Sunsynk F42: AC Line Low Voltage
F42 means the grid voltage your Sunsynk sees has fallen below the allowed range, so it disconnects to protect itself and the grid. This is usually a grid-side event like a brownout or a weak supply, and the inverter reconnects on its own once the voltage is back to normal.
Sunsynk and Deye are the same platform Sunsynk hybrids are rebadged from the same hardware family as Deye, so F42 means the same fault on both and these steps apply either way. The practical difference is the app you read it in: Sunsynk uses SunSynk Connect, Deye uses SolarMan. If your unit is actually a Deye, see Deye F42.
Grid, your wiring, or the inverter?
- A brownout or weak grid. The utility voltage genuinely sagged. Most common, and it self-corrects.
- Voltage drop on a long or thin AC cable between the inverter and the connection point, which can pull the reading low under load.
- A loose AC connection adding resistance.
- Standard limits. The inverter follows grid-protection rules, so it trips even on dips your appliances would shrug off.
How to handle it
- Wait for it to clear. If the grid voltage was just low for a while, F42 clears by itself when it recovers. A brief, occasional F42 is normal on weaker grids.
- Notice how often it happens. Rare and brief is fine. Frequent or long F42 events point to a genuinely weak supply or a wiring issue.
- Check for obvious causes. If your whole area has low or unstable power, it is the utility. If only your system reads low, the AC cabling may be the cause.
- For persistent F42, call your installer to check the AC cable sizing and connections, or contact your utility about the supply voltage.
Quick decision flowchart
F42 appears
↓ is it rare and brief, clearing on its own?
✅ Yes → normal grid dip. Nothing to do.
❌ Frequent or long-lasting → weak supply or AC wiring issue.
↓
Persistent F42: installer checks AC cable sizing and connections; utility checks supply voltage.
Related Sunsynk codes
FAQ
My appliances work fine, so why does the inverter complain?
Grid-tied inverters must follow strict voltage limits set by standards, tighter than what household appliances tolerate. So the inverter can trip on a dip your lights barely notice.
F42 happens a lot in the evening.
Evening demand can pull neighbourhood voltage down. If it is frequent, mention it to your installer and utility, since a chronically low supply is not ideal for the system.
Helpful guides
Sources
- Sunsynk Hybrid Inverter User Manual (F42 = AC line low voltage; verify grid voltage range and check AC cable connections).
- Sunsynk fault-code references and installer documentation.
⚠️ Safety disclaimer. Solar inverters carry lethal DC and AC voltage even when "off". This guide covers observation only. AC cabling and connection checks must be done by a licensed installer or electrician.