I had a Vauxhall Antara and have had an intermittent problem with my fuel gauge dropping off occasionally to read empty even if the tank is full. Unfortunately, now on start up the gauge reads just over half, when full, when I initially put in my key and then drops off to empty when I start the engine. I understand there are two fuel sender units in my car. Could one of these be the problem? If so how do I determine which?