Please help, sister's 1.6 R reg golf oil pressure light started flickering/staying on on her way to work, sent me text, and went over and changed the oil and filter, light did not come after a quick spin, and she drove it home no probs, all is well, then she has texted me tonight and the light has come on again, are they sender units prone to failing ? or have i got to drop the sump and clean the strainer if there is one ?