TPMS Explained: That Tire-Pressure Light, Demystified

Direct TPMS puts a battery-powered pressure sensor inside each wheel that radios its reading to the car. Indirect TPMS has no sensors — it infers a low tire from the ABS wheel-speed data, because an under-inflated tire rolls at a slightly different rate.

Direct systems are more precise and more common on newer vehicles; they are also why a second set of rims needs its own sensors.

TPMS sensors run on a sealed battery that typically lasts 5 to 10 years; when it dies the sensor is replaced. After mounting new tires or swapping rim sets, the system often needs a relearn so it knows which sensor sits where. We program and relearn as part of the install so you leave with the light off.

TPMS sensor supply and install starts at $50 per sensor with balancing and installation included. Most vehicles take four, but you replace only the ones that have failed — one dead sensor does not mean buying a set of four, and any shop telling you otherwise is selling rather than diagnosing.

Sensors fail in two ordinary ways. The first is simply age: the internal battery is sealed and not separately replaceable, so somewhere between the five- and ten-year mark the sensor stops transmitting and is replaced as a unit. The second is physical — the sensor sits inside the wheel at the valve, so corrosion around the valve stem (road salt does this) or damage during a careless mounting can kill an otherwise healthy sensor.

That second cause is worth knowing about in a New Brunswick winter. Salt attacks the aluminium valve stems used on many sensors, and a stem that corrodes through will leak, which shows up as a tire that keeps going soft rather than as a warning light. Replacing the service kit — the grommet, nut and valve core — when tires are changed is cheap insurance and is part of a proper mount.

The single most common TPMS call we get in Fredericton is not a broken sensor. It's the first genuinely cold morning of the season, when half the city starts the car and finds the pressure light on.

This is physics rather than a fault. Air contracts as it cools, at roughly 1 PSI for every 5 to 6°C the temperature drops. A set of tires inflated correctly on a mild 15°C afternoon in October will genuinely be several PSI low at -10°C, and that is easily enough to cross the warning threshold — which is typically around 25% below the placard pressure.

So the light is telling the truth: the tires really are low, and they should be brought back to the pressure on the placard in the driver's door jamb. Two things follow from that. Set your pressures when the tires are cold, meaning before driving rather than after, because a warm tire reads high. And if the light comes back within a few days of a proper fill, you're no longer looking at temperature — that's a slow leak, and it's worth finding before it becomes a flat somewhere inconvenient.

No — and this surprises people, because the assumption is usually that it must be. There is no federal Canadian regulation requiring tire-pressure monitoring on passenger vehicles. In the United States it has been mandatory on new passenger vehicles since the 2008 model year, and because manufacturers build to a shared North American platform, most Canadian-market cars simply inherit the system.

Practically, that means a failed sensor is not the automatic inspection problem that a bald or damaged tire is. But we'd still fix it rather than live with it, for a reason that has nothing to do with regulation: a dash light that is permanently on is a warning you have trained yourself to ignore. The next time it comes on because a tire is genuinely going down, you won't see it.