I would never, ever give my engineering code to a customer, but would happily reset the code to default on demand (chargeable as appropriate based on response required) or during a service if the customer didn't want us to visit again.
Again, no decent co. will ever give out their own engineer codes, and would never agree to a 'shared code' on anything they are taking service or efficacy responsibility for.
Your company should have defaulted the system at day 1.