That Orange Horseshoe on Your Dashboard: TPMS Explained

Of all the warning lights on a modern dashboard, this is the one drivers most often cannot name. It is orange, roughly horseshoe-shaped, with a jagged bottom edge and an exclamation mark in the middle. Some people read it as a plug socket. Some assume it is something to do with the engine. It is your Tyre Pressure Monitoring System — TPMS — and it is worth understanding, because it is one of the few warning lights that is directly about whether your car stops and steers properly.

Here is what it is telling you, why inflating your tyres sometimes fails to turn it off, and why it matters at MOT time.

What the light actually is

The symbol is a cross-section of a tyre viewed from the front — the jagged bottom is meant to be the tread deforming under load. The exclamation mark is doing the obvious work.

TPMS became mandatory on new passenger cars sold in Europe from November 2014, and many manufacturers fitted it well before that. If your car was registered in the last decade or so, it almost certainly has it, whether you have ever noticed the light or not.

Why it exists

Under-inflated tyres are one of those problems that hides in plain sight. A tyre can be significantly down on pressure and still look completely normal — the sidewall bulge you would need to see by eye only becomes obvious once the tyre is dangerously flat. Meanwhile:

• Braking distances get longer and the car feels vague and slow to respond in an emergency swerve.
• The tyre wears out faster and unevenly, scrubbing away at both shoulders and shortening its life considerably.
• Fuel economy drops, because a soft tyre takes more energy to roll.
• The tyre runs hot — a soft tyre flexes more, flexing generates heat, and heat is what causes a blowout at speed.

Tyre pressures fall naturally, roughly a couple of psi a month, plus more as the weather turns cold. Very few of us check them monthly. TPMS is there to catch what we do not.

There are two completely different systems

This is the single most useful thing to know, because it determines what the light means and what fixing it involves.

	Direct TPMS	Indirect TPMS
How it works	A battery-powered sensor inside each wheel measures actual pressure and radios it to the car	Uses the ABS wheel speed sensors — a soft tyre is fractionally smaller, so it rotates slightly faster
What it can tell you	Often the exact pressure in each individual tyre	Only that one tyre is behaving differently to the others
Hardware in the wheel	Yes — a sensor with a sealed, non-replaceable battery	None
Typical fault	Flat sensor battery, damaged or corroded sensor	Needs resetting after inflating or rotating tyres
Blind spot	None significant	All four tyres losing pressure evenly — nothing looks different, so nothing is flagged

If your dashboard shows you four pressure readings, you have a direct system. If it only ever shows a warning light, you probably have an indirect one — though plenty of direct systems keep their readings hidden in a submenu.

Solid light or flashing light? They mean different things

Most owners never learn this distinction, and it is genuinely useful.

• A steady, solid light means the system is working correctly and is reporting low pressure. Check your tyres.
• A light that flashes for around a minute on start-up and then stays on solid means something quite different: the system itself has a fault. It is not telling you a tyre is soft — it is telling you it can no longer see one of the sensors, or that the module has a problem.

That flashing sequence is the one that will not go away no matter how much air you put in, because the pressures were never the issue.

Why the light comes back after you’ve inflated everything

Assuming the pressures really are correct — checked cold, against the sticker in the driver’s door shut or fuel flap rather than the maximum figure moulded into the tyre sidewall — there are a handful of usual suspects.

The sensor battery has died

Direct TPMS sensors run on a small lithium cell that is sealed inside the unit during manufacture. It cannot be replaced on its own; when it goes, the sensor is replaced. They typically last somewhere between five and ten years, which means an awful lot of cars on the road right now are reaching the end of their first set. They also tend to fail in a cluster, because they all started life on the same day.

The sensor was damaged fitting a tyre

The sensor lives on the inside of the wheel, attached to the valve. A tyre machine operated carelessly can crack the body or snap the stem. It is one of several reasons to have tyres fitted somewhere that takes care over it.

