The OE sensors on some German vehicles with early TPMS systems didn’t have LF coils. So the parts can’t be read using the wireless LF signal that TPMS tools use.
Examples include: Audi, BMW, Mercedes, Porsche & Volkswagen vehicles that come equipped with OEM sensors made by Huf/Beru.
If shops have trouble testing TPMS sensors on 2000-10 German models, then the techs can try deflating the tire for a few seconds, during tool testing. The sensor should pick up the pressure drop inside the tire and transmit its data readings to the tool.