I bought my scanner online. I bought the ACTRON CP9180 for close to US$200.00. Great investment since I have three cars including the van.
I do not want to be offensive about your mechanic. But in my opinion, any shop dedicated to do a good job should have at least one of these or equivalent.
The scanner reads the temperature from the computer. The temperature sensor goes connected to the computer. The computer uses it for various functions.
There is another way of reading the temperature by reading the voltage of between terminal 59 at the ECM (computer) and ground. Initial voltage should be between 3.12 and 3.52V for the LEFT mark. As the engine warms up, when the voltage drops between 1.10 and 1.36V the RIGHT mark should be aligned with the roller on top.
If the mechanic can identify terminal 59 at the ECM, then he could adjust the fast idle cam. The only care he has to take is not to make any short circuit in the ECM when reading terminal 59.
My experience with Firestone places or similat has not been pleasant. Therefore, I do not recommend them at all. I just trust them for wheel alignment and balancing and still check everything of my auto before and after their work.
Hope it helps.