F42 on a Sunsynk ECCO single-phase hybrid SG05LP1: Grid voltage fault (AC line low voltage)
Short answer
On a Sunsynk inverter, F42 means: Grid voltage fault (AC line low voltage). The inverter stops until the fault clears or is reset.
Reviewed 24 September 2026 · 1 source
What it means
The manual says: Grid voltage fault (AC line low voltage).
Part involved: grid voltage.
When it shows: Grid voltage below limit.
Likely causes:
- Grid under-voltage; wiring connections; settings (also listed as Parallel_Version_Fault in parallel systems).
Is it safe?
| Does the inverter stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
The inverter stops until the fault clears or is reset.
Resetting itself: yes.
Manual reset: Check voltage vs spec and grid set-up page; seek help.
What you can check
- Read the full code and message on the inverter screen or in the Sunsynk Connect app and note the date/time for your installer.
- Check the inverter's AC isolator next to the inverter and the solar/battery breaker in the consumer unit are ON (do not reset a breaker that trips again - call your installer).
What the engineer will check
- Check grid voltage and AC cables; check grid set-up (UK G98/G99); in parallel systems align firmware versions.
Who to ask for: Installer, with DNO (network operator) if grid supply is at fault.
Certification: None for homeowner checks; any grid-protection setting change needs installer and DNO consent (G98/G99).
What you might notice
Sources
- Fault Codes Causes & Solutions Version 1.0 ↗ · Version 1.0 · page 9 · Reviewed 24 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 24 September 2026