I had a similar problem some time ago, maybe this here works:
Try to switch into another truck (One of your drivers or new bought).
Get out of your office after this action so that you sit inside the other truck, than get into your office again to switch back into your own truck.
Everything should work fine again then.
If not:
Do you have this problem since the beginning or just since the end of the career?
@Mutko
Continue looking in other service stations, some parts are really hard to find. Don't give up your hope so fast.
