The early Ford Mustang Mach-E electric cars are affected by a software issue that might result in a depleted 12V auxiliary battery and inability to drive or even enter the vehicle.

According to The Verge's article, there is a problem with the software on the powertrain control module, which prevents the 12V auxiliary battery from charging if the car is plugged-in and charging the main traction battery.

In other words, the small 12V battery will get some juice from the main battery (through a DC/DC converter) only when unplugged/driving. It's an obvious bug and in many cases it will lead to a depleted auxiliary battery.

According to an official service bulletin filed by Ford with the National Highway Traffic Safety Administration (NHTSA), the problem concerns cars "Built on or before 3-Feb-2021" (at least several thousand).

"Issue: Some 2021 Mustang Mach-E vehicles built on or before 3-Feb-2021 may exhibit the 12-volt battery becoming discharged while the vehicle is plugged in during the high voltage charging process. This may be due to the parameters in the powertrain control module (PCM). To correct the condition, follow the Service Procedure to reprogram various modules starting with the PCM." - service bulletin

Since the Ford Mustang Mach-E is envisioned for over-the-air (OTA) software updates it should be an easy fix. However, Ford's OTA are not yet ready (beta phase) so at least for now owners of the affected cars will have to visit dealers to get a new software update.

The service procedure:

  1. Connect a battery charger to the 12-volt battery.
  2. Reprogram the PCM using the latest software level of the appropriate Ford diagnostic scan tool.
  3. Check the availability for software updates for other modules.

We guess that at least for more advanced customers that can check the voltage of their battery and monitor it over time, it's not a necessary update - at least in theory they can wait for the OTA/regular maintenance at the dealership.

Those who, on the other hand, were surprised by a dead 12V battery must first recharge (jump-start)/replace it. It might be tricky, as without the auxiliary power, you can't get into a closed Mach-E.

There are already videos how to do it in the case of Mach-E specifically. Basically, any roadside assistance should be able to help.

Various issues with charging the 12V lead-acid batteries affected multiple electric cars in the past. One of the most recent models with 12V problem was the Volkswagen ID.3 (solved).

Hopefully in the future manufacturers will be more careful on this topic, include OTA as soon as possible and maybe gradually will switch to lithium-ion batteries of higher capacity/higher voltage to generally improve the auxiliary system.

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