In short. A solid tyre pressure (TPMS) light means the system thinks at least one tyre is significantly low: stop somewhere safe as soon as you can, check all four with a gauge and inflate to the figure on the door label. A light that flashes for about a minute when you start the car and then stays on means the system itself has a fault, so it may not warn you of a real loss: check the pressures by hand and have the system looked at. If it comes on after new tyres or wheels, the usual causes are a reset or sensor relearn that was not done, or wheels without working sensors.
Solid or flashing: what each one means
The symbol is a tyre cross-section with an exclamation mark, usually amber; some cars add a separate lamp marked "TPMS" for faults.
| What you see | What it means | What to do |
|---|---|---|
| Lit briefly at start-up, then out | Bulb check | Nothing |
| Steady, from start or while driving | At least one tyre below the threshold (on some cars, too high) | Stop when safe, check all four cold, inflate to the label |
| Flashes about a minute at start-up, then steady | The system has a fault | Check pressures by hand; have the system diagnosed |
| Same, just after a tyre or wheel change | Sensor missing, damaged or not learnt, or system not reset | Back to the fitter; see the handbook's reset or relearn |
| Separate "TPMS" lamp lit | Fault, on cars with a dedicated fault lamp | As for flashing |
In the United States, Federal Motor Vehicle Safety Standard No. 138 allows one lamp to carry both messages. On a fault it "flashes for a period of at least 60 seconds but no longer than 90 seconds" after the ignition is switched on, then stays lit while the fault exists, and repeats this at every start until the fault is corrected (FMVSS 138, S4.4(c)(2)). It must also light as a bulb check at ignition (S4.3.3).
UN Regulation No. 141, used in Europe and the UK, says the same without a duration: a combined lamp flashes to signal a fault, then quickly stays on while it lasts, at every ignition (UN R141, 5.5.4). It also lets the lamp flash to give information about resetting the system, as the owner's manual describes (5.5.5): a flash right after you press reset is not necessarily a fault.
The MOT manual tells British testers the lamp "will generally illuminate and go off again" at ignition, stays on after an earlier pressure loss, which "does not automatically mean the system has a malfunction", and in a malfunction "may flash a number of times and then remain on" (MOT inspection manual, 5.2.3). Tesla's Model 3 handbook: if the indicator "flashes for one minute whenever you power on", a TPMS fault has been detected (Tesla owner's manual).
When does the light come on? The thresholds in the rules
The light is a legal backstop, not a gauge: each rule fixes the latest point at which it must warn, and your car may warn earlier.
United States. FMVSS 138 covers vehicles with a gross vehicle weight rating of 4,536 kg (10,000 lb) or less, except those with dual wheels on an axle (S2). The lamp must light within 20 minutes once a tyre is 25 per cent below the maker's recommended cold pressure, or at 140 kPa (20 psi) for standard-load passenger tyres if that is higher (S4.2(a), Table 1). After a phase-in from October 2005, "all vehicles manufactured on or after September 1, 2007 must comply" (S7.3).
Europe and the UK. UN R141 covers M1 cars up to 3,500 kg and N1 vans, except those with twin wheels on an axle (1, 5.1.1). The lamp must light within ten minutes of cumulative driving after one tyre loses 20 per cent of its operating pressure or reaches 150 kPa, whichever is higher, and within 60 minutes for the same 20 per cent loss, or 150 kPa, in one or more of the four tyres (a slower loss, 5.3.1); the system must work from 40 km/h (25 mph) (5.1.3, 5.2.1). A fault must be shown within ten minutes (5.4.1); the US allows 20 (S4.4(a)).
