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I have had this error code come up P0420 - Bank 1 CAT system efficiency to low.The engine light comes on after a 6-8 miles with the error code after i erase it.I am hoping it is one of two faulty Lambda but not sure which one.Is there a way of testing them OHMs ie multimeter ,There is nothing about fault ellimination,where as fault P0422 says check CAT.I can see them attached but not sure where the plug ends are in the engine bay compartment though going by the diagrams they appear to be quite short.Any advice would be great .Thanks

If there is no extra faults being logged to point to either of the Lambda probes being faulty, then you are just guessing, using a DVM will not give any useful info.

B1 S1 is the first and a controlling/regulating sensor, it can alter the base injection period, B1 S2 is only a monitoring sensor to check what is happening at the second cat.

It would be helpful if you used a proper fully VW Group compliant scan tool, like VCDS, OBDeleven or Carista as they can "see" all available VW Group fault codes, most cheap generic scan tools can only "see" emissions related fault codes, ie the end "end result/effect".

Thanks for that info,sorry for late reply but it has been one of those weeks where all seems to go haywire .