The check engine lite came on in my wifes C280 and the codes are as follows"34, 30,84,35(indicating lean mixture).

My mechanic isn't sure of the fix and suggests we start by replacing the oxygen sensor before the CC first and then the one after if need be.

Both sensors are original(108K miles) but I'm reluctant to try his "let's see if this works" plan without more assurance, although he's probably right given the mileage.

Comments and suggestions please.

Many thanks.

 

You're right to be concerned about an approach that throws parts at it. Given the mileage, both the O2 and cats could be concerns, but find a mechanic who knows how to figure it out better as there are several things that should be checked first.

Here's a link to give you an overview of what he should have been thinking about - see the section on causes of lean fuel trim issues:
http://www.engine-light-help.com/oxy...sor-codes.html