How tyre pressure monitoring system (TPMS) works | Auto Expert John Cadogan

Here’s a question from Tim, regarding the TPMS on his 2017 Subaru Impreza S.
"The manual says that if I get my tyres rotated or if my sensors are reading one value and my own tyre pressure gauge reads another I should take my car to the stealership for calibration. Isn’t this illegal in Australia? The manual doesn’t list any way the end user can reset or re-calibrate the TPMS system. "
Tim says he has a bunch of tyre pressure gauges and they all read pretty much the same but the Tyre Pressure Monitoring System reads about 3-5psi higher than all those gauges. In other words, the tyres are actually at 32, Tim says, but the TPMS says 35-37.
The barbed wire enema in this situation for Tim is (quote): “Having to fork out dollars at the stealership every time I get my tyres changed or rotated to get the TPMS reset.”
Obviously you need a quick reset when you get the tyres rotated - because otherwise the car has no way of knowing the sensor that was formerly on the left front is now actually on the left rear, or whatever. The recalibration process here is merely a matter of telling the car where the sensors are now. It’s, like, a five-minute job with a scan tool.
I’m not so sure - at least in every case - that individual sensors can be recalibrated in the context of adjusting the reading they send to the car, for any given pressure. The ‘recalibration’ process there is likely - at least in some cases - to be replacement of the sensor.
So just to put this in perspective: TPMS might save your life by preventing a blowout (we’ll get to that) but the feedback effect - the bad news component - is that occasional calibration is required.
Whoever does it: They’ll need to capture the signals from the in-wheel sensors and plug some scan tool or PC into the car via the onboard diagnostics port and let let the mad recalibrating voodoo happen.
If any car is displaying real-time pressures, you’ve got direct TPMS - the kind with the pressure transducer in the wheel. It’s essentially a sensor (which is a pressure-voltage transducer - converts pressure to electricity), plus an analog-to-digital converter and an RF transmitter, which an ECU connected to the CAN bus in the car receives wirelessly and decodes.
In the case here, where there’s merely a minor difference between the TPMS indicated pressure and the real pressure, why bother calibrating? Live with the offset.
Tim knows that 35-37psi indicated by onboard TPMS equals 32psi actual. Just be aware that ‘normal’ equals a displayed pressure in that region of 35-37. So if there’s suddenly a reading of 28, that’s bad.
So I’d suggest that you don’t need to calibrate your TPMS in this situation. Because you simply don’t need to know your tyres’ exact operating pressures in real time. It’s worthless obsessing about some minor offset.
What you really need to know is if one tyre starts to lose air significantly. If you’ve got three tyres reading 38 and one reading 24, that’s bad. Do something about it now. This is like a blowout looking for a place to happen - and on a freeway you may not feel anything until it fails catastrophically.
The big strength of TPMS is that it will alert you to that slow leak - which is the major cause of blowouts on the highway. Gradual pressure loss makes the sidewalls sag. They flex a lot more than they’re designed to, and they get hot - thanks to hysteresis, which we covered recently - link above - which is the classic blowout failure mechanism. Overheated sidewall.
I’d suggest one needs a TPMS calibration - and possible replacement of malfunctioning sensors - only if the actual pressure versus the indicated pressure varies wildly.
Definitely keep checking manually with a gauge, every two weeks. That’s a great idea too. I wish everyone did it. In addition to saving your life, potentially, it will certainly also save you money because underinflated tyres wear out fast.
And while you’re [DOWN THERE] you can look for signs of uneven wear, which is a red flag you need a wheel alignment, or mechanical damage to the tyre itself - like a dirty big chunk missing from the sidewall, or something.
Like most things on any car that you drive regularly, it’s important just to keep track of what’s normal and what’s strange, in the context of gauge readings and behaviour generally. ‘Strange’ is usually bad - strange readings, strange vibrations, strange noises, strange pool of ‘whatever’ under where you park the car overnight.
At the very least, this kinda stuff warrants expert investigation. And if you get onto it early, the repair bill is usually a lot less.