I got error 28 on the Yamaha PW series (with the eco+ mode) motor system yesterday. I got this Procedure to get rid of it and to reset the system:
1. Turn off (if not possible to turn off remove battry and put it back again).
2. Turn on.
3. Then push the on/off button again and keep pushing it until high, std and eco light turns on
4. Select the eco mode with arrowbuttons and confirm with the light on button
5. Push light button and both the arrow buttons at the same time
6. System wil turn off. Wait 5 minutes while the system restarts.
7. If this not clear the error then go to your dealer.