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Fault code index · Fox ESS H1(G2) inverter & battery

Fox ESS Fault Code Index

Every Fox ESS inverter alarm from the official H1(G2) manual and Fox Cloud — what the fault code means, the most likely cause, and what to do next. Find your code below or browse by system area. Covers H1, H3, KH, and AC1 series.

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Sourced from official Fox ESS H1(G2)/AC1(G2) user manual V1.0.8 Linked to full diagnostic guides where available Covers H1, H3, KH, AC1, and T-series hybrid inverters

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Grid / AC Ground / safety Solar / PV Battery / BMS DC bus / hardware Relay / EPS Communication
Fox ESS fault code you can't resolve?

If your Fox ESS inverter is showing a fault code in Fox Cloud or the red LED is lit and you're unsure what to do next, a remote diagnostic identifies the root cause from your monitoring data — typically within 30 minutes.

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Fox ESS hub BMS comm fault Grid code fault Battery not charging EPS not working Fox Cloud offline
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How to find your fault code. Open Fox Cloud (app or web portal), select your device, and check the alarm history. Each fault has a numeric code and an LCD name — for example, Error 14 appears as BMS Lost on the inverter display. The red indicator LED (B) on the control panel lights up when the inverter is in fault mode. On the LCD, navigate to History → Error log for a timestamped record of all past faults.
Grid & AC faults

Fault codes triggered by mains supply conditions — voltage out of range, frequency shifts, or total grid loss. The inverter disconnects automatically and attempts to reconnect once conditions return to normal. Grid Lost, Grid Volt, and Grid Freq faults frequently appear together during the same grid event.

Grid Lost Fault Fox Cloud Error -1 — no AC supply
The inverter cannot detect a grid connection. Check the AC isolator below the inverter is switched on and the MCB/RCBO on the solar circuit at the consumer unit has not tripped. Fox ESS recommends a 20A MCB for 3.7kW models and 32A for 5–6kW models. If an RCBO is fitted, it must be bi-directional and rated at 100mA — a standard 30mA RCD will nuisance-trip. Often appears alongside Grid Volt Fault and Grid Freq Fault during power cuts. Grid fault guide →
Grid Volt Fault Fox Cloud Error 26 — voltage out of range
Grid voltage has exceeded the upper or lower limit set by the G98/G99 grid code. UK nominal is 230V with a permitted range of 216–253V. If voltage is consistently high (above 250V), this may be a local DNO supply issue — particularly common in rural areas with long cable runs. The inverter will attempt to reconnect once voltage returns to normal. If it triggers in daylight hours only, the inverter's own export may be pushing local voltage above the limit. Grid fault guide →
Grid Freq Fault Fox Cloud Error 27 — frequency out of range
Grid frequency has drifted outside the permitted 49.5–50.5 Hz operating window. This is almost always a grid-side event outside your control — it typically resolves within minutes as the national grid rebalances. If it appears alongside Grid Lost and Grid Volt faults, it indicates a complete grid outage. Persistent frequency faults in isolation are rare and may indicate a fault in the inverter's frequency measurement circuit.
10min Volt Fault Fox Cloud Error 28 — sustained overvoltage
The rolling ten-minute average grid voltage has exceeded the upper limit. Unlike Grid Volt Fault which triggers on instantaneous spikes, this code indicates a sustained high voltage condition. The inverter typically reconnects after 15–45 minutes. If this appears repeatedly during peak solar hours, an export limit may be needed to reduce the inverter's contribution to local voltage rise. Contact your DNO if voltage readings consistently exceed 253V.
Over Load Fault On-grid overload
The inverter has detected an overload condition in on-grid mode. Check whether connected load power exceeds the inverter's continuous rated output. If the system is exporting at full capacity while simultaneously supplying household demand, the total throughput may exceed the inverter's rating. Reduce demand or check for a wiring issue that is routing unexpected loads through the inverter's output circuit.
Ground & safety faults

Safety-critical fault codes related to insulation resistance, residual current detection, earth connections, and arc faults. These cause the inverter to shut down immediately to protect people and property. Do not repeatedly clear these without investigation.

Iso Fault ⚠ Fox Cloud Error 3 — insulation fault
The inverter has detected low insulation resistance between the DC circuit and earth. Common in wet weather when moisture enters PV panel junction boxes or cable connectors — often clears when conditions dry out. To test whether the fault is PV-side, isolate the PV switch on the underside of the inverter. If the fault clears with PV off, the issue is on the DC string side. Persistent Iso Faults in dry conditions require a megohmmeter test on each PV string. Do not ignore this code — it indicates a potential safety issue.
DCI Fault ⚠ DC component in AC output
DC components have been detected in the AC output exceeding the permitted residual current limit. The inverter disconnects from the grid to prevent DC injection. Disconnect PV, grid, and battery, wait 15 minutes, then reconnect. If the fault returns immediately, there may be a fault in the inverter's output filter or a wiring issue. Do not continue operating with a persistent DCI Fault — contact a qualified engineer.
Ground Fault ⚠ Earth connection failed
The ground connection has failed or the inverter has detected a voltage between neutral and PE (protective earth) that is outside the acceptable range. Check the AC wiring — verify the neutral-earth bond is correct and there are no loose connections on the earth conductor. Disconnect PV, grid, and battery, then reconnect. If the fault persists, the AC installation may have an earth fault that requires an electrician to diagnose.
Res Cur Fault Residual current high — software detection
The inverter's software has detected residual current exceeding the threshold. Check PV cable insulation — this fault shares the same root causes as Iso Fault but is detected by a different measurement path. Wait to see if it clears as conditions change (e.g. after rain dries). If persistent, have the PV string insulation tested professionally.
Res Cur HW Fault Residual current detection device failed
The inverter's internal residual current detection hardware has failed. This is separate from Res Cur Fault — it indicates the monitoring device itself is faulty rather than detecting actual residual current. Disconnect PV, grid, and battery, wait 15 minutes, then reconnect. If the fault persists, the internal RCD monitoring board requires professional repair or replacement.
Solar & PV faults

Fault codes related to the PV string inputs — voltage out of range, overcurrent, and reversed polarity. These typically cause the inverter to stop generating from solar while battery and grid functions may continue.

