I had an oil sensor fail on an '01 T4 - the evidence was the low pressure warning light coming on continuously after a moderate run, but good oil level and obvious circulation under the breather cap.
On very close inspection there was a pinprick of oil around the electrical connector to the pressure sensor above the oil filter. The diaphragm underneath must have punctured. Simple to replace. Earlier models have two oil sensors - a second at the RHS of the head. There's a bit more diagnosis involved in that case.
HTH
Bookmarks