Mystery Button?

Looks like to tire pressure monitor reset button. Turn car power on (position 1, no engine running). Hold the button for 20 seconds until the tire pressure logo comes on the small screen on the dash. Start car and drive. It learns pressures from driving over the next few minutes.

 

Useful to reset after manually-checking during seasonal temperature changes.

 

So on my car I randomly loose connection to the sensors and receive a no signal error. To be safe, I replaced all four sensors in the tires and still sometimes get a lost signal. I just reset and move on. I typically only drive on weekends and tend to check tire pressures before leaving the house so I cannot comment on different sensor readings verse the interior display.

 

It is actually kind of nice to have the reset button on the 2005. I threw in 3rd party sensors when I got new tires and working perfectly well with the button reset.

 

The TPMS system monitors pressures for a nominal value +/- a tolerance. My guess is that the +/- tolerance is ~3psi. So, if your nominal value is 35 psi, you'll get an alert if the pressure drops below 32psi or goes above 38psi. This is useful for alerting driver of pressure needing to be corrected. It's also the reason why you get all of the tire pressure alerts when the temps drop appreciably during the fall/winter.

As far as your manual gauge vs the TPMS measurement, it's doubtful that these will agree to any level of accuracy. That being said, push the reset button once you've established your chosen pressure with the manual gauge.

Hopefully this makes sense / is helpful.

 

My '08 C63 has a reset, but only accessible via menu. Of course, it's old and regularly loses connection to the in-wheel sensors. The joys of older cars, eh?

 

Batteries likely shot since Mercedes used the same Beru/HUF-Beru sensors. They typically only last 5-8 years.