[AWJDThumper]

I've just come back from holiday involving a 300 mile journey each way. Before I left, I checked the tyre pressures manually and they were all about 2.5 bar (36 psi in old money). However this didn't stop the car from telling me that I had slightly low pressure on one wheel. I had previously reset the TPMS and assumed that I had done that wrong and the warning was because the system was incorrectly sensing the tyre pressures.

At that point, I realised I didn't really understand how the TPMS reset worked and so sort help from the internet. That led me to believe that the reset relied on me inflating the tyres to the correct nominal pressures (assumed to be 2.4 bar / 35 psi at the front and 2.2 bar / 32 psi rear), after which, the TPMS ECU would equate the TPMS sensor voltage outputs with those pressure - ie, it calibrates itself.

Armed with this better understanding of how the TPMS works, I inflated all the (cold) tyres to 2.5 bar. After running the TPMS reset, I expected to see 2.4 and 2.2 bar reported but, what I got, was somewhere in between.

The questions I have are:

1. Is my understanding of what the TPMS reset correct?

2. If so, what tyre pressures is my X1 expecting me to put into the types before I reset?

3. How does it take into account the rising tyre press as the tyres heat up during the reset?

4. Why does it take the TPMS ECU a relatively long time to do the reset and what exactly is it doing during that time.

Any answers to these questions would be gratefully received.

[Qsilver7]

Hopefully this will help: