My CLK320 airbag light came on recently, I had it re-set and sure enough the light came on again after a few days driving. They gave me a printout of the fault (Ignition circuit with component R1 2/20 left front side airbag ignition squib has high resistance). Do I need a new left front door Airbag ?? How come they are able to reset it??

I doubt you need a new airbag. The failure rate for MB airbags is virtually nil. If your car were in my shop, the first thing I'd do, (assuming everything checked out OK) would be eliminate the intermediate connector between the airbag and the SRS control unit. That connector is most likely adding just a small amount of resistance to the circuit, causing the issue you have. It's actually a fairly common fix which I've used many times on a variety of MB's.

Thank you for your reply and care. Is that resistance measurable??

Since the ignition squib is built into the airbag it can't be directly measured. DO NOT use an ohmmeter to measure the resistance of the airbag!!!

It's not a terribly complicated procedure, but I'd suggest letting your local dealer fix it. No doubt they've done many of them. It shouldn't cost much, maybe an hour or two of labor. I wouldn't recommend DIY for a system as critical as SRS.