What that means at four label pressures:
| Label pressure (cold) | US: lamp by (25 % down, minimum 1.40 bar) | UN R141: lamp by (20 % down, minimum 1.50 bar) |
|---|---|---|
| 2.2 bar (32 psi) | 1.65 bar (23.9 psi) | 1.76 bar (25.5 psi) |
| 2.5 bar (36 psi) | 1.88 bar (27.2 psi) | 2.00 bar (29.0 psi) |
| 2.9 bar (42 psi) | 2.17 bar (31.5 psi) | 2.32 bar (33.6 psi) |
| 3.5 bar (51 psi) | 2.62 bar (38.1 psi) | 2.80 bar (40.6 psi) |
Calculation by the Onyx Forged design office, 1 bar = 14.5038 psi. The UN text measures the loss from the warm operating pressure of its test drive, so its column, set against the cold label, is an approximation. Either way, when the lamp is obliged to light, the tyre is already a fifth to a quarter down. The statement every US owner's manual must carry says that when it lights "you should stop and check your tires as soon as possible", and that the TPMS "is not a substitute for proper tire maintenance" (S4.5(a)). How and when to check is in our tyre pressure guide.
Is it safe to drive with the light on?
With a solid light, drive gently to the nearest safe place to check, not to your destination. The required US statement says driving on a significantly under-inflated tyre "causes the tire to overheat and can lead to tire failure". If a tyre is visibly flat, bulging or damaged, do not drive on it: change the wheel somewhere safe, never on a motorway hard shoulder, following the handbook's jacking points and never reaching under a car held only by the jack, or call for help. If one keeps losing air, read slow punctures: causes and fixes and have it inspected before you take the car back on the road.
With a flashing-then-solid light, the car is telling you it cannot be trusted to warn you. You can drive, but check pressures by hand until it is repaired: FMVSS 138 requires the manual to say that with the fault lamp lit "the system may not be able to detect or signal low tire pressure as intended" (S4.5(a)).
Why the light is on when the tyres are full
The reading was taken hot. Tesla says about 1.6 km (one mile) of driving warms tyres enough to raise the pressure, and that a hot tyre at or below the recommended cold pressure "is dangerously underinflated".
It got colder. Tesla gives about one psi lost for every 6 °C drop; Transport Canada says large temperature changes can bring the warning on. We checked with the gas law (constant volume, from 20 °C):
| Label pressure | Lost for a 6 °C fall | Lost for a 20 °C fall |
|---|---|---|
| 2.2 bar (32 psi) | 0.07 bar (0.95 psi) | 0.22 bar (3.2 psi) |
| 2.5 bar (36 psi) | 0.07 bar (1.04 psi) | 0.24 bar (3.5 psi) |
| 2.9 bar (42 psi) | 0.08 bar (1.16 psi) | 0.27 bar (3.9 psi) |
Calculation by the Onyx Forged design office; a real tyre stretches a little, so the figures are close, not exact, and they agree with Tesla's. Cold alone rarely lights the lamp on a correctly set tyre: a 2.5 bar tyre set at 20 °C reaches the UN threshold only at about -22 °C. Cold on a tyre already low does: set 10 per cent low (2.25 bar), it falls to about 2.03 bar at 0 °C, 81 per cent of the label, at the edge of the UN threshold. Hence the light on the first frosty morning.
The light has not caught up. Tesla notes it "does not immediately turn off" when you adjust pressure; FMVSS 138 keeps it lit until the pressure is corrected or the system is manually reset (S4.2(b)). On some cars the lamp only goes out after a short drive.
An indirect system needs resetting after you inflate (see below).
A tyre is too high. Tesla's indicator warns if a tyre is "underinflated or overinflated".
You did not check them all. The US manual statement says each tyre, "including the spare (if provided)", should be checked monthly when cold. Whether your system watches the spare is in the handbook.
If the pressures are right, cold, and the lamp flashes at each start, it is a fault: a sensor with a flat battery, a damaged sensor or valve, or a receiver problem. Continental describes the sensors as battery-powered and integrated into the valve (Continental UK). We found no maker's figure for battery life in what we read, so we give none.
