I have a flashing yellow light on a husqvarna 550 epos mower. I cannot turn the mower off underneath the mower and it the start-stop button does absolutely nothing. I cannot update the mower at all either. Any ideas?
Have you tried to Bluetooth into it?
Cannot connect to it via bluetooth. This is not unusual in and of itself as this seems to happen with a lot of mowers. It is the complete lack of response from the on off switch that is the most unusual. I think it is brain dead.
Try going through the process of adding it as a new mower and see if you can bt in
Husqvarna are by far the worst with bluetooth
I can’t get it to flash blue at all. It only does the flash yellow. 
Lights don’t matter, just try to search for it using the plus button.
The other option is to take off the console where the button is and manually depress the mechanism.
Did you get this resolved? I’ve had it before but can’t remember exactly what I did. Flashing yellow indicates it needs the pin code, but when you can’t connect there’s no way to enter it.
Possibly try a wired/USB connection with autocheck3 if you have that available?
Turn the power switch to off. Open the mower, hold up the rear and disconnect the ribbon and the cord from the top half. Wait about 30 seconds and plug the cord in first and then the ribbon. Only replace a screw on the rear and front in case you need to open it up again. Check for a response from the mower.
If still not working, open the mower and disconnect the batteries for 30 minutes and try again. If you lay the top to the rear , then you don’t need to disconnect the ribbon or cord.
If that doesn’t work, then you need a new main board.