2013 Blue Topaz Volt with 80K miles. Two instances of reduction in power. First case, doing sustained 80 mph in 100 deg.F the drive power dropped suddenly. Being by an exit I steered off and proceeded to coast to a light and eventually pulled under a gas station canopy. It felt like motor B had shut off, and it was using A, because it ran at really low speed and felt like molasses. I think the ICE was still running, but can't be sure. The left console had 2 messages, the initial one said a main propulsion unit was off-line, (or something like that), then another more permanent one saying "propulsion unit unavailable". This instance I was in Mountain mode at 50% battery and had filled up with gas about 50 miles back.
I shut off the car and opened the hood, waited a few minutes then powered on. What I eventually discovered was that my Volt starts the ICE if the hood is up, whereas it starts electric (normal) with it down. Lets not deal with that issue. On-Star got involved and ran a diagnostic which passed OK. I also looked for messages but found none.
After an hour or so cool-down it appeared OK again so I proceded another 150 miles driving normally. My suspicion is the shut-down was heat related, rather than a fault.
Second case, 2 days later, coming back at 80 mph, in 100+ deg.F, it again lost power. This time it was definitely loss of ICE, as it switched to electric and posted a "propulsion unit unavailable" followed by "engine not available, service soon". I reduced speed to 40 mph, and drove about 3 miles to an exit. It was not sluggish like the first time but the messages were similar. Before switching off I noticed there was a high-pitched whining like a fan going gangbusters. Electric was at 25% on "hold" when the ICE quit. Still 100 miles from home, I began hopping between charge stations to ensure I didn't get stuck if the ICE quit again. As the day cooled off I tried the ICE again and it worked fine. My suspicion was possibly normal operation but I'd expect a warning before quitting.
Next day trip to the Chevy dealer I asked them to check if any faults had been logged that shed light on the shut-downs. Here is an excerpt from the service receipt:
Scan for codes w/gds2. Found multiple DTCS
ECM - U0293
Hybrid Power Train Module 2 - U0293
Transmission Control Module - U0293
U0293 Lost Communication with Hybrid /EV Power Train Control Module
Scan tool communication with module, intermittent lost communication.
Recommend replacement of power inverter module. <<< This was done.
And on into the 90K service list.
Time will tell if my Volt now behaves differently, but I don't often commute at 80 mph in 100+ weather. I will try to avoid it or drive below the speed limit. I have also taken a hack-saw to the number-plate holder reducing the radiator overhang by 2".
Despite the scariness of losing power, the Volt never actually broke down entirely, and seemed to recover OK once it cooled off.
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