Could be either one, or more likely the wire from the sender is shorted to ground.
First test, Unplug the sending unit and see what the needle does when you turn on the key. If it still goes high you have a gauge or wiring issue. If it does not then change the sender.
If the needle still spikes, check the senders wiring for an area where it is shorted to ground as that is the most likely problem.
Wednesday, April 19th, 2017 AT 9:27 PM