Direct and indirect systems
Transport Canada describes both: one "uses ABS sensors to measure the rotational speeds of the wheels", as an under-inflated tyre turns slightly faster; the other "directly measures the pressure with a wheel mounted pressure sensor in each wheel" (Transport Canada). EU law allows both: requirements "should allow both direct and indirect tyre pressure monitoring systems" (Regulation (EU) 2019/2144, recital 7).
| Indirect (wheel speed) | Direct (sensor in each wheel) | |
|---|---|---|
| What it measures | Differences in wheel rotation, through the ABS sensors | Pressure, by a sensor on the valve |
| Pressure of each tyre on the dashboard | Usually not | Often |
| After inflating | Usually reset by the driver | Usually reads the new pressure after a short drive |
| After new tyres or wheels | Reset needed | Sensors fitted and, on many cars, learnt by the car |
| Typical fault after a change | Reset not done, or done at the wrong pressures | No sensors, a sensor damaged in fitting, sensors not learnt |
Our summary of the sources in this guide; the handbook says which your car has. The German Road Safety Council wrote in 2016 that indirect systems "have to be reset in case of pressure change, e.g. due to changed loading conditions or wheel/tyre combination", and that after a change on a car with direct TPMS "the driver may have to visit a service center" (DVR).
How to reset the light, by system
First, inflate all four tyres, cold, to the label figure. The EU regulation requires systems "designed to avoid resetting or recalibration at a low tyre pressure" (2019/2144, Article 5(2)), and the UN test inflates all tyres to the maker's cold pressure before resetting the system to the maker's instructions (UN R141, Annex 3, 2.8). An indirect system reset at the wrong pressure learns the wrong baseline and stays silent until the tyre is lower still.
Read the handbook. The UN rule requires it to say the car has a TPMS and how to reset it, where that is possible (6.1.1): the button or menu, and any drive needed afterwards.
Indirect systems. Usually a reset from a button or the settings menu, then normal driving while the system learns.
Direct systems. The lamp goes out once the car receives correct readings. After a wheel change the car must learn the new sensors. On some cars this is automatic: Tesla says that after replacing wheels (not tyres), sensors "reset automatically within 10 minutes of driving over 25 km/h", and asks you to select the new wheels in the wheel configuration menu so the car can learn them and give more accurate status updates. On others a fitter uses a tool, as the DVR's service-centre note suggests. Tesla adds that after another workshop repairs or replaces a tyre, "the tire sensor may not work until Tesla performs the setup procedure"; more in our Tesla Model 3 and Model Y wheel guide.
A flashing light is not a reset problem. Pressing reset will not cure a fault; it needs a diagnosis.
After new tyres or new wheels
The FMVSS 138 manual statement says faults may come from "the installation of replacement or alternate tires or wheels on the vehicle that prevent the TPMS from functioning properly", and to "always check the TPMS malfunction telltale after replacing one or more tires or wheels" (S4.5(a)). Transport Canada adds that the fault warning comes on if tyres are changed or rotated without a reset, and that winter tyres may not work properly with the system unless "the correct type of pressure sensors for your vehicle" are fitted.
What to check, in order:
- Do the new wheels have sensors? A winter set on a car with direct TPMS needs its own; see storing tyres and wheels between seasons.
- Are they the right type for the car?
- Has the car learnt them, by itself after a drive or with the fitter's tool?
- Was an indirect system reset at the right pressures, after the fitting or a tyre rotation?
- Was a sensor damaged in fitting? A fitter can read each one.
On new wheels, settle the sensor question before ordering. Onyx Forged does not recommend a size or an offset: sizes, pressures and TPMS parts for your car are in its handbook and the maker's documents.
