98 E320 codes read

Fellow NG members:
Purchased a code reader and pulled the following from my 98 E320 w/ 61,000 on the clock:
P0173, P0170 and P040.
Reads as though all the O2 sensors capped out, but I'm not sure. Here's
why: The check engine light flutuates between on & off, depending upon how much gas is in the tank. I changed the gas cap. No effect.
Thanks in advance for your help.
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The P0170 and P0173 are the codes for fuel trim malfunction in Banks 1 and 2 respectively. They are the ones that I was getting as indicted in my previous messages. I think that the P040 code appears to be missing a digit. See the following site for Mercedes codes applicable to cars 1996 and on:
http://www.carsoftwares.com/mercedestroublecodes.html
Dan

how
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First check if for EGR valve operation/flow.. Code is P0400
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Sorry for the typo, AJ The third code was P0400...
I filled the gas tank, made sure the gas cap was affixed tightly and removed the codes. I still suspect the gas cap was in some way involved; previously, before purchasing a new gap, if I jiggled it the CE light would immediately extinguish.
Question: Assuming the error codes return, and the old gas cap was not the source, how would the problem impact operation of the car? Lower fuel mileage? Uneven top side acceleration?
After diagnosing this, I can decide whether to take it to my reliable independent or punt and go to the stealership.
Again, thanks......

how
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EGRcode will set trim codes.
Common problem is egr tube blockage or sticking erg. A simple DIY test for egr system flow/operation is to supply vac to the egr valve vac actuating port [ small hose] with a hand pump while the engine is running at idle . If the valve opens and the pipe is clear, the engine will run rough...if not , the valve is not opening or the pipe is plugged.
When releasing vac to egr, it should Snap shut. If it hangs up, it is not closing all the way [ sticking] ...
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I've got an appointment with my idependent. Looks like new 02 sensors are in the offing, though he promised to check the Air Mass Meter.
I'll keep you guys informed.
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