The valve stem has corroded

Many sensors use an aluminium valve stem with a separate nickel-plated core. Road salt gets in between the two dissimilar metals and corrodes the joint, which causes a slow leak, then a snapped stem. Fitting a fresh service kit — new core, nut, seal and cap — whenever tyres are changed is cheap insurance and is exactly why those kits exist.

A new sensor has not been programmed to the car

A replacement sensor is not plug-and-play. It has an ID that the car has to learn before it will accept its transmissions. Fit one without programming and the car simply reports a missing sensor — the light stays on and everything looks broken, when it is a five-minute job with the right tool.

You have swapped to a different set of wheels

Winter wheels, a spare set of alloys, or a space-saver in use will all trigger it if the second set has no sensors, or has sensors the car has not been taught.

The system simply needs a reset

This is the common one on indirect systems. They learn what “correct” looks like and then watch for deviation, so after you inflate the tyres, rotate them, or fit new ones, you have to tell the car to re-learn its baseline. On some cars there is a button or a menu item; on others it needs a diagnostic tool. Until you do, it carries on comparing against the old readings.

TPMS and the MOT

This is where it stops being an annoyance and starts costing you a retest.

For vehicles first used on or after 1 January 2012, a TPMS malfunction indicator that is illuminated or otherwise indicating a system fault is a major defect — which means an MOT failure. A missing or obviously inoperative sensor counts too.

Testers see this constantly, and it is a thoroughly avoidable failure. The car is mechanically perfect, the tyres are excellent, and it fails on a warning light nobody got round to investigating. Worth sorting before the test, not during it.

What we do about it

At Dom’s Garage Services we read the system properly rather than guessing. Our TPMS equipment interrogates each sensor individually so we can see which one is reporting, what pressure it thinks it is seeing, and — crucially — how much life its battery has left. That last figure often turns a repeat visit into a single one, because if one sensor has died at seven years old, the other three are usually not far behind and you would far rather know now.

From there it is straightforward: replace what has failed, fit fresh service kits, programme the new sensors to your car, and reset the system so the light goes out and stays out. If the fault turns out to be a slow puncture rather than the sensor, we will tell you that instead.

Frequently asked questions

Can I just ignore it?

You can drive with it on, but you are driving without the safety net it provides, and if your car was first used from 2012 onwards it will fail its next MOT. You are also very likely masking a genuine slow puncture.

Should I still check my tyres by hand if I have TPMS?

Yes. TPMS is a warning system, not a substitute for maintenance — most systems only alert you once a tyre is meaningfully down, by which point it has already been running soft for a while. A monthly check with a decent gauge, done cold, is still the gold standard. It also lets you spot uneven wear, damage and nails in the tread while you are down there.

Do sensors need replacing every time I change my tyres?

No — the sensor usually outlives several sets of tyres. But the service kit should be renewed whenever the tyre comes off, and if a sensor is already several years old it is often sensible to change it while the tyre is off the rim rather than pay to have it removed again later.

My car came with a repair kit instead of a spare — does that change anything?

It makes TPMS more important, not less. If you have no spare wheel, early warning of a deflating tyre is what gives you the chance to get somewhere safely rather than being stranded. It is also worth knowing that some tyre sealants can foul a sensor, so mention it if you have used one.

The light came on when it got cold. Is something wrong?

Probably not. Air contracts as it cools, so a sudden cold snap can knock pressures down enough to trigger the warning across the whole car at once. Check and inflate to the correct cold pressures. If it keeps returning after that, it is worth investigating.

Sort your TPMS light in Letchworth

If that orange horseshoe has been on for a while, or it keeps reappearing after you have inflated everything, bring it in. We will find out which sensor is at fault, tell you how long the rest have left, and get it cleared properly before it becomes an MOT problem.

Dom’s Garage Services, 11 Protea Way, Letchworth, SG6 1JT. Book online or give the workshop a call.