot see a twenty per cent pressure loss by eye, and that is the range where the consequences start.

Sensors, servicing and tyre changes

On a direct system the sensors are consumable, and there are a few practical points worth knowing.

Sensor batteries typically last several years and are not usually replaceable on their own, so a failed sensor means a new sensor. The sensor is part of the valve assembly, so it is worth having the valve serviced with a new seal kit when tyres are replaced, because a corroded valve is a common cause of both a slow leak and a sensor warning.

A new sensor also usually needs registering to the car so it knows which wheel it is on, and wheels moved during a rotation may need the positions relearned. Neither is difficult, but both are steps that get skipped and produce a warning light on a car with perfectly good tyres.

The habit that makes it redundant

Check pressures monthly with your own gauge, cold, and after any significant temperature change. It takes five minutes and it covers the seasonal false alarms, the slow leaks and the fuel and wear costs in one go.

Treat the warning light as a backstop rather than as the system that manages your tyres. It is watching for the thing you missed, and the fewer things there are to miss, the better it works.