Thank you, That explains a lot. I do know about the Nitrogen use, as it doesn't change pressures with temperature changes. I saw a nitrogen Generator on Leno's Garage website. It is great to prevent moisture(rust) on steel rims and wooden rims. I do have a Nitrogen Bottle in my shop, but the cost of the Nitrogen Generator is a bit high for me since I own a Collision Center and not a Mechanic Shop. I do plan to aquire one in the future if possible. Usually the GREEN Valve caps means it has nitrogen. Nitrogen is very dangerous to deal with as for the asphixiation dangers.

I will try to adjust pressures to find which one is acting up.

 It's great to have this type of Forum to get help when needed.

Ok here is how it works,the sensor does have a battery in it which is sealed into the unit and it works like a rechargable battery.when the car is moving that rotation of the wheel is what recharges the battery,when the car has stopped and is no longer moving the unit shuts down after a few minutes,and stays that way until the wheels are moving again.Here at the Honda Dealership where i work we also started using nitrogen in all our new cars because customers were always complaining about the TPMS light coming on.That has been cut so drastically now that it hardly ever happens any more,now thats not to say that the particular unit may have a bad battery or the sensor itself may be bad,try switching the tires around and see if the same thing happens on that wheel or does it change.There is also a main receiver unit mounted in the roof of some cars or in the main cabin of the vehicle.Hope that helps. Minsuki.

Where can I find the replacement batteries? Is there a part number for it? It's not a problem to pull all four tires off and change them out.

Oops, disregard my post

The 2002 to 2006 MINIs used the ABS wheel speed sensors.

The 2007+ have actual TPMS units in each wheel - special valve stems as the sensor is the inside portion of the valve stem.

I've had both (2002 MCS & 2008 Clubman-S).

I am fairly certain that on these cars there are no TPMS "sensors". Using the ABS wheel speed sensors, the ECM looks for the tire that is spinning faster than the others past a certain threshold. The tire with less air pressure in it effectively has a smaller diameter than the others, so it must spin faster to cover the same distance as the other tires. This is how it works on the R50 & R53, and I'm pretty sure it's the same setup on the R56.

The issue you are experiencing could be related to a problem with the ABS system or its sensors.

The batteries in the TPMS sensors are 'supposed' to last 7-12 years, but I had to replace them in one of my other cars after 5 years. 

4 years seems a bit early, but they do tend to start giving false readings when the battery runs low...

Ken

I had all the tires checked at my mechanic shop ( No Charge) and he set all at 40 PSI. In the morning when I get in my car the sensor warning is on. I will try a few more things. I just wondered if there is a bettery in each sensor, if so I would like to replace them all after 4 years. Thanks for the reply.

Are you sure it is not your gauge that is wrong? I had this exact same thing a few weeks ago - gauge said all were at 38psi - in reality they were at 31psi.

An accurate gauge is a great investment.

 I own a 2008 Mini Cooper S and lately my TPM warning comes up, yet my pressures are fine. It does reset. Is there a Battery in each sensor and does it need to be replaced? I figured it may be a low battery or one is damaged. If I have to pull all 4 tires off to check them I would like to have new batteries for each or a replacement for the one acting up. Can anyone help me with this problem? Since the car is 4 years old it is out of warranty even though I only have 17K miles on it.