Help Needed! CLK 500 O2 sensors and Air Mass

[mserpe]

I have a couple of questions about my Mercedes CLK500. The "Check Engine" light just came on last weekend when I drove to Reno and I just had it checked at a local Mercedes shop (German Auto Kraft in Belmont).

They said, apparently based on using a diagnostic code error reader, I need 4 Oxygen sensors (Two before the converter, two behind) and an Air Mass replacement. The Oxygen sensors runs $220 a piece and with labor about $1000, the Air Mass runs $380 and labor, so I'm looking at $1500 in repairs.

Question is, based on the code diagnostics how can all 5 of these items suddenly go from good to bad and trigger the check engine lamp? It does not seem like that makes any sense. Are the codes not specific enough for a shop to know which sensor exactly? It seems like the shop wants to just replace all of them instead of one at a time to see which one is bad.

thoughts????

many thanks!