check engine light killin me

[thekid]

I have a 2000 ml 320
my light came on and stayed on took it to a shop they pulled 3 codes
p0135
p0155
p0173
they told me it was an o2 sensor problem had them replace. everything was ok for a couple of days but now light is on again shop says the same codes are back. they are try to fix problem, they have had my truck since yeasterday, i dont want to back blind about the problem.
can anyone give me a hand