If you have owned a Tesla Model S or X for a while, you have probably heard about the dreaded issue of the MCU failure that is bound to happen after a certain number of write cycles committed to the on-board eMMC module. Typically, this affects older vehicles or ones with high mileage on them. I was surprised then that my car was starting to exhibit the symptoms that eventually led to complete MCU (main computer unit) failure. It was only 2+ years old and I barely crossed the 50,000 miles mark few months earlier.

It started with me not being able to connect to the vehicle using the mobile app. I contacted Tesla’s support online and we performed all the steps suggested by the associate, to no avail. The steps included:

• complete restart of the MCU (with the brake pedal pressed),
• powering off the vehicle and leaving it off for few minutes,
• reinstalling Tesla app on the phone,
• un-pairing the phone from the car and re-pairing it,
• trying to install the app on the other phone.

None of the above fixed the issue, but I was not too concerned. I had an upcoming appointment with the Tesla Service Center for some unrelated concerns, so I just added this issue to the list of things to be addressed (this was also concluded and suggested by the online support).

As the date of my appointment was drawing near, I hadn’t observed any major issues with the MCU, other than the fact that it stopped displaying cover images for the music on my USB stick. Ironically, on the day of my appointment, the main screen went completely blank and the car took a minute to power on when I was leaving for work in the morning. The screen stayed blank though (yes, I tried to reboot it again, in case one wonders). The IC (instrument cluster) was working, so at least I had some visibility into the vehicle’s status. The A/C was running as well, so it didn’t seem like a big deal. For now.
Unfortunately, it was one of those rare days in Texas where the weather was very humid, misty and it started to drizzle. There was still fog on the ground when I was leaving and my windows were starting to fog up as well. Without access to advanced climate controls (which includes the sunroof – you are unable to control it with the scroll wheel when MCU is down!) it was quickly getting worse, so much so that I decided to get back home and drive to work using my wife’s car instead. My appointment with Service Center was not until later in the afternoon, so I knew the weather should clear up by then and I would be able to get there even if A/C would fail completely. In the worst case I could still roll the windows down :).

Not being able to control the sunroof or temperature settings using the scroll wheel was probably the biggest surprise, but here are some other things that I discovered:

• no sound – not just the music playback, but no feedback from turn indicators, navigation or proximity sensors around the car when parking,
• the energy efficiency information was not updating (the chart was gone, too),
• the odometer stopped refreshing the mileage while driving,
• TPMS information was not available,
• the outside temperature was not showing either.

• 

Overall, the MCU plays a more vital role than many realize. It is not just the entertainment console, but it controls many of the vehicle features. Luckily, I have purchased the Extended Service Agreement, so the parts have been replaced free of charge (minus the standard $200 deductible for each service visit while under ESA), but I was still baffled as to why it failed at the first place. The only reasonable explanation I could come out with is that my original MCU was already replaced once as a goodwill gesture due to screen discoloration. That was before the UV treatment was available. It must have been a refurbished unit (so was the one I received this time).

It is worth mentioning that if this happens under warranty, Tesla will replace the MCU for free and the replacement comes with 4-year warranty. If you are out of warranty, this can be an expensive proposition.