What the law says in each country
| Where | Rule | What it requires | At the periodic test |
|---|---|---|---|
| United States | FMVSS 138 | TPMS on light vehicles up to 4,536 kg made from 1 September 2007; warning at 25 % below cold placard pressure; fault lamp | Inspections set state by state |
| European Union | Regulation (EU) 2019/2144, Article 5; UN R141 | "Accurate" TPMS; M1 up to 3,500 kg and N1 covered by R141 | Set by each member state |
| Great Britain | Regulation (EC) No 661/2009, Article 9(2), as kept in UK law | M1 vehicles equipped with an accurate TPMS | Cars first used from 1 January 2012: TPMS "malfunctioning or obviously not working" major; tyre obviously under-inflated minor |
| Canada | Transport Canada guidance | No fitment rule found in what we read | Not covered in what we read |
| Australia | Not established | No Australian Design Rule on TPMS found in what we read | Not covered in what we read |
Sources: FMVSS 138; 2019/2144, Article 5 and Annex II (C13); 661/2009, Article 9; MOT inspection manual, 5.2.3. The DVR dates the requirement for all new European M1 cars to November 2014. Before R141, TPMS sat in UN Regulation No. 64, which the EU regulation still lists as "Temporary-use spare unit, run-flat tyres/system (and tyre pressure monitoring system)".
When it is unclear whether the lamp shows a fault or low pressures, the MOT manual asks the tester to add an advisory. The rest of the wheel checks, and Northern Ireland's own dates, are in new wheels and the MOT. In Canada and Australia, we did not establish a national fitment rule; your handbook is the reference for how your car's system behaves.
Frequently asked questions
What is the difference between a flashing and a solid tyre pressure light?
Solid: a tyre is low (on some cars, too high). Flashing for about a minute at start-up, then solid: the system has a fault. The US rule sets the flash at 60 to 90 seconds; the UN rule sets no duration.
Why does my tyre pressure light flash and then stay on?
That is the fault signal: often a sensor with a flat battery or damaged during a tyre change, wheels without sensors, or sensors not learnt. While it lasts, check pressures by hand.
Can I drive with the TPMS light on?
Solid: only to a safe place to check, since the rules let the lamp wait until a tyre is 20 to 25 per cent down. Never on a flat or damaged tyre. Flashing: yes, but check pressures yourself until it is repaired.
Why is the light on when my tyres are full?
Usually a warm reading, a temperature drop, a lamp that has not yet updated, an indirect system awaiting a reset, or an over-inflated tyre. If it flashes at each start, it is a fault.
How do I reset the tyre pressure light?
Inflate all four, cold, to the door-label figure, then follow the handbook: a button or menu and some driving on indirect systems; on direct systems the lamp usually goes out after a short drive.
Why did the light come on after new tyres?
The required US manual statement names replacement tyres and wheels as a cause. Sensors may be damaged, missing or not learnt, or an indirect system was not reset. Ask the fitter to read the sensors and do the relearn or reset.
Does cold weather turn the TPMS light on?
On a tyre already a little low, yes. Tesla gives about 1 psi per 6 °C, as our calculation does; 20 °C colder takes about 0.24 bar (3.5 psi) off a 2.5 bar tyre.
Will a TPMS light fail the MOT?
On cars first used from 1 January 2012, a system "malfunctioning or obviously not working" is a major defect, a fail. A lamp lit for low pressure is not automatically a fault, but an obviously under-inflated tyre is a minor defect.
Where to go next
If you are planning a set of wheels, describe your car, its tyres and its pressure sensors when you configure them; Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards.
Sources
- 49 CFR 571.138, FMVSS No. 138, Tire pressure monitoring systems, sections S3, S4.2 to S4.5 and S7 (Legal Information Institute).
- UN Regulation No. 141, tyre pressure monitoring systems (E/ECE/324/Rev.2/Add.140, 30 January 2017; French text), sections 5, 6 and Annex 3.
- Regulation (EU) 2019/2144, recital 7, Articles 3 and 5, Annex II; Regulation (EC) No 661/2009, Article 9, UK version.
- GOV.UK, MOT inspection manual, section 5.2.3.
- Transport Canada, Tire pressure monitors.
- German Road Safety Council (DVR), Tyre pressure monitoring systems (24 May 2016).
- Tesla, Model 3 owner's manual, Tire Care and Maintenance (English edition).
- Continental UK, Tyre pressure monitoring system.
All pages were read on 16 October 2026. Calculations are by the Onyx Forged design office and are shown in the text.
The Onyx Forged design office.