TPMS sensors for tires: what they are and how they work

TPMS sensors for tires

Surely (and unfortunately) some of you will have found the tire of your car excessively flat or even punctured and having to resort to replacing it with the famous spare wheel (temporarily) or use one of those kits inflating device which is increasingly found on board our cars lately. Is it possible to predict any failures of this type? Yes, approximately, with the TPMS pressure monitoring system.

Since 1 November 2012, the TPMS system has been present on all new homologated models of cars and campers sold in the EU and 2 years later (from 1 November 2014) it is mandatory for EU registration. But what exactly is it about?

TPMS: what is it

TPMS is a tire pressure monitoring system (Tyre Pressure Monitoring System) or an electronic device integrated which is adopted to check the tire pressure of a vehicle. It provides real-time information to the driver, alerting him when the inflation pressure of one or more tires is not as indicated.

The correct pressure, to clarify, is generally indicated on a label placed on the "beat" of the car door. driver and indicated in bar. The TPMS then monitors the tire bars and warns the driver with sound or light signals (pictograms or warning lights) directly on the instrument panel in case of problems. There are two types of TPMS on the market, those with indirect (passive) and direct (active) control, which differ according to their own functioning and communication methods with the control unit.

Indirect TPMS (passive)

The indirect TPMS uses the ABS system sensors to evaluate tire pressure; in essence, it is based on the principle that an incorrectly inflated tire will have a smaller diameter and, consequently, a higher angular velocity than one that operates correctly. In this case it is therefore not possible to measure the pressure directly, but only by exploiting the ABS calculation.

When changing tires or performing operations on the tyres, it is always necessary to reset the system; it takes about 30 minutes for the system to recognize the change and register it to the system. A small button on the dashboard or driver's side is usually always available to initiate the reset procedure.

Direct TPMS (active)

Unlike the variant mentioned above, the direct TPMS calculates and measures the pressure using sensors installed directly on the valve of each individual tire and equipped with a battery. Unlike their indirect counterparts, these sensors can also measure the temperature inside the tyres.

The presence of a battery makes the sensors more susceptible to malfunctions or simply problems related to the charge of the battery itself. In general, the life of each individual battery is around 5 or 10 years, but in case of excessive cold it is likely that the duration will run out sooner. Unfortunately the battery is almost always welded so it will be easier to replace the sensor than to change the battery. However, there are "advanced" models powered by an electromagnetic induction system; this aspect overcomes the problem of the short life of battery-powered sensors. On the other hand, the cost can be significantly higher. Furthermore, while those with batteries are installed inside the rubber, those with electromagnetic induction are applied externally and can be of the cap or flow through type, and both positioned directly on the valve stem.

Original and universal TPMS

When buying a new car it is generally a good solution to buy a second set of winter tires perhaps accompanied by a set of rims, so as to make the tire change faster and cheaper in the long run. In winter, with salt and ice, it's not difficult to ruin alloy wheels and buying an aesthetically less satisfying set is never a bad idea. If you buy everything from your dealer, you will certainly also be offered a set of TPMS sensors to complete the set and avoid having warnings on your car.

But is it necessary to use original TPMS? What changes?

original sensors: these are pre-programmed devices with the settings of the vehicle for which they are intended. They always work and have a slightly higher list price than the universal ones; universal sensors: they are very popular on Amazon and in the major specialized shops in the sector. They are not pre-programmed and can be installed in any vehicle. To make them work, however, the workshop (or tire specialist) must have an additional programming tool that is used to calibrate the sensor to the vehicle's specifications.

How are new TPMS recognized?

After installing new TPMS sensors, the vehicle must detect the sensors and establish a stable connection between the control unit and the sensors. Depending on the make, model and year of production of the vehicle, there are different technologies in the field:

self-learning : at a speed between 20 and 30 km/h, for a distance of a few kilometres, the vehicle automatically recognizes modifies it and therefore records the new pressure; manual learning: unlike the previous procedure, the system learns the change from a standstill without using the car; programmed reset (via OBD): perhaps the most complicated to carry out. Here it is necessary the intervention of someone who is familiar with and has access to an OBD II diagnostic system and reset the modification directly from the tool.

TPMS warning light on: how it works and what to do

The TPMS warning light generally has three operating modes so as to communicate to the driver the type of problem detected.



If the warning light turns on (fixed) while driving, it means that the pressure of at least one tire has undergone a change while driving. Always check the pressure as soon as possible to avoid long-term problems. If it turns on and off it means that the pressure level is approaching the minimum value (generally 20% of the normal value) perhaps due to the harsh ambient temperature. If, on the other hand, the light flashes for a period of 90-120 seconds and then stays on, there is probably a communication problem. So no flat tire, just a problem in communicating between the car's ECU and the monitoring system. The causes in this case can be the most varied; from a valve malfunction to the dreaded dead battery. Unfortunately, in this case, the only solution is to go to the tire specialist or dealer and check the condition of the battery with the appropriate tools or by disassembling the tyre.

TPMS: frequently asked questions

Every How advisable is it to check the pressure of your tires?

There is no set period, we recommend checking the correct pressure at least once every 2-3 months. We remind you that a small hole can compromise the tire even progressively over time (also due to variations in ambient temperatures). Better an extra check than a nasty surprise on the road. Do pressures occur cold or hot?

It is always a good rule to check the pressure with cold tires so as not to have false readings. Can pressure change with temperature or altitude?

Certainly. In the mountains, by the sea, in the cold or in the heat: the pressure inside the tire changes constantly. As mentioned above, check it often. I bought a new set of rims, can I move the sensors?

Not always. If they are direct, active sensors, it is not possible. Conversely, those installed on the valve can be moved from set to set. Is worth? We leave this up to you to decide, but given the learning procedure to be done every time, we believe that the game is not worth the candle. Which TPMS sensors cost the least?

As mentioned, the original sensors certified by the manufacturer tend to cost more. But do not think there is an abysmal difference. On a set of 4 sensors, the savings with a universal kit can be 20-30 euros. What to do if a TPMS sensor comes on when driving

Simple, pull over safely in the first available area. Don't panic, pull over, check for any screws or nails inside the tread and evaluate the situation. We specify that, unless the pressure is really (really!) low, it is impossible to evaluate the pressure of a tire with your own hands. It's not a soccer ball, the thickness of the tread (and of the shoulder) is demanding and you will hardly understand if the rubber has a problem. Does the TPMS sensor detect overinflation?

No, the TPMS sensor detects if there is a leak or variation less than the factory default. My car does not have the legally required TPMS sensors, should I install them?

No. Only vehicles that were equipped with direct or indirect TPMS systems during construction are required to comply with the TPMS obligation; however, all post-2004 vehicles are equipped with these devices. What is advisable to keep in the car to check the pressure?

If you don't trust the TPMS sensors and want a second opinion, we recommend keeping a small portable pressure gauge or a compact car compressor on board. In this regard, at this address, our buying guide is available with the models we most recommend.