FoxESS Inverter Fault Codes: What They Mean and What to Do

Most FoxESS fault messages fall into five groups, and the group tells you more than the exact wording. Grid faults usually clear by themselves once the mains supply is back to normal. Faults about insulation, wiring or the inverter’s own electronics need an engineer. Communication and battery faults sit in between: some are a dropped connection, some are a failing part.

This guide uses the fault names in FoxESS’s user manual for its H1 and AC1 series, the single-phase hybrid and AC-coupled inverters fitted in many UK homes. Other FoxESS models use similar wording, but names and numbers vary by model and firmware, so the manual for your inverter is the final word.

Key points

• Grid faults (Grid Lost, Grid Volt, Grid Freq) mean the inverter has disconnected to protect the network. It reconnects automatically when the grid is back within limits.

• Iso Fault and Res Cur Fault point to an insulation or earth leakage problem on the solar side. One after heavy rain can clear; repeats need an engineer.

• Meter Lost and BMS Lost mean the inverter has lost contact with its meter or the battery. The system may keep running, but charging and export control can go wrong.

• Battery (BMS) faults and internal hardware faults aren’t homeowner fixes. Note the message, the time and how often it happens, then call your installer.

• Never open the inverter or battery, and don’t switch DC isolators unless an engineer has told you how and in which order.

Before anything else: is it still showing?

Open the FoxESS Cloud app or look at the inverter’s screen and check whether the fault is active now or only in the history. Many faults are logged, clear within minutes and never return. A fault that appears once during a storm or a local power cut is usually the inverter doing its job.

Write down three things: the exact fault message, the date and time, and whether it keeps coming back. That is what an engineer, or FoxESS, will ask for first, along with the model and serial number from the label on the side of the inverter.

Grid faults: usually the grid, not your inverter

These are the most common faults and usually the least worrying. The inverter constantly measures the mains supply. If the voltage or frequency moves outside the range it is set to accept, or the supply disappears altogether, it disconnects. UK connection rules require this, so a solar system never feeds power into a network that engineers may be working on.

• Grid Lost Fault: the inverter can’t see the mains supply. Expected during a power cut. The app may show this as “No Utility”.

• Grid Volt Fault: grid voltage is outside the accepted range.

• Grid Freq Fault: grid frequency is outside the accepted range.

• 10min Volt Fault: the average grid voltage over ten minutes has been outside the accepted range.

In each case the manual’s advice is the same: the system reconnects once the grid is back to normal. If a voltage fault keeps appearing on sunny afternoons, the cause is often high local voltage rather than a faulty inverter. Our guide to inverters that shut down on sunny days explains why, and how it can be reported to the network operator.

If the house has power but the inverter still shows Grid Lost, check whether the solar breaker in your consumer unit (often labelled “PV” or “solar”) has tripped. Resetting a tripped breaker once is fine. If it trips again, leave it off and call an engineer.

Insulation and wiring faults: call an engineer

These faults are about the solar panels, cables and connections rather than the grid.

• Iso Fault: the insulation between the solar wiring and earth has dropped too low. Damp is a common cause, so one after heavy rain that clears as things dry out isn’t unusual. Damaged cables, water in a connector or a failing panel can cause it too.

• Res Cur Fault: the inverter has detected leakage current to earth above its limit.

• Pv Volt Fault: the voltage from the panels is outside the inverter’s range.

• PvCon Dir Fault and BatCon Dir Fault: the solar or battery connections are reversed. These normally appear straight after installation work, not out of nowhere.

• Bat Volt Fault: the battery voltage is outside the normal range.

Repeated insulation faults matter. Left alone, they cost generation on wet days, and a damaged cable on a roof is a safety issue in its own right. Solar panels produce DC voltage whenever there is daylight, including on dull days and at dusk, so testing the array is work for an engineer with the right meters and protective equipment.

Communication faults: a link has dropped

• Meter Lost Fault: the inverter has lost contact with its meter or CT clamp. Without it, the inverter can’t see what the house is using or exporting.

• BMS Lost and Bms Ext Fault: the inverter has lost contact with the battery’s management system.

• Bms Int Fault: the battery modules aren’t communicating with each other, or a setting on the battery is wrong.

