Gen4 Buyer’s guide? Looking at this one:

Welcome Tom,
Mark the coolant level on the coolant bottle & check every 2 weeks or before and after a long drive. If you catch the problem early, you won't damage anything. If you allow the problem to persist to the point the CEL is telling you something is wrong - your risking major repair costs.
Re: Dr Prius app & scanner; an honest shop wouldn't mind if you scan the car or they will use their own scanner to show you what you need to know. I understand why a shop wouldn't let customer's plug-in their cars - someone can really disable or damage the car if they know or don't know what they are doing - so have the shop mechanic available when you go look at the car. It will be his option to use your or his scan tool.

Hope this helps...