[WopOnTour]

Loss of Isolation (LOI) issues can be some of the most complex, difficult, and time consuming to diagnose (in ANY EV) due to the nature of fault and the vast number of components and amount of high voltage circuitry that may be involved.
So to maintain the continuity of the diagnostic process I certainly wouldn't suggest you visit another dealer just because they are encountering some difficulty in isolating the root cause. Bad advice IMO.

But I would suggest you make sure the dealer is aware of and is/has been following preliminary bulletin PIC5920 for additional assistance in reaching the correct LOI conclusions based on observed voltage offsets (read from the GDS2 scanning equipment) that can help to identify the most likely cause. An insulation tester (a special tool all Volt dealers have) is also used to test all high voltage cables and components.
But unfortunately it's not an infallible process.

Let's just say replacing the A/C compressor was certainly not out of the question as the potential root cause. They would have also flushed the A/C system at that time as well as any introduction of incorrect refrigerant oils can create these same LOI issues. It's already happened a few times already that I am aware of when the Volt's A/C was recharged with the incorrect equipment and resulting in LOI DTCs. Even incorrect coolant (or coolant concentration) in either the battery or electronics cooling systems can also trigger these LOI DTCs.

What year is your Volt?
Have you owned it since new? or did you purchase it used?
Has it ever been involved in a roadway accident?
Has it ever had any body work performed?
Has it had any major work performed on any of the primary high voltage components? (battery, drive unit, power electronics?)
Has the cooling systems or A/C system ever been serviced by any non-dealership service facility?
Has anyone ever serviced or added coolant or water to any of the underhood reservoirs?
Has the dealer asked you all of these questions already?

WopOnTour

 

[WopOnTour]

And if it takes long enough to find the problem, the faulty component likely will release it's store of smoke and
then the problem will be easy to find !!

However "smoke" would require a completed circuit between the isolated 360V positive and negative DC rails in order to create an actual current flow that would certainly blow one of the high-voltage fusible elements within the circuit and creating some significant arc flash damage as well that would visibly simplify diagnosis.

In this case however the detected loss of isolation to chassis ground would be only 1 leg of either 360+ or 360- terminals which on their own represent only an incorrect voltage potential that exists. These DTCs indicate a loss of normally very high resistance (>400Mohms) between one of these legs to chassis ground. The fault detection system for this type of isolation issue are part of the motor control circuitry within the power inverter module and therefore have little to do with physical proximity to the actual short- part of what makes them somewhat difficult to isolate, especially if intermittent. So while there certainly COULD be a short to the transaxle case within the motor drive unit itself, it's replacement should NOT be based on the fact that the DTCs appear to call out MG A/B/C in the code description.

Hopefully they will use the aforementioned bulletin, and follow the well documented process which also demands Volt technical assistance be involved in the decision making process.

WOT

PS> Once the repairs are complete I suggest you speak with your Volt Advisor as to any possible goodwill treatment with respect to how long you were without your car. Others here have experienced some appreciated responses from GM and/or the dealer in this regard.

 

[WopOnTour]

Update 2 - Tech informed technical support of the scenario leading to the CEL. Tech support sent him a new Coolant Temperature Control module, should have it back today or tomorrow. Will definitely have the chance to test if it is fixed as it is getting down to 8 degrees here this weekend.

I suspect they are actually replacing the K10 Coolant Heater Control Module as it is a potential failure component of excessive loss of isolation. Hopefully they used the bulletin to complete an included work-sheet that documents various voltage levels at specific points of the high-voltage circuit as reported from various modules. Then based on those results, the offending component that represents the "leak" can be more accurately identified.
Hopefully they'll have if fixed up and back to you very soon.
WOT