The gas tank access panel has locked and do not know how to unlock. Had the car 8 years and never happened before. Is there a lock/unlock button I am unaware of???? Can't get to the gas cap to fill up.

SOURCE: Door locks don't work

Disassemble the paneling on left side ,under the steering wheel. Remove all screws and pull out the panel until you can you can reach behind it to remove the two Black Relays, on the side closest to Steering wheel. Remove the Relay covers, Clean the contacts with a 1/8 fine strip of 600 wet/dry sand paper. Only place the paper between the contacts, and pull out while holding slight pressure on the relay Armature. Turn over the sandpaper and repeat once again. Very gently press the TOP resting contact pad closer to the armature, to lessen the gap between the contact points, so the armature doesn't have to travel as far to reach the closed contact position. Repeat the above with the other2nd Relay. Replace the covers, reinstall the relays And test the Door locks prior to buttoning up the paneling. Note, a pair of long needle nosed pliers will help in removing the relays. Just Grip the relay and rock in Forward and Backward as you apply upward pressure.

SOURCE: Tighten Gas Cap Error Message

seems that your vent hose on the tank is plugged so the tank is over pressurrizing sending the error code and not allowing you to fill it all the way unclog the vent tube and this will solve the problem

SOURCE: 2002 MB C320 gas cap problem

I had the exact same problem, went to the Mercedes dealer and they fixed it for free with a pair of pliers. They just pulled up the metal clips and turned them back into place.

SOURCE: 2007 chevy silverado slow filling gas tank

vent tube must be blocked off, check the tube first, thanks robert b.

SOURCE: Filled up gas tank and gauge still shows empty. I

ethanol fuel effects the fuel sending unit ,even just 10% ethanol.New sending units from gm adress this.