Error Code 15 is caused by either low or higher voltage than is normal. SP-2.3 protects the Printed Circuit Board by tripping this E15 when there is sufficient "Brown Power" or a voltage spike. Sometimes this happens rarely, a few times it has been linked to large motors, welders, or power uses elsewhere. It may be linked to improper breaker size. The first fix is to turn the breaker off at the electrical panel to reset SunPump.
The solution has been to turn the water temperature down, from 60C to 52C worked in our test to reduce sensitive protection. Reset the Breaker to clear the E15 from display. If iE15 persists, the water is too hot, it has to be used up or cool down first.
E15 shown above was caused by leaving the electric heater element on manual using the rocker switch, not turning it off, then having the High Limit thermal protection snap switch pop off. It is a simple bimetal switch triggered at 158 F. Use some hot water and it will reset automatically when the temperature falls back to the Heat Pump start temperature near 120 F. You cannot start the heater during an E15 overheat protection.
If E15 persists then you may want to ask your Power Utility for assistance. Most instances happen in smaller population rural areas.
This problem is not a SunPump issue, it is external. If you need to fix it but do not, it can lead to equipment damage that may not be covered under warranty. Dealers should consider installing power protection in areas with known Brown Power issues, like the Gulf Islands. Attached is an accessory to consider installing if E15 happens too often.
Note: E15 electrical current protection for the 6 foot tall SP 2.3 is not the same as E15 for the older 3 foot tall SP 2.2 that is caused by low water flow.