Optima XM6?

my optima xm6 system was on a twelve month contract, but the engineer has now ceased to trade and will not attend any callouts. i am suspicious that he has placed an engineers code into the machine as the day my contract was due to run out it started to fault. (the system goes into alarm mode even though it is not armed) i would be grateful if someone would provide me with some advice on how to prevent this from occuring. Thankyou Mark