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Need some input please. Have two recent problems with my '97 C280's (w/65K mi) and I am trying to determine their relevance to each other:

- Three days ago my check engine light came on (first time ever).

- Dealer tells me by phone it’s normally the rear 02 sensor and/or the mass air sensor.

- My local Union 76 gas station read the code on a Snap-On OBD2 reader. It's error P0171. They reset the light, but it comes on again 50 miles later.

- From my research, code P0171 = "System too Lean (Bank 1)) faulty air mass flow sensor"

- I install a new set of Bosch plugs and a new Bosch mass air sensor ($238).

- I drive it for 15 miles, then take it back to the gas station for a read and a reset.

- Now it reads P0171 and P0170. They decide not reset it. The mechanic viewing the OBD2 reader tells me the front 02 sensor is working, but seems "slow." He says the problem is likely a dirty injector, an intake manifold leak, or a vacuum line leak.

- I consult my error code chart, finding this (verbatim):
P0170 = Fuel Trim Malfunction (Bank 1)) check vacuum leaks first or...
P0171 = System too Lean (Bank 1)) faulty air mass flow sensor

- I add a can of LubroMoly fuel injector cleaner, fill the fuel tank, inspect the fuel cap and, later at home, disconnect the battery (for 15 minutes) to reset the CE light and inspect the engine bay for any obvious vacuum leaks.

- The car is driven for 30 miles. So far the CE light is OFF but there is now a new problem -- the fuel gauge reads empty and the low level light is ON. I turn the ignition off and back on and the gauge reads full (as it should). Later, it goes to empty again and now stays there no matter what.

Please help me with these questions:
1. Is there a well-known place to check for vacuum leaks on a ’97 C280?
2. Is there any relationship between the CE error codes and the fuel gauge?
3. After the new MAS do I have to have the dealer reset the "fuel trim" whatever that means? Or does disconnecting the battery do the same thing?

Any help would be greatly appreciated.
 

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C43, E320, G500
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I've had P0170 and P0173 and recently P0450 codes that are causing the CEL to stay on. I've changed out the MAS and no change. I use a cheap scanner from Autozone. My MB mechanic used his scanner and only found fuel level sensor problems (P0460). The car have been running fine. So, I'm not too worried about it.
 
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