On this page
- What does the tyre pressure warning light mean?
- What should you do when the light comes on?
- Direct and indirect TPMS: what's the difference?
- How do you reset the tyre pressure warning light?
- Steady or flashing light: what's the difference?
- Is TPMS compulsory?
- Warning light on and a flat tyre in Milan?
- Frequently asked questions
What does the tyre pressure warning light mean?
The tyre pressure warning light means the monitoring system (TPMS, Tyre Pressure Monitoring System) has detected a pressure lower than expected. The most common symbol is a horseshoe-shaped tyre cross-section with an exclamation mark in the middle.
The drop can be caused by a puncture, a slow leak (valve, rim, small hole) or even a fall in outside temperature, which lowers the pressure. Either way it needs checking: it's not a warning to ignore.
What should you do when the light comes on?
When the light comes on, slow down gradually and stop as soon as you find a safe spot.
- Look at all four tyres: if one is visibly deflated or flat, don't set off again (see can you drive on a punctured tyre?).
- If none looks deflated, check the pressure with a gauge at the first opportunity, with the tyres cold.
- Restore the pressure specified by the manufacturer: you'll find it in the handbook and often on a label on the door pillar or inside the fuel filler flap.
- If the system requires it, carry out the reset (see below).
- If the light comes back on shortly afterwards, there's probably a leak: have the tyre checked.
Direct and indirect TPMS: what's the difference?
The difference lies in how the system "measures" the pressure.
| Direct TPMS | Indirect TPMS | |
|---|---|---|
| How it works | A sensor in each wheel (usually on the valve) measures the pressure and transmits it by radio to the control unit | Uses the ABS wheel-speed sensors: a deflated tyre has a smaller rolling diameter and turns faster |
| What it shows | Often the pressure of each individual wheel | Usually just the warning, with no numerical value |
| After a tyre change | The sensors must be handled carefully; on some vehicles a relearn procedure is needed | A reset/recalibration is normally needed |
| Maintenance | The sensors have an internal battery and can fail | No additional sensors in the wheels |
How do you reset the tyre pressure warning light?
The light is reset using the manufacturer's procedure, which is usually found in the on-board computer menu (under the tyre pressure item) or via a button. The exact procedure is described in the owner's handbook.
- With indirect TPMS the reset must be done after bringing all the tyres to the correct pressure, with the tyres cold: the system stores that condition as its reference.
- With direct TPMS the light often goes off by itself after the pressure is restored and a few minutes of driving; some vehicles require a specific procedure.
Don't reset the system without first checking and correcting the pressure: you'd be cancelling a genuine warning.
Steady or flashing light: what's the difference?
On many vehicles a steady light indicates a drop in pressure, while a light that flashes for a while and then stays on signals a system malfunction (for example a faulty or unrecognised sensor). The exact behaviour varies from manufacturer to manufacturer: check the handbook.
A faulty system won't warn you of a puncture: check the pressure manually until it's fixed.
Is TPMS compulsory?
Yes: a tyre pressure monitoring system is compulsory on all new cars registered in the European Union from 1 November 2014, under Regulation (EC) No 661/2009.
It's also essential on vehicles with run-flat tyres, which can lose pressure without you noticing. Find out more in run-flat tyres: how they work.
Warning light on and a flat tyre in Milan?
If the light comes on in Milan or the surrounding province and you find a deflated or flat tyre, there's no need to take risks: SOS Tyre reaches your vehicle in 30 minutes on average, 24/7. Read the page on a deflated tyre or call +39 377 326 0999.
Frequently asked questions
The tyre pressure light comes on when it's cold: is that normal?
It can happen: the air pressure in tyres falls with temperature, so on cold mornings it can drop below the system's threshold. Check the pressure anyway and bring it back to the value specified by the manufacturer; if the light comes on often, have a possible slow leak checked.
Can I keep driving with the tyre pressure light on?
Only after checking that no tyre is deflated. If a tyre is visibly low or flat, don't carry on. If the tyres look fine, drive carefully to somewhere you can check the pressure with a gauge and restore it.
Does TPMS show which tyre is deflated?
It depends on the system. Direct TPMS, with sensors in each wheel, often shows the pressure of each tyre and identifies the one affected. Indirect TPMS, based on the ABS sensors, usually only tells you there's a problem: you'll need to check all four wheels.
Do I need to reset the TPMS after replacing a tyre?
With an indirect system, normally yes, after bringing all the tyres to the correct pressure. With a direct system it depends on the vehicle: some recognise the sensors automatically, others require a relearn procedure. The correct procedure is described in the handbook.
Does inflating with nitrogen stop the light coming on?
No. Any gas used for inflation can escape through a hole, a faulty valve or the edge of the rim. The light signals a drop in pressure regardless of the gas: the cause still needs to be checked.