So about a month ago I put 2.0bar into the front tyres and 2.1bar into the rear and held down the tyre pressure sensor button on the centre console till I heard a beep. This resets the sensor according to the manual.

And today I check my pressure, and it's 1.8bar on the front and 1.9bar on the back. The only thing I can think of besides the monitor not working is maybe I put air in a month ago when the tires were warm. I had only driven 10km when I put the air in. This might explain the exact delta on the front and rear..Doesn't tyre pressure read higher when they are warm ?

AFAIK the TPM works via the ABS system and senses different rotational speeds on the wheel when tyre pressure is lost

Thanks...well I just pumped them up today and have not reset the monitor yet, so I maybe it will detect something is up seeing as it probably compares the readout from the abs to what's stored in memory. Then again maybe a 0.2 bar delta is not enough to trigger it. I worry too much

I think the tyre pressure monitor is more designed to detect a difference of one tyre being lower than the other three (using the ABS sensor as suggested above).

In your case all four tyres were, relative to each other, at the same pressures, just lower than a month ago. This can easily be attributed to different ambient temperature/pressure to when they were last checked and now. I checked mine yesterday and were all 0.5 bar lower than a couple of weeks ago; the difference I know is that it was around 20-22 deg C then, and yesterday was only 15-17 deg C.

Additionally all four pressures will rise and fall with various driving conditions, which will naturally not trigger the TPM warning; but the loss of pressure from one tyre will not.

A good reason why pressures should be regularly checked.

it works by counting the revolutions of all the wheels, when one goes around alot faster to the rest it sets it off. If you put the crappy spare on one of the wheels it probably go off.

Interestingly I got my first (slow) puncture the other day and knew from the handling of the car that something wasn't right - the tyre monitor didn't flag up any problem. When I stopped the tyre was almost completely flat.

Popped the spare on and continued my journey...couple of mins later BEEEEEP - the tyre pressure monitor light appeared. Better late than never I suppose.

I've not properly read the manual - but I'm guessing it only works at higher speeds (i.e. - 2 mins later when I rejoined the motorway). It come on then becuase it detected the spare wheel was a different size.

After the tyre was replaced and the original wheel went back on, the pressure light come on again when I joined a motorway, again due to the change in size.

I remember the bit from the manual about resetting it with the button - so done that. Think it would only be useful if you got a flat on the motorway.

Mine has come on twice in the past week or so, after checking the tyres it seems the O/S/R tyre is down from 31psi to 22psi. Both times it was the O/S/R tyre thats been going down. Its going in tomorrow to have it checked out.