355 high revs, 'slow down', burning smell and problems with the lambda results

Hi guys, new to the forum. I own a '97 355 5.2 imported from Germany.
When the car was delivered to me, the delivery guy said when he picked it up from the showroom it revved up to 6k revs so to stand back when it was getting unloaded.
He fired it up and it did not face any issues, it fired up and idled perfectly. However, while driving home the 'slow down' light kept on flashing and getting out the car there was a horrible burnt plastic smell coming from the rear of the car, which every now and then you can still smell.
Anyways, after an inspection from underneath, I noticed that one of the sensors fitted in the catalytics wasn't screwed on properly and was sorta bent. I've ordered a new part and we'll see what change that will make.
Other issues I faced have been a slight leak from the power steering reservoir (as per every other 355).

Now, I took the car for an MOT and it did not pass for a too high lambda value, apparently it was double what it was supposed to be, and they guys that tested it said that there is a slight misfire, needing some sorta service.

Could that be due to that bent and poorly screwed sensor? Or could it be linked to that burnt plastic smell in some way? What do you guys think?

Cheers
Fab

 

Notenough info. If your mech inclined check your st and Lt fuel trims and fix the sdl. Check the throttle peddle, carpet floor mat,maybe engine oil level. If you just bought the car take it too your mechanic and make sure your not hurting anything major. Do lots or reading on this site and others. Welcome and good luck