I've developed a CEL when filling up with gas. Oddly enough, the CEL clears by itself at half tank or so. Codes range from P0150 P0300 P0302 P0177.

This has happened on 2 tanks pf gas already

just listing some possibilities...

loose gas cap
o2 sensors going bad

I was thinking o2 sensors but why would it clear itself at half tank. Brought it to the dealer twice and they say most likely 02 sensors but couldn't understand why it clears itself when the fuel level goes down

what year is your car?

i don't know much but i've got a 2010 and i've had to replace 2 o2 sensors after 6-7 years...

since your cel comes and goes with your fuel level, it may be possible that your gas cap isn't sealing up tightly when closed... maybe it needs replacing...

2010. I'll go check both the gas cap and o2 sensors. Whats really crazy is it started acting up as soon as it got home from the dealer after a Service B

I got my car back after some service last week and after filling up full tank had CEL. Took back to dealer who said o2 sensor and its been there since so I wonder whats up.

Hope they fix it because my warranty is about to end soon. 2013 48k miles

Even if the gas cap looks fine - its a cheap first step to buy another one, toss it on and see if anything changes.

yes

i would also recommend clearing your codes afterward and monitor it for a while... if cel comes up again, check the codes and see if they are the same as before.

There's another potential cause: overfilling of the fuel tank when pumping gas.

This happens on many cars (not just Mercedes). It is a result of excess gasoline flooding a vent system. This manifests on many vehicles as a trouble code. The trouble code would indeed go away after a portion of the overfilled tank has been consumed, as the vent system gets cleared of gas AND the trouble code is usually one that goes away after the ignition is cycled off.

Overfilling can sometimes be "non-obvious":
- Pump shutoff is incorrectly set or too slow
- Pump delivery rate is set high so even a timely shutoff keeps the fuel flowing a bit too long
- Car is not level because front or rear axle is "off the concrete pad" that surrounds the pumps in an otherwise paved lot and so is lower
- Owner does not realize that you MUST stop pumping when the pump clicks off, not try for the next "even dollar amount"
- Some vehicles are also just too sensitive as a result of stacked manufacturing tolerances or marginal sensors

Try stopping the fuel flow a little early and see if the problem goes away.

Jim G

interesting... never knew about this... i always stop when it automatically clicks... i cringe when i see people keep forcing it... click after click after click...