The manual’s fix is to check the communication cable, which is an engineer’s job if it means opening covers or handling cables near live equipment. What you can do is think about what has changed: building work near the meter, a new consumer unit, or someone moving things in the loft or garage. A Meter Lost fault often explains odd battery behaviour, so it is worth reading alongside our guides to a battery not charging and a battery not discharging.

One thing that isn’t on this list: the app showing the system as offline. That is usually the Wi-Fi link to FoxESS Cloud, not a fault in the inverter. Our guide to getting a monitoring app back online covers it.

Battery (BMS) faults: contact your installer

Faults starting “Bms” come from the battery’s own management system. They cover high or low voltage, high charge or discharge current, high or low temperature, cell imbalance, internal sensor faults and battery modules that don’t match. For every one of them, FoxESS’s manual says to contact the battery supplier.

Low-temperature messages are the ones worth understanding. A battery in a cold garage can hold back until it warms up, which is protection rather than failure. If it happens only on frosty mornings and clears by midday, mention it at your next service. If it keeps happening, the battery may need a warmer spot. Our guide to where a home battery can be installed covers location.

Internal faults: the inverter itself

The last group covers the inverter’s own electronics: current and voltage protection (SW Inv Cur, HW Inv Cur, SW Bus Vol, DCI Fault), relays that have stuck open or closed (Bat Relay, Main Relay, Eps Relay, and S1, S2, M1 or M2 Close Fault), memory faults (Inv EEPROM, ARM EEPROM) and the inverter’s internal cross-checks (GridV Cons, GridF Cons, Dci Cons, Rc Cons, RDSP SPI, RDSP Smpl).

For these, the manual suggests a full restart, then contacting FoxESS if the fault doesn’t clear. A restart means switching off the battery, the solar DC isolators and the AC supply in a set order and back on again. The order differs between models and battery types, so don’t attempt it from memory or a forum post. Ask your installer to talk you through it, or let an engineer do it. A fault that returns after a restart usually means a repair or a replacement, and our page on inverter replacement explains the options, including for older systems on the Feed-in Tariff.

What you can safely check yourself

1. Look at the app or the screen: is the fault active now, or only in the history?

2. Note the exact message, the time and how often it appears.

3. Think back: has there been a power cut, a storm or work on your electricity supply?

4. Check the solar breaker in the consumer unit. Reset it once if it has tripped; if it trips again, leave it off.

5. Photograph the label on the inverter (model and serial number) and the screen showing the fault.

Don’t open the inverter, the battery or any isolator covers, unplug cables, or switch DC isolators on a guess. If you smell burning, see scorch marks or hear crackling, switch off at the consumer unit if you can reach it safely and call an engineer.

Who to call

Start with the company that installed your system. FoxESS handles warranty claims through installers and distributors, who carry out the first diagnosis. If your installer has gone out of business, our guide on what to do when your solar installer has gone bust explains your options. We inspect and repair FoxESS systems whoever fitted them, through our solar servicing team.

Frequently asked questions

My inverter showed Grid Lost Fault overnight and is fine now. Should I worry?

Usually not. It most likely saw a brief interruption to the mains supply, disconnected as it should, and reconnected when the supply came back.

Is an Iso Fault dangerous?

A single Iso Fault after heavy rain that clears as things dry out is common. Repeated or persistent Iso Faults mean an insulation problem on the solar side that an engineer should find, because damaged DC cabling can be a fire risk.

Can I clear a FoxESS fault from the app?

Many faults clear on their own once the cause has gone. Clearing a persistent fault without finding the cause only hides it. If a fault keeps coming back, it needs diagnosing.

What do the numbers in the app mean?

The FoxESS Cloud app sometimes shows a number alongside the fault name. FoxESS doesn’t publish a public list matching every number to a fault, so go by the name and your model’s manual, and give both to whoever investigates.

Do you repair FoxESS inverters you didn’t install?

Yes. We service and repair FoxESS systems whoever fitted them, across London and up to around 50 miles out.

Book a solar system inspection. If a FoxESS fault keeps coming back, we’ll find the cause and tell you whether it needs a setting change, a repair or a replacement.