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Are you having a starting and or stalling concern?
Your right about the code meaning and if you are not having any starting and or stalling issues then it may just be a old stored fault code. History has shown that ECM-720A (P1670) can be falsely set when the engine dies, or doesn't start which may have been an issue before you got the car. I would clear the code and see if anything reset. You can clear the code with your scan tool or also by disconnecting the negative battery cable for a few minutes. There is also a service bulletin released by Volvo calling for replacing the antenna ring for the keys if this code returns. Lets clear it first and see if any thing returns. See the bulletin as well in the ink below for reference if needed.https://dl.dropboxusercontent.com/u/104923897/P1670Oct%2012.pdf
As long as the system is showing a fault code its not going to pass inspection. If the code returns after resetting then that means there is a problem and needs to be addressed.
You can reboot it by disconnecting the negative battery cable for a few minutes but if the problem is still there then the code will reset until its addressed and repaired.
In most cases if the thermostat is sticking open the computer will set a fault code for that. If you not getting any cooling system codes its not likely. As for the idle concern, you may want to have a look at the throttle body and see if there is any sludge build up and if so give it a good cleaning and see if that will help.
A phone call will cost you $26.00 .
I do not know of any other way, it has to run through there system.