Checked the zone wiki and it said the following;

" To test, first unplug the C101 and earth pin 4 (brown/purple wire). Now if you switch the ignition on, the gauge should go to full hot. If it does, repeat the test at the brown plug by removing the cable from the brown plug, and earthing the terminal with a piece of wire (for older cars with two-pin terminals, just bridge them together). If it now works, the issue is normally corrosion of the C191 plug."

only problem is there is no pin 4 on my c101 its empty

any suggestions greatly appreciated!