The oil level gauge should work when you turn on the ignition, and immediately after starting, and should be inactive while driving. But don't rely too much on this gauge: Check the oil level with the dipstick regularly.

If the oil level gauge does not work, you can take it out and clean it, and also clean the contacts. To get to it, you have to loosen the diesel filter holder and place it carefully next to the spare wheel (remember to remove the cable).
The error can also be caused by a poor contact on the instrument console.