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Discussion Starter #1 (Edited)
While traveling 450 miles from home, I awoke the second morning to the engine starting as soon as the car was turned on, even with significant charge on the traction battery. There was a CEL and a message on the display saying "AC not on due to high engine temp (or similar wording)." AC did work in spite of wording. I checked the coolant level and hood latch, and I tried a computer reset by disconnecting negative terminal of 12V battery. No luck.

I called OnStar and retrieved the DTC of P0116 and they emailed me the description of the DTC:
P0116 The Engine and Transmission System is not performing as expected. An issue has been detected in the Engine Cooling System which monitors and controls engine temperature to maintain optimal operating temperatures and avoid overheating conditions. If you are driving under normal operating conditions, please check the temperature gauge on the instrument panel. If the needle is in the red warning area, the engine is too hot. Pull off the road, stop the vehicle, and turn off the engine as soon as possible.
I was fearful that if I let the battery fully deplete, and the engine was eventually disabled due to a perceived high temp, I could be facing a tow. Running in either HOLD or MOUNTAIN seemed to be consuming a lot of fuel. I figured the mixture could well be way rich as the computer was trying to reduce the perceived high engine temp. I was scheduled to drive home the next day. So, I found a dealer.

One hour after arriving at Lamb Chevrolet in Prescott, AZ, I had a diagnosis. I thought they were blaming it on me being a "pack rat" and then I realized they were referring to a real animal. Who knew there is really an animal with that name . . . I thought it was merely an endearing term for human hoarders. Being paranoid about them perhaps seeing my WOT sensor and not seeing the OEM sensor, I asked to see the problem harness pigtail. I was shown a loose wire from the temperature sensor hanging out from under the engine cover and some nesting material under there. Seems that on my second night there, a Prescott rodent had found my car. The technician said that the rats like the Volt (GM?) wiring as much as gummy bears. So, since I was facing a diagnosis charge either way, and fuel was being consumed at way to high a rate with questionable throttle response, driving back home and repairing (or trying) it myself wasn't a good option. $510 later, everything was good again except my wallet. $41 for the part and the rest for labor (3.3 hrs?)

Back in her cubicle, my svc writer said that some people had reported success using a product that emits up to 105 db ultrasonic sound and has flashing lights to make the under-hood area inhospitable for nest-building by the critters. I later found the product to be the MouseBlocker Pro. I guess the unit is hardwired to the battery, and runs anytime the voltage is between 11V and 13.2V. Although the a majority of reviews are positive, it's evidently not a guaranteed fix for everyone, as one purchaser reported that a mouse/rat had built a nest right on top of the actual device (who knows whether it was installed correctly and/or working). If I were to park outside on a regular basis in that area, I'd probably try the $90 device to mitigate the odds of having another $510 expenditure. I know there are tapes you can use to wrap the existing wires, making them unappetizing to the rats, but there are LOTS of wires under our Volt hoods.

If anybody else sees that DTC, you might want to look under the cover for this issue. Good luck.
 

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Odd that a code for engine not running at optimal temperature forces you to drive it using the engine and not the battery - You would think the opposite

Don
 

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Discussion Starter #3
Odd that a code for engine not running at optimal temperature forces you to drive it using the engine and not the battery - You would think the opposite
As soon as it happened, my host and I scratched our heads over that irony as well. I did find an old post on another forum regarding the same DTC by a GM tech who tried to explain it by saying that despite the wording on the display, GM had chosen to use the same message for low temperature readings as well. ???
 

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Discussion Starter #4
An important point for anyone who, down the road, might encounter these symptoms . . . if there is any chance it could be rodent damage, call your automobile insurer. I just found that my assumption that a part had "failed" and taking it to the dealer for diagnosis will probably cost me my comprehensive insurance reimbursement. Since I didn't take it to a partnering facility, USAA will limit any reimbursement to the amount one of their partners (probably not dealers), would have charged, minus the deductible.
 
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