citroen piccaso ?

i'm assuming this is a petrol engine. if the warning is there on idle but goes when you raise the revs, then the oil filter is probably broken. remove filter cover, remove filter insert, and check down the middle of it (it's a tubular style filter) and see if the cross-shaped bit of plastic is broken (bet it is). this bit of plastic holds an oil pressure valve open and allows oil pressure to be maintained at idle. replace filter with a genuine citroen item cos aftermarket filters tend to break a bit more readily.

slipdiff
March 2010