Mercedes-Benz S400 Check Fuel Cap light is on Inspection at your home or office.

Common reasons for this to happen:

There are four primary reasons for why the Check Fuel Cap light might come on:

• Fuel cap is not on: The Check Fuel Cap light exists to remind you that you forgot to put the fuel cap on. It’s not uncommon for drivers to take off the fuel cap at the gas station, fuel their car, and then drive off without ever screwing the gas cap back on. If your Check Fuel Cap light comes on, it may simply be that you blanked and forgot to put the cap back on the tank.

• Fuel cap is not securely fastened: The Check Fuel Cap light can come on even if the gas cap is on. If the fuel cap is not completely fastened, the vehicle will not recognize that the cap is on. Some fuel caps can be a little tricky to securely fasten, so make sure that your gas cap turns until it clicks, or until it feels perfectly tight.

• Dirty gas cap O ring: The fuel cap has an O ring that is an important part of your car recognizing when the cap is on. Sometimes this O ring needs to be cleaned and lubricated in order for the car to tell that the gas cap is on.

• Faulty Check Fuel Cap indicator: It’s possible that your vehicle’s electrical unit is malfunctioning, and that the Check Fuel Cap warning light shouldn’t actually be illuminated. This is a rather rare occurrence, but it can happen.