[Electric Tire Shredder]

Letting the 12V battery die has caused many different issues for other owners, even after replacing it with a good fully charged one.

If I were you I'd put the car to sleep*, then leave the error-causing humidity sensor disconnected, then disconnect the negative terminal for 30 seconds. If that fails, try for 30+ hours (or as long as you can without inconveniencing anybody.

Humidity Sensor SUCKS! (until you unplug it)

It activates the range-reducing A/C compressor, without even lighting the A/C button! It deactivates recirculate, which can also reduce range by heating/cooling outside air instead of pre-conditioned cabin air. It can cause false alarms including ABS, RBS, HSA, ESC, PRND, SRS (airbag), etc...

www.fiat500owners.com

* Key out for 2 minutes without opening or closing any door or hatch. It's okay to leave one door open (such as the left, to pop the hood) as long as you don't close it until after disconnecting the battery.

 

[Electric Tire Shredder]

No.

It's a different situation:

• What you accidentally did was expose the system to 10V, 9V, 8V, 7V, 6V, 5V, etc, which is apparently a bit hard for it to deal with, creating issues that aren't auto-corrected even after 12 volts is reestablished.
• Disconnecting the negative is just a more complete reboot of the onboard computer, by switching from 12 volts to zero volts, & back to 12.

Even if it did re-create the same situation, you'd be no worse off.

 

[Electric Tire Shredder]

Many other users, including myself, have disconnected the 12V negative, always with good results. I've only had to do it once for replacement, & once to reset, which fixed the issue I had. However I've disconnected it about 30 times for other reasons (click here if you're curious & check post #2).

All it ever did, besides reboot the computer, is zero out the trip gauges & the date/time. It didn't even zero out the radio stations, audio settings or menu preferences such as the charge timer.

Other users have had that timer auto-default to "active", so just check later that it's still set to your own personal preference.

 

[Electric Tire Shredder]

I am pretty nervous...

Here's the list of proven solutions.

You can start with whichever option makes you the least nervous. If you're lucky, the first one you pick will work:

• With car asleep, disconnect/reconnect negative terminal
• With car asleep, disconnect humidity sensor, & don't reconnect
• Clear error codes with $17 WIRELESS OBD dongle & free AlfaOBD Android/Windows app (I consider those tools essential, since they can sometimes revive a "dead" 500e)
• 2-minutes-off, & then turn the key on, then off another 2 minutes, etc. 10 times is usually enough, but apparently 50 can be required to clear some errors if you don't use OBD.
• With car asleep, disconnect HV plug 5 minutes, under back seat*
• Take it to a dealer (like Fiat"Cares" usually suggests), some of whom charge $600+ just for the OBD scan that you can do yourself with the $17 dongle.

* Just make sure the black handle is straight up perpendicular prior to reconnecting. It's easy to get wrong, but also very easy to get right, if you just check the pics at the right end, so I included both full-size AND "click to enlarge" versions:

Click to enlarge:

 

[Electric Tire Shredder]

...aftermarket bulbs don't always work...
I went to our local Fiat dealer and bought the same "W21/5W" and "P21/5W" bulbs... these new bulbs are working ...

I'm glad you got it fixed, & that you confirmed the easy @Skipyg fix.

It's pretty weird though. Generic bulbs have always worked fine on my 2013 500e (both headlights & both brakelights) & also on my friend's 2016 brakelight.

 

[Electric Tire Shredder]

I've never had any issues using bulbs of any brand from AutoZone or O'Reilly or PepBoys or any auto parts store, as long as they're the same bulb number, printed or stamped into the bulb itself.

If you don't have the original bulb that worked, the bulb numbers are in the manual. There's a copy of nearly every year's manual in item 13 here:

Electric Tire Shredder 500e Website Map (Click Here)

 

[Electric Tire Shredder]

@leob52 stated "the exact P21/5W did the trick". In other words, the problem was fixed by getting a bulb with the exact same number as the original bulb printed/stamped on it.