TPMS & Valves

Two possibilities dominate. A sensor battery has reached the end of its life, which typically happens after several years and often to more than one sensor within a short period, so the system reports a missing signal rather than a low pressure. Or the vehicle has lost its record of which sensor sits at which corner, usually after a wheel swap, and needs a relearn. Have the system read with a tool that can interrogate each sensor individually before ordering anything.

It is the procedure that tells the vehicle which sensor identity belongs at which wheel position. You need it after replacing a sensor, after swapping wheels between corners, and after fitting a second set of wheels. Depending on the vehicle it is done with a tool that triggers each sensor in turn, or automatically by driving above a set speed for a period, or through a menu in the vehicle's display. Check which method applies before the wheels go back on.

On the overwhelming majority of designs, no. The battery is sealed inside the sensor housing to survive years of heat, vibration and moisture, and there is no service access. When it expires the module is replaced. Since sensors fitted at the same time tend to fail within a short period of one another, many owners choose to replace the set during a tyre change, which avoids paying for tyre removal several times over.

If your vehicle uses a direct system, yes, or it will report a fault every time the second set is fitted. The usual approaches are a second set of sensors permanently fitted to the winter wheels with a relearn at each changeover, or moving one set between wheels twice a year, which costs less in parts and more in labour and risks damaging the sensors during fitting. On vehicles with indirect systems, no sensors are needed and a reset through the menu is enough.

Corrosion, usually between dissimilar metals. Many clamp in sensors use an aluminium stem with a nickel plated valve core and cap, and fitting a chrome plated brass cap creates a galvanic cell that corrodes the stem from the outside in. Road salt accelerates it. The stem then breaks at the nut, often while a tyre is being inflated. Use the correct cap material, replace the seal and nut whenever a tyre is changed, and do not overtighten the stem nut.

Valve caps sit right at the bottom of the range, and pressure monitoring modules are modest, though a set of four adds up alongside the tyre fitting labour. The live minimum and maximum are shown in the band at the top of this page. Because sensor replacement requires the tyre to be broken from the rim, the fitting cost is a real part of the total, which is the argument for replacing a set during a tyre change rather than individually.

Only if you can safely break the tyre bead from the rim and reseat it, which needs a tyre machine rather than levers on most modern low profile tyres. The sensor itself is simple to fit: seat it correctly against the rim, torque the stem nut to the specified figure, and avoid clamping it at an angle. The greater risk is damaging the sensor while the tyre is manipulated, which is why this is usually left to a fitter who will also perform the relearn.

Often, and with a caveat worth understanding. Programmable sensors are blank units configured with a tool to emulate a specific manufacturer's protocol, and when programmed correctly they behave like the original. What they need is the correct protocol for your exact vehicle and model year, and a fitter with current software. Where you want certainty, the factory reference removes the variable. Match the OE number here and the sensor arrives already speaking the right language.

No, and this is worth being blunt about. Most systems warn at around a quarter below the recommended pressure, which is well past the point where handling, tyre wear and efficiency have already suffered. The system is a safety net for a puncture, not a maintenance tool. Check pressures with a gauge monthly and before long journeys, when the tyres are cold, against the figures on the door pillar placard rather than the number on the tyre sidewall.

Because pressure falls with temperature, at roughly a tenth of a bar for every ten degrees Celsius. A tyre set correctly on a mild afternoon can be measurably low on a frosty morning and trip the warning, then recover as the tyres warm on the journey. That does not mean the warning is false: the tyre genuinely is underinflated at that moment. Set pressures when the tyres are cold, and add a little heading into winter rather than chasing the warning each morning.

It is less informative rather than useless. Indirect systems compare wheel rotation speeds and detect a tyre losing pressure relative to the others, which works and requires no sensors, batteries or relearns. What it cannot do is give an actual pressure reading, name the corner reliably, or detect all four tyres deflating equally over time. It also needs resetting after any pressure adjustment or tyre change. Direct systems cost more and tell you more.

Yes. Sensors differ by radio frequency and by protocol, and the clamp in and snap in stems are separate references again, so the less common versions are quoted. Send the number and say what your wheels use. Replace all four together once the first battery fails. The sensors were fitted on the same day and reach the end of their life within months of each other, and the tyre removal cost is paid once instead of four times. Confirm your workshop can run the relearn procedure, because a new sensor that has not been registered leaves the warning lamp on.