The VW uses two sending units -- one for the coolant temperature gauge and one for the oil gauge. In the absence of gauges, the senders send a signal to the dummy lights, and turn them on when the oil or coolant is low. The sensors' sending units are located in the intake manifold or top of the block for coolant, and in the middle or lower block, for oil, depending on the year and model VW. The units are held in with one or two bolts, or may be screwed into the block, and have a wiring harness connector tying it into the main wiring harness.
Locate the sending unit that is malfunctioning. Unplug the wiring harness connector, using your fingers by pulling the tangs on the side of the connector away from the plug, then pulling the plug off the sensor.
Remove the retaining bolt or bolts, using the appropriate socket. If the sender's body has a hexagon shape, it is a screw-in sender. Turn the sender counterclockwise, using a wrench, to remove it. Pull the sender out of the block.
Insert the new sender and tighten the retaining bolts firmly, if it is held in with bolts. Do not over tighten the bolts, as you could damage the sender. If the sender is the screw-in type, screw the sender in by hand as far as it will go. Tighten it firmly, using a wrench. Plug in the wiring harness connector, ensuring that it snaps into place.
