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Honestly I'm not terribly surprised. I used to be in the camp of 'Drive it until the plug-in capacity is dead and drive it as a hybrid vehicle' but over time have realized that's not really how it works out in reality. This became especially prevalent when I had a somewhat self-inflicted issue come back up recently with the EV+ mode. But the short version is that with even a somewhat aged battery in my 2013 with a little bit of degradation, it gets cranky when it is deeply discharged. Those with more severe levels of degradation I can certainly see running into issues just driving around once the charge level gets low. In my case I only experience issues after EV+ has done its thing at a destination I won't be charging at. Next few drive cycles the CEL is on and ICE is on full time until the battery is brought back up to a reasonable level after it was deep discharged and sat. But it's all about a few damaged cells that will drop significantly out of the normal voltage range when discharged and pulls the rest of the battery down with them.   Ford just cut too many corners with the overall battery management and EV operation thresholds trying to make it run like a full EV when it really isn't that capable. Add in the lack of owner education on their part and doubling down on not warrantying any degradation just adds salt to the wound. I've said it in a few places but because of this combined with owner ignorance (using the term literally, not in a derogatory manner), I don't have high hopes of these vehicles (the Energi's at least) being good buys on the used market in the coming years. I certainly wouldn't buy one without a super thorough inspection and battery health check.   That all said, I've been super happy with mine. 2013 Energi SEL. Fully loaded. Sitting at 96k miles. All of the major known quirks have been resolved either by the previous owner or myself so she's a 100% reliable vehicle. Did the Sync 3 upgrade which was well worth it and I'm perfectly happy with what I've got now and don't see a need to upgrade anytime soon. Also did the 4G modem upgrade which has been working great despite the neutered functionality for our vehicles. But at least there's no monthly/yearly subscription to worry about.   But I've also educated myself over the now nearly 8 years of ownership on all the quirks and nuances in order to better maintain the overall health of the vehicle but it's really 95% keeping the battery happy.

I don't know how useful this is to others, but thought I'd mention I just swapped out the ACM on my 2014 Ford C-Max for one that I purchased on ebay from a 2013 Ford C-Max (due to the same popping issue).  I'm running into the same volume problem with my new ACM.  The popping has stopped, but I have to crank the volume to max to get a decent level for radio (still a bit low TBH).   A bit of a discussion on reddit is here: https://www.reddit.com/r/cmaxhybrid/comments/1hqpb1g/replacing_acm/  

My 2016 Energi died at 143,000 from this....  The imbalanced battery pack was causing the power train to fail. So the car is totally unreliable on the road. Salvage. There are no battery packs in California, and it would cost $8000 to $9000 to repair it. Good riddance. It ain't a Toyota. A project car for Ford that failed the long haul.

Mine died this week, at 143,000 .. my son put 48,000 miles on it as a used vehicle. The sad part is that Ford has no HV battery packs in northern California, even if I wanted to replace the battery pack. Cost at the dealer is roughly $8000-9000, but again no parts. The dealer is not saying much they are mum on this issue, I wonder if Dearborn wants it that way. The vehicle is essentially salvage. I wonder what the percentage is that fail between 100,000 and 150,000 (maybe because of bad thermals, or other issues that may be supplier related).  The C-Max was a throwaway vehicle for me. Sad!  

My 2013 C-Max energi 89k has been having this fuel door won't open now for over a month, including during a 2,500 mile trip over Thanksgiving.  I tried the various techniques mentioned above, then tried replacing the Fuel Door relay in the back fuse box.  Still no luck but now the 12V battery runs down if I don't use the C-Max every day to keep it charged.   Now I think(?) is hear a transformer-like buzzing near there -- a stuck solenoid perhaps?  I just pulled the relay back out -- will update soon.   Happy New Year  --  btw we love this car !!!  

The transmission can be removed without pulling the engine; however, you should use a "cherry picker" to support the engine from the lift bracket found on top of the engine, then disconnect the motor-mount on the passenger side of the engine bay.  This allows the 1/4" movement (towards the passenger side) you will need to get the transmission out.  Unlike the Ford Fusion Hybrid, Ford engineers did not leave enough space in the engine bay to allow safe removal of the transmission with the engine fully-installed.  It is literally that tight in the C-Max engine bay, and that is why Hybrid & Electric Car Sales out of Franklin, TN, charges $500 more than the identical FH35 transmission repair service, compared to the Fusion.  If anyone wants to attempt a repair of their HF35, I'm happy to share what I learned going through the entire process, in my garage, with the help of none other than my amazing brother-in-law, Steve.

We finally removed and split-open the HF35 in our 2013 C-Max Energi after 222,500 miles.  The "grinding" noise had started ~6 months prior (~12,000 miles prior).  This is the first serious problem we have had with this car since new.   NOTE: I had changed the transmission fluid twice (first time @ 99,659 miles, the second time @ 200,657 miles).  The interval recommended by Ford in the owners manual is every 150,000 miles(!).    Three of the four roller bearings in our HF35 had severe spalling.  I replaced all four roller bearings as well as the pump filter.    In hindsight, I wish I had changed the transmission fluid every 50,000 miles; maybe that would have extended the life, maybe a lot, maybe a small amount.  I think 100,000 miles was asking too much out of Mercon LV in the HF35 application, let alone 150,000 miles which Ford recommended.  We ordered a 2025 Ford Maverick Hybrid (CVT), and I'll be replacing the transmission fluid every 50,000 miles, religiously.     I have about ~$250.00 into repairing the HF35 in our C-Max.  Transmission has been reinstalled and car has been test-driven:  No more grinding noise.  Hoping to get another 100,000 miles out of this car.   If you are mechanically-inclined, have a "cherry-picker", transmission jack, and are willing to spend ~$100 on bearing puller tools (if you don't already own them), this can be done with the help of a second mechanically-inclined friend/relative, and not that much $$... 

This data in late 2024, looks different now, I think Consumer Reports found some transmissions with issues maybe built around 2015 (MY 2016) and in early 2013 releases, but overall the eCVTs are very reliable. Consumer Reports is stringent with giving out double green arrows. And if you do a bit of research on eCVT from Toyota, these things are iron clad. If Ford used Aisin (Toyota's transmission house) then this should not be a grave concern, outside of what is noted. 

Hi is this still available?

I just "rebuilt" the HF35 in our 2013 C-Max Energi (222,5000 miles) in that we replaced all four roller bearings, which were badly spalled.  We did not index-mark any of the gears, and since they are all concentrically uniform, I can't forsee any issues.  Learned a LOT from this process.  If I could find all of the related torque-values somewhere, I could start a side-hustle.  NOTE: Be sure to buy Ford-branded bearings.  I didn't, and the BT4Z-4221-A equivalent I had purchased would not seat on the gearshaft, even using a 20-ton press.  Removed that POS and replaced it with a new Ford P/N BT4Z-4221-A, and it dropped right on (heat bearing to 375 degrees F in an oven and chill the gearshaft with a chunk of dry-ice, for 30 minutes.