Solar Inverter Faults & Error Codes: What They Mean & What to Do
Red light on the inverter, or an error code you don’t recognise? Here’s what the common faults actually mean, which are safe to wait out, and which need an engineer — fast.
The short answer
Most solar inverter faults fall into a few families: grid-side faults (voltage/frequency out of range, “no utility” — often not your fault and self-clearing), isolation and ground faults (moisture or damaged wiring — safety trips that stop generation), overheating and DC over-voltage, and communication/firmware glitches (often just the monitoring, not the system).
What’s safe to do yourself is limited: note the exact code, check if it self-clears, confirm the AC isolator is on, and try one careful power-cycle if your manual allows. What’s not safe: opening the inverter, touching DC wiring, or repeatedly resetting a safety fault (isolation, ground, arc) — that overrides a protection system flagging a real hazard. Persistent or recurring faults need an MCS-certified engineer.
What this guide covers
An inverter fault is one of the most common — and most alarming — moments in solar ownership. A red or amber light, an unfamiliar code on the display or app, and generation dropping to zero. The good news: many faults are harmless and self-clearing (the grid wobbled, moisture is drying off), and even the serious ones are usually straightforward for an engineer to diagnose. The key is knowing which is which — and crucially, which faults you should never simply keep resetting.
This guide translates the common faults into plain English, tells you what’s safe to do, and pairs with our repairs guide (costs and who-pays) and replacement guide (when a fault means the inverter’s reached the end).
First: is it actually broken?
Before assuming the worst, rule out the false alarms — a surprising share of “faults” aren’t:
- Is it just the app/monitoring? A dropped WiFi dongle shows “offline” or a comms error while the system generates perfectly. Check whether the inverter display itself (not just the app) shows a fault, and whether generation has genuinely stopped.
- Is it night, or very low light? Inverters power down when there’s no meaningful generation — a blank or “waiting” state at dusk is normal, not a fault.
- Has the grid just wobbled? Many faults are momentary grid events the inverter rides out and clears within minutes.
- Is the AC isolator on? The switch (often a grey box near the inverter or meter) can be knocked off — a common cause of a “no utility”/”no grid” fault.
What’s safe to do yourself (and what isn’t)
The golden rule: note the code, one reset, then stop
Safe: read and photograph the exact error code, check whether it clears on its own, confirm the AC isolator is switched on, and — if your manual permits — perform one careful power-cycle (off at the AC isolator, wait a minute, back on). Not safe, ever: opening the inverter casing, touching or inspecting DC cabling (it’s live whenever there’s daylight and can deliver a lethal shock), or repeatedly resetting a fault that keeps coming back — especially isolation, ground or arc faults. Those are safety systems. If a fault returns after one reset, that’s your signal to stop and call an engineer, not to reset again. The same safety rules apply as any solar work: DC is dangerous, and no saving is worth a roof fall or a shock.
Grid-side faults (often not your fault)
These relate to the mains supply, and many are outside your control and self-clearing:
- Grid voltage fault / over-voltage: the mains voltage has risen outside the inverter’s safe window (a known issue in some areas with high solar uptake). The inverter shuts off to protect itself and clears when voltage normalises. Persistent over-voltage is worth reporting to your installer or network operator — it may be a grid issue they can address.
- Grid frequency fault: mains frequency out of range — usually momentary, clears itself.
- “No utility” / “no grid” / “no AC”: the inverter can’t detect mains — check the AC isolator first, then whether there’s a power cut (do the neighbours have power?). If both are fine and it persists, contact your installer.
The reassuring theme: grid-side faults are frequently the grid’s problem, not your equipment’s. Note them, and only escalate if they’re frequent or persistent.
Isolation & ground faults (safety trips — take seriously)
These are the faults people search most, and they matter:
- Isolation fault / ISO fault / Riso fault: the inverter has detected the DC side isn’t properly insulated from earth. Usual causes: moisture ingress in a connector or junction box (very common — hence it often appears on damp mornings and clears as things dry), damaged cable insulation, a degrading panel, or rodent or bird damage to cabling. A one-off that clears is usually moisture; a persistent or recurring one needs tracing.
- Ground fault / earth fault / GFDI: current is leaking to earth — from damaged wiring, water ingress, animal damage or a faulty panel. This is a genuine safety trip and should never be repeatedly reset or ignored.
Both are the inverter doing its job — protecting you. The fix is diagnosis: an engineer isolates which string, connector or panel is causing the leakage, often with insulation-resistance testing. Left unresolved, the underlying cause (water, damage) tends to worsen, so prompt attention is cheaper than delay — as our repairs guide shows, a connector reseal caught early is far cheaper than a damaged inverter later.
Arc, DC over-voltage, over-temperature & EEPROM faults
- Arc fault (AFCI): the inverter has detected an electrical arc — a potential fire risk from a loose or damaged DC connection. Take this seriously; don’t reset repeatedly. An engineer should inspect the connections.
- DC over-voltage: the string voltage exceeds the inverter’s maximum — often a design/configuration issue (too many panels per string) or very cold, bright conditions. If it recurs, the array configuration needs checking.
- Over-temperature: the inverter is too hot — poor ventilation, direct sun on the unit, or a blocked area around it. Ensure airflow around the inverter isn’t obstructed; persistent over-temp in normal conditions suggests a cooling fault.
- EEPROM / internal / F-codes (e.g. F51, F11): internal electronic faults, often firmware-related. Sometimes cleared by a firmware update or power-cycle; if persistent, they can indicate the inverter is failing — see our replacement guide for the repair-vs-replace maths.
Reading brand-specific fault codes
Every manufacturer uses its own code system, so the exact code plus your manual (or the manufacturer’s fault-code list) is the fastest route to meaning. Common brands and where their codes crop up: Solis, Growatt (whose “standby mode” sticking and comms errors are frequent, usually firmware-fixable), Sofar, ABB/Aurora, GivEnergy, SolarEdge and Fronius. Two practical tips: photograph the code and the screen (invaluable for an engineer diagnosing remotely), and quote the exact code when you call — “it’s showing a fault” tells an engineer little; “isolation fault, code XX, clears then returns each morning” tells them almost everything. If your inverter is a brand whose manufacturer has changed status, code support may route through your installer rather than the maker.
When to call an engineer
| Situation | Action |
|---|---|
| Fault cleared on its own, hasn’t returned | Monitor — likely a transient |
| Grid fault, neighbours also affected | Wait / contact network operator |
| Fault returns after one power-cycle | Call an engineer — stop resetting |
| Any isolation, ground or arc fault persisting | Engineer — safety issue |
| Burning smell, visible damage, scorch marks | Isolate (AC off) & call urgently |
| Generation down with no obvious cause | Diagnosis (£80–£150, often credited) |
A professional diagnosis typically runs £80–£150, usually credited against any repair — and it’s the safe, definitive route for anything beyond noting a code and a single reset. The repair-vs-replace decision follows from what they find: a reseal or firmware fix is cheap; a failed 12-year-old inverter is usually a replacement.
Inverter faults: the do’s & don’ts
✓ Do
- Note & photograph the exact error code
- Check if it’s just the app/monitoring offline
- Confirm the AC isolator is switched on
- Try one careful power-cycle (if manual allows)
- Check if neighbours have power (grid faults)
- Call an engineer for anything persistent
✗ Don’t
- Repeatedly reset a recurring safety fault
- Open the inverter or touch DC wiring
- Ignore isolation, ground or arc faults
- Reset through a burning smell or visible damage
- Assume “fault” = broken (often it’s the app)
- Climb on the roof to investigate yourself
Frequently asked questions
What does an isolation (ISO) fault mean?
The DC side isn’t properly insulated from earth — usually moisture in a connector or junction box, damaged insulation, or a failing panel. It’s a safety trip. A one-off that clears as things dry is often moisture; a persistent one needs an engineer to trace the source.
What’s a ground fault?
Current leaking to earth from damaged wiring, water ingress, animal damage or a faulty panel. It’s a genuine safety trip — never repeatedly reset or ignore it. Have an MCS engineer diagnose it.
Why is my inverter showing a fault?
Common causes: grid voltage/frequency issues (often not your fault), isolation or ground faults (moisture/damage), overheating, DC over-voltage, or comms/firmware glitches. Note the code, check if generation actually stopped, and if it persists, get it diagnosed.
Can I fix it myself?
Only the basics — note the code, check for self-clearing, confirm the isolator’s on, one power-cycle if the manual allows. Anything involving DC wiring, opening the unit, or repeat-resetting a safety fault is an engineer’s job. Solar DC is live and dangerous.
What’s a “no utility” or grid voltage fault?
Grid-side: “no utility” means no mains detected (check the isolator, check for a power cut); grid voltage/frequency faults mean the mains is outside the safe window. Many clear when the grid stabilises. Persistent ones — contact your installer or network operator.
Should I reset the inverter?
One careful power-cycle is fine for a one-off if your manual allows. Repeatedly resetting a recurring fault — especially isolation, ground or arc — overrides a safety system flagging a real problem. If it returns after one reset, stop and get it diagnosed.
Note the code, reset once, never fight a safety fault
The bottom line: Many inverter faults are harmless and self-clearing — a grid wobble, drying moisture, or just the app offline. The ones that matter (isolation, ground, arc) are safety systems doing their job, and the dangerous mistake is resetting them on repeat instead of diagnosing them. Note the code, try one reset, and if it returns, get an engineer.
Get my free diagnosis quote →The verdict
A faulting inverter looks scarier than it usually is: a large share of faults are transient grid events, drying moisture, or a dropped monitoring connection — nothing that needs more than noting the code and a single reset. The genuinely important skill is recognising the faults you must not simply reset: isolation, ground and arc faults are safety systems flagging real problems, and overriding them repeatedly is how a small issue becomes a dangerous one.
So: photograph the code, rule out the false alarms, try one careful power-cycle — and if the fault returns, or it’s a safety trip, get a professional diagnosis. It’s £80–£150, usually credited against the fix, and it’s the safe, definitive answer. Free diagnosis quotes connect you with MCS-certified engineers who can trace it safely.
Get your free diagnosis quote from MCS-certified engineers →
Important
Going Solar UK is an introduction service, not an installer or electrical contractor. This guide is general information, not electrical or safety advice: solar DC wiring is live and potentially lethal, and diagnosis or repair of inverter faults should be carried out by a qualified, MCS-certified engineer. Never open an inverter, touch DC cabling, or work on your roof yourself, and never repeatedly reset a safety fault. If you smell burning or see scorching or smoke, switch off at the AC isolator if safe to do so and seek urgent professional help. Fault behaviours and codes vary by manufacturer — always consult your specific inverter’s manual.
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