Just had my timing belt changed and my 3 litre.

Just had my timing belt changed and my 3 litre engine would not idle when warm. Adjustments were made. I have been told this should not happen by the belt being changed and that the camshaft slipped by a couple of teeth and the timing is out and that is why it would not idle. Just passed an emissions test, if the timing is out would it pass that test. Do I have any reason to doubt my mechanic and the test was done elsewhere.
Thanks