I almost choked on my water when he said "B-S-M" verbally. I've heard some stories from customers, but damn. Yes, let's get advice on how to go about dealing with an issue with a highly complex 5-figure value component on a vehicle from a guy who clearly doesn't even know what a battery management system is. (This isn't the only clear instance I've seen of a solid lack of knowledge.)
ANYWAY.
Specifically with the two most common errors facing older vehicles: most of these are triggered when the car knows that it can not trust the readings from the cell sense boards for one reason or another, meaning the BMS can no longer tell for sure that cells are within valid safe limits. Its temporary solution to this is to narrow the window of usable voltages to be within what it expects to still result in safe voltages, despite a growing error rate. Hence the "maximum charge level reduced" message that accompanies them. This lets you operate the vehicle sufficiently to get it somewhere to get it taken care of instead of leaving you dead in the water. Firmwares prior to about 2019.16.x would instead just leave you stranded. Current firmware at least gives you a way to use the vehicle for a bit.
This is TEMPORARY however. Eventually the amount of error involved will grow until the maximum charge level allowed is 0.
If you just find someone to reset/clear this, you've now just told the BMS, "All the readings are good, no worries! Trust them!" Then, you're going to charge your car, and the BMS is going to work with the readings that it had previously determined to be invalid as if they were real. In reality, you're now likely over-charging or over-discharging the battery pack, and because you effectively told the BMS it was wrong about the readings being inaccurate, it has no idea about this yet because it takes a non-zero amount of time and use for it to gather the data required.
Over-discharging, not usually a huge problem. You're just damaging the pack more.
Over-charging? Well, I hope you have good insurance because eventually your car and whatever it's parked next to will be a smoldering pile of rubble if the BMS doesn't re-catch the issue before the pack ends up overcharged.
Fortunately the BMS is absolutely excellent, and it will usually pretty quickly start catching on and immediately start adjusting usable range accordingly, at least in the background (which is one of the reasons why you find people who have had non-fixes like this done on their vehicles getting stranded with double digit miles of range showing on the dash). Eventually you'll be back in the same boat, since the issue hasn't been fixed.
And if you're an a*****e and are just doing this to sell the car and scam someone else into dealing with the problem, then honestly, **** you. I absolutely can't stand that people like this exist and are actively doing this. I've now had to tell at least 10 people over the past several months that they were scammed by sellers of vehicles like this, where the logs clearly show what was done too. Only one scammer so far had the foresight to wipe the logs entirely on the gateway, but the BMS keeps some records of its own. I've offered to help these people at zero cost with any lawsuits they bring against the scammers, one of which is a dealership who even admitted to previously having the error cleared. Several have taken me up on that. I actually have a deposition scheduled for next week, actually, for one of them.
So don't mess around.
And anyone who ends up in this boat (bought a 2012-2015 S, soon saw a BMS error like this), let me know if you want to see if you were scammed so you can act accordingly. I don't have infinite resources, but I hate scammers and will do what I can.
TL;DR - Don't clear BMS errors without fixing the issue first (which would normally clear it for you anyway).
