How does TPMS work?

iambu

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Does anyone know how the TPMS (Tyre Pressure Monitoring System) works on the B9 A4?

I retrofitted the genuine TPMS system to my VW Scirocco... This works by using the ABS system (I think) to monitor the rolling circumference of each wheel/tyre. If a change in the rolling circumference is detected, it assumes a change in pressure, and warns you to check the tyre pressures. However it doesn't tell you specifically which tyre to check. Once you have checked all the pressures and they are all correct, you reset the system by holding down the TPMS button.

Does the B9 use this same system, or does it have pressure sensors installed in each wheel rim?
 
On my old A5 it told you which specific wheel, I assume it's the same with the A4