Well start by having a catalytic converter pressure tested remove oxygen sensor put a gauge in if the pressures high that means the cat's clogged up. But if not that generally it means turbo is broken. Realize subs are endless money pits one of the big reasons they don't make them anymore people just got sick and tired of them breaking all the time

Usually when a turbo gives out you'll see obvious symptoms like lots of oil smoke pouring out of the exhaust pipe due to the failed seals.

There's a lot of plumbing associated with the turbocharger as well as the BPC control valve (the valve with 3 lines going into it) operated by the computer, the "hooter" valve (bypass valve), and a wastegate. That all has to be working for the turbocharger to function. It could even be as simple as the retaining ring on the wastegate control rod rusting off and the rod coming loose. Also check that all hoses are properly connected and electrical connections on control valves are solid.

Unless the converter is actually clogged (have that tested as Scotty recommended) the P0420 may not be related to this problem. Beyond the generic OBD2 code a Tech2 or equivalent scanner can reveal more data.

Lots of information on troubleshooting the 9-5 can be found on Saab-specific sites:

https://www.saabnet.com/tsn/bb/9-5/

https://www.saabcentral.com/forums/9-5-workshop.19/