2000 CLK 430 "check engine light."
 

My check engine light comes on and stays solid, without blinking on and off. After a week or so, light goes off. Car runs well without any hesitation or missing. Light goes off after being on for a week. Gas mileage is fine. Any thoughts as to what I should do. Mechanic checked diagnostics which showed air mass sensor defect. However, the symptoms are not there. Thanks for any advice.

You probably indeed have symptoms, but they will be things like increased fuel consumption, bad exhaust mixture sent to cat converters, etc. The light doesn't always mean the engine will stop running and the like, but it always means you'll fail an emissions inspection.

As mentioned a bad air mass sensor can cause a lot of $$$ problems, esp the catalyst failure issue!

Have it replaced!