oxygen sensor rich. (code 45)

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i've had this problem before, had engine vaccum and brake booster replaced.
then my car ran good for about two weeks.
now its back. i ran a codes test and got 45, like last time
my car runs fine for now. nothing really happens like last time, which is weird.
except for exhaust braking not quite the same.

can anyone direct me as to where the problem could be?

thanks guys.

Vroooom!

[hako]

Can you give a bit more in the way of what is the problem like what do you mean by exhaust braking.

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aaaaah...

its not smooth, doesnt pop like it used to.
if that helps?