Originally Posted by pmckechnie

I have seen this problem many times over the years. Do a simple test, turn the ignition on but don't start the engine. If the gauge reads 3 bar then you have an electrical problem, possibly the sending unit or its connection. If the gauge reads 0 and goes to 3 as soon as the engine is started (after it is warmed up) it still could be the sending unit. My first guess is the wire on the sending unit was bumped and lost its connection when the oil was changed.

PaulM