Any chance your VW Passat is equipped with an aftermarket stereo system? There is a K-wire which connects to the VW stock radio from the ECU (engine control unit), and is used to communicate radio system errors/faults. Aftermarket systems do not have a K-wire attachment. With the K-wire disconnected, the vehicle can not communicate with a smog machine.
We recommend, if possible, installing an original factory stereo, or finding a professional aftermarket stereo installer who can wire your VW's radio to function properly with the ECU. It can often be done by installing an interface adapter between the aftermarket stereo and ECU. Search or ask about "Smart Radio Wiring Harness" for newer Volkswagen models.
If this is not the case (your VW Passat has its original stereo installed) then you'll need to dig a bit deeper into why there is a communication failure. A smog check repair station will need to check the DLC (data link connector) pins to ensure none have been pushed in, as well as check the power, ground, and ECU fuse and signal wire to the DLC. The fault can and should be found, but you'll probably need a smog check repair station which is familiar with Volkswagen.
