Oil Level Warning But Level OK - W168 A Class

[Aletank]

A mate has a W168 A Class, every now and again he will get a message on the dash saying add 1.5 litres of oil and gets a warning noise also.He has checked the oil level with the dip stick and all is fine, he will get the warning then it may be OK for a couple of days/weeks.
Is there any kind of sensor maybe that can go faulty/play up on these?

Any wisdom greatly appreciated!

 

[jaymanek]

oil level sensor does play up on these...

 

[Aletank]

oil level sensor does play up on these...

Thanks for the reply,
Do you know where these are located?
How easy they are to fit ?
How much they may cost ?

THanks again

 

[jaymanek]

On oil pan, held on with two torx, think you have to lower the engine out to get to it..