is it possible to check the O2 sensors somehow without having to replace them?

Question

I'm working on a P0420 code on the Audi A3 TDI and understand that the O2 sensors may be the fault. Is it possible to check the installed units for proper operation somehow without replacing them haphazardly?

Answer 1

Yes, but you need special scanner that can read the signal from the O2 sensor to the Engine Control Module (ECM). You could tested with an ohm meter but the it's not very accurate.