Acquired a 2004 S-Type back in June. Drove it 350 miles with some cylinders misfiring. It has been in the shop since then but the mechanic has put me off several times, and I may have to resort to fixing it myself.
I know I need a new catalytic converter and oxygen sensor. How can I tell which side?
How can I tell which cylinders are not firing properly.
What other issues might be causing it to misfire?

Thanks in advance,

Mark

 

Read codes using an OBD tool (from $5). Each code has info you want.