B8 S4 - TPMS
 

In my 2011 S4, the TPMS (tire pressure monitoring system) has never seemed to be accurate. When I would physically measure the tire pressure, it would be less than the recommended pressure, but the TPMS would never display anything. So I kind of would ignore it and continue measuring it manually (from time to time). About 1 month ago I replaced my tires (got only 17K miles on the original Pirellis). This morning I received a "TPMS system failure" message and so I reset the TPMS system thru the MMI. After that it seems to be fine. My one thought was that whatever was done during the new tire installation, may have become undone or something. Is this something to be concerned about though?

This system does not use pressure sensors, it uses the ABS to measure rotations of the wheels. You should ALWAYS reset it when you add or remove air from the tires and allow it to recalibrate. The system will not go off when the pressure is a couple PSI lower than whatever it was set at, it cannot detect this small a change, but will typically go off when the tire pressure has changed ~20% (so at 35PSI you would see a 7psi loss before alert, meaning you are at 28PSI and would not be in danger of anything more than loss of fuel economy prior to then.)

Clarify more
 

Like was said, it uses the ABS sensors to sense wheel spin speed, rather than actual pressure. What matters is change in rotational speed. So for instance, if you get the car back from the tire shop, they most likely did not reset the system. So you need to go into the MMI and reset the base level. You should do this if you manually adjust pressures too. Then if one wheel starts spinning at a different speed, it will set off the alarm.

Is it that it can't detect it or it is programmed to ignore small changes?

I say this because temperature changes affect pressure and maybe its a way to ignore this fluctuation.

Not sure, but with enough temp changes it will detect it, I am sure there is some "slop" built in for false positives, but there has to be a substantial enough change in pressure to lower the rolling diameter of the wheel and allow the ABS to detect this.

I don't believe it will trigger an alert with lower pressures due to temp drop because all the tire's pressures will drop at the same rate, with a system that uses ABS sensors it's looking for a speed difference in a single tire compared to the others, also there must be a difference in speed for that tire for a particular amount of time, think about going around a tight turn where the inside tires will be rotating at a slower speed then the outside tires for the duration of the turn (and this doesn't even trigger the ABS system), must be some complex software in there.

It will if the pressure in the tires drops 8 psi from where the tire pressure was set. That probably won't happen JUST from temperature alone. But if the tire pressure is down from set PSI, additional loss from temp can trigger a warning. It doesn't matter if all tires are affected equally by temp. It has to do with being 8 psi below set pressure.