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Very likely the battery is failing under high load and the car is falling back to hybrid mode as a safety. Probably have at least one or more cells that are seriously degraded and are falling below a given voltage threshold under load. No different than a dying 12v battery under engine starting conditions on a traditional ICE only vehicle. Pulling codes when this happens could help confirm this, but that is my educated guess what is happening. High EV load climbing a hill with the heater going is going to be a significant load on the battery. Do you follow the recommended battery management tasks many owners have already laid out and recommend? These vehicles unfortunately have poor management of the battery by default in 'Auto' mode and do require manual intervention to help maintain them and prolong their life. Here's a short abbreviated list of recommended tasks to prolong the battery life. It's not exhaustive and there have been a plethora of posts covering this not only here but on other sites that you can seek out: 1) Never drive in Auto/EV Now mode above ~50mph or so. Switch to EV Later. 2) Never use Auto/EV Now on the highway/freeway. Switch to EV Later. 3) In the winter if it is extremely cold and you use cabin heat, you may wish to start in EV Later and let the ICE do the bulk of the work to heat up the car. Once the coolant temp gets high enough that you see the climate use drop to 0, then you can drop back to Auto. You may even want to wait until the cabin reaches temp and the HVAC fan slows. The heaviest load on the battery from the climate system is trying to warm the coolant from low ambient temps. Once the ICE has done most of the work, just maintaining the coolant temp on battery is a low enough load to not be a huge issue. 4) In hotter weather especially if you live in a hotter climate like Arizona or South Florida, monitoring the battery temp is crucial. The battery is air cooled and even then it does a piss poor job at it. Even with the A/C going full blast, it makes little change in the battery temp. Once the battery temp gets above 90 or 100f, EV Later/hybrid mode should be used. The vehicle won't give you the battery temp by default, you'll need an aftermarket tool to view this either in the form of something like a ScanGauge or using a mobile app like Torque or ForScan with a cheap Bluetooth OBD dongle. As an addendum on the charging side: During hotter weather, you may want to set up charging to only occur overnight when the temps are cooler. The battery has ZERO cooling while charging. The fan you may hear is just for the the actual charging module and not the battery. Value Charging is often used to set the vehicle to start charging at set times. You can also find EVSEs with scheduling capabilities or use simple outlet timers (as long as they can support the amperage pulled by the vehicle).

Hi Cr08, Thank you for taking the time to explain and confirm this....I checked the code that comes up and you are right with one or more cells that are most likely degraded. The code that comes up is BECM P0DE1:00-28 Code: P0DE1 - Hybrid/EV Battery Cell Balancing Circuit N Stuck On and then I think the screen said cell balancing in progress. Since I have owned this from day 1 I have tried to prolong the integrity of the battery as you mentioned above but I think the hills portion I was not as good with. I am not ready to throw in the towel on the car just yet, will still drive it with care and fun but knowing its tendencies is very helpful. Thanks for all your help troubleshooting! Corby

That should be pretty clear just based on location of the sound. The evap leak test you'll hear from the rear of the vehicle in about the same place you hear the charging fan from when plugged in (if an Energi model). If it is anything up front in the engine compartment, it isn't that. From the engine compartment, the list of parts that could make noise without the vehicle running are the vacuum pump for the brakes, ICE water pump, aux water pump for EV heating use (this is another Energi only thing), the aux transmission fluid pump (Energi only), and the cooling pump for the hybrid inverter. For the items in the engine compartment, here's the rough locations: ICE Water pump and aux water pump: Passenger side Vacuum pump, aux transmission fluid pump, and hybrid inverter cooling pump: Driver side. It has been noted previously, though I don't have any threads to link to atm, that it isn't terribly uncommon (and not common either) for water to ingress in some of the harnesses and short some of the connectors causing things to run continuously even at key-off. I think the most common part to experience this is the ICE water pump if memory serves.