Pv Volt Fault PV string voltage out of range
PV string voltage is outside the inverter's acceptable input range. Check PV1Volt and PV2Volt in Fox Cloud — the H1(G2) maximum input voltage is 580V DC per MPPT. High string voltage from cold panels in winter morning sun is a common trigger (cold cells produce higher open-circuit voltage). If the string design is close to the maximum, removing one panel from the series string may be necessary. Low voltage faults indicate a panel, connection, or shading issue.
SW Pv Cur Fault Software — PV input current high
The inverter's software has detected PV input current exceeding the rated limit. This can occur if the PV array's short-circuit current (Isc) exceeds the inverter's maximum input current per MPPT. Check the array design — the combined Isc of parallel strings on each MPPT input must not exceed the inverter's specification. Disconnect PV, grid, and battery, wait 15 minutes, then reconnect.
HW Pv Cur Fault Hardware — PV input current high
The inverter's hardware protection has detected PV input overcurrent. This is a hardware-level detection (faster response than SW Pv Cur Fault) and indicates a more severe condition. Disconnect all sources and wait 15 minutes. If it triggers repeatedly, the PV string configuration may exceed the inverter's rating, or the current sensor on the PV input board may be faulty. Contact a qualified engineer.
PvCon Dir Fault ⚠ PV connection reversed
The PV string positive and negative poles are connected in reverse at the inverter's MC4 input. This is an installation error. Do not attempt to swap the connectors while the panels are generating — PV strings produce DC voltage whenever exposed to light. A qualified installer must correct the polarity, ideally at dawn/dusk or with the string covered. Reversed polarity can damage the inverter's MPPT circuit if left connected.
Battery & BMS faults

Fault codes related to the battery management system, battery voltage and current conditions, relay status, and temperature protection. These apply to Fox ESS ECS, HV2600, and compatible third-party batteries connected via the BMS communication bus.

BMS Lost Fox Cloud Error 14 — BMS comms interrupted
The inverter has lost communication with the battery BMS. Check the RS485/CAN cable is securely connected to both the battery BMS port and the correct port on the inverter — the H1(G2) has multiple RJ45 sockets that look similar. Verify the DIP switch on the battery master is set to the correct slave count. On V2 ECS batteries, leave the DIP at position 0 for auto-negotiation. Power cycle both inverter and battery if cabling is correct. BMS comm fault guide →
Bms Ext Fault External BMS communication interrupted
Communication between the BMS and the inverter has been interrupted on the external communication bus. This is functionally similar to BMS Lost but reported by a different detection path. Check the communication cable between the battery and inverter is correctly and firmly connected. BMS comm fault guide →
Bms Int Fault Internal BMS — DIP switch or inter-battery comms
The DIP switch is at the wrong position or communication between battery packs in a multi-battery setup has been interrupted. Move the DIP switch to the correct position for the number of slave batteries connected. Check the communication cable between battery packs is correctly and firmly connected. Power down all batteries, correct the DIP setting, then power on in sequence — master first, then slaves. BMS comm fault guide →
Bat Volt Fault Battery voltage out of range
The battery input voltage is outside the inverter's acceptable range. Check the battery voltage reading in Fox Cloud — it should be within the battery's nominal range (e.g. 200–430V for HV systems). If voltage is high, one or more cells may be overcharged — check the battery firmware and update if available. If voltage is low, the battery may have over-discharged. Contact a qualified engineer if the voltage reading is significantly outside the expected range.
SW Bat Cur Fault Software — battery current high
The inverter's software has detected battery current exceeding the rated limit. Check the inverter's max charge and discharge current settings match the battery specification. Disconnect PV, grid, and battery, wait 15 minutes, then reconnect. If it persists, the charge/discharge current limit may need adjusting in the inverter settings or there may be an underlying battery or BMS issue.
HW Bat Cur Fault Hardware — battery current high
The inverter's hardware protection has detected battery overcurrent. This is a hardware-level detection and indicates a more severe condition than SW Bat Cur Fault. Disconnect all sources and wait 15 minutes before reconnecting. If it recurs, the inverter's battery port circuit or the battery itself may have a fault requiring professional inspection.
Bat Power Low Battery charge depleted
The battery charge level is low and the inverter has stopped discharging to protect the cells. Wait for the battery to recharge from solar or grid (depending on your charge schedule settings). This is normal protective behaviour and should clear once the battery SOC rises above the minimum threshold set in the inverter's battery settings. If the battery never charges above this point, check that charging is enabled and the charge schedule is correctly configured. Battery not charging guide →
BatCon Dir Fault ⚠ Battery connection reversed
The battery's positive and negative poles are connected in reverse at the inverter. This is an installation error that can damage the inverter's battery port circuit. Disconnect the battery immediately and have a qualified installer correct the polarity. Check the positive and negative battery cables are connected to the correct terminals before re-energising. The inverter may require professional inspection even after the polarity is corrected.
Bat Relay Open Battery relay stuck open
The inverter's internal battery relay is not closing, preventing the battery circuit from connecting. