I bought this 1990 300SEL that had been sitting parked for over 2 years.
As part of getting it in shape over the last 2 months I changed the oil & filter (twice), replaced the fuel filter, and it is on its 3rd tank of fresh premium gas.
Each time, I poured a bottle of fuel system cleaner in the gas tank when I was filling up with gas.

The car idles nice and seems to run strong, so I took it for the emissions test. It failed.
Hydrocarbons 1.4136 GPM (maximum allowed is 1.4000) and
Oxides of Nitrogen 2.7471 GPM (maximum allowed is 2.5000).

1. Is it possible that the oxygen sensor is bad even though the car runs nice and the check engine light never comes on ?

2. Is there some heavy duty stuff I should pour or spray directly somewhere to get rid of built up carbon or other junk?

3. I purposefully didn't change the spark plugs yet because if I need to run some heavy duty stuff through the car I wanted to have it foul the existing plugs and THEN change to nice clean plugs. I didn't think changing the plugs yet would make much difference in the emissions since the car runs well - am I wrong about that? Can changing the plugs reduce the emissions that much?

Any help much appreciated, especially with step by step directions that a moron can follow.

Thank you.