98goldes30

Hey looking for advise from other lexus owners regarding a cel code po300 up to 306. I checked the spark plugs 3 days ago got one out to see and it does in deed have a black tip... however its not from a burn, its carbon as i found some deposits on my exhaust. I was told I was running "rich"... I replaced the air filter and because i havent yet taken the spark plugs out the code still fires. A close friend told me he was 90% sure it was my cat. converter and the 02 sensor that follows. Im now going to check that sensor because it could just be that. If anyone has any ideas what would cause my car to run rich please respond.

BTW the dealership told me it would be around $300 to fix... so im a little bit mechainically inclined so im doing it myself. just wondering what you think is the problem.