PCA SDR

My TPC system on a 2007 997 is acting up. It began with one wheel not registering (---), then two and now intermittently two or occasionally none.
Also the "gong" sounds and the icon comes on saying "TPC Unavailable" (or something like that). I assume failures/battery issues with the sensors and nothing systemic, but wanted to ask before I go charging off to replace sensors. Is there some more investigation I should do before spending money? Thanks.

One simple thing to check - most TPC systems are measuring not tire pressure, but the rotation of the tire with respect to the other tires and with respect to the baseline (usually you reset after checking tire pressures).
The system uses magnets on the wheels which rotate past a fixed sensor. If the sensors were knocked out of place, or the magnet knocked off, or (I heard of this) the magnet picked up a bunch of iron filings, the TPC system can't get a reading from that wheel.
Check around the wheel sensor area and see if it needs to be cleaned - particularly the wheel that first started failing.

Kim is right for BMW and other types but not Porsche
The 997 has pressure sensors with Batteries.

They are likely just dying.
They are good for 4 years and that is a 6 year old car.
A Porsche diagnostic tester will tell you for sure.

But an easier diagnosic is to go to a discount tire store.
They have very sophisticated magic wands they can wave over the wheels to read battery level.