Okay, I have a 1994 C220. Took it in for smog today and it failed. The said the hydrocarbons were too high. Max is 50, mine was 70. Anyone have a clue what this means? How much will it be to fix? The smog shop couldn't help. The don't work on Mercedes. Any help would be appreciated. We already put in new spark plugs and changed the oil. Thank you.

 

Ηι,
Means that your engine/katalyt ,some small amount (ppm) of gasoline, can't burn it,and this comes out unburned, and pollute the air.
There are many reasons for this,but is probably not practical for you to
know all,cause i gess you are not a DIY's.
Generally speaking must check:a)Injectors(clean),b)O2 sensors,c)katalytic converter (have you ever change it since 1994 ?),d) The AI FILTER!!, and a lot more, which are all workshop job.
There are two limits for HC, in Europe,[e.g Greece: idle<120ppm, 2300rpm<100ppm,(by the way which part of world you live?)
Yours 70ppm are in idle or 2300rpm one? Anyway 20ppm difference is not big deal(from an engineer's point of view !) From my experiance, i recomment you to drive your car fast (using high revs /5500rpm throu gears before a test ,because this give better HC & CO marks,especially when a car runs in cities traffic.
I hope that i gave you an idea,
The costs depends from what is realy fault,from few $ to a lot.
Also there are some tricks!

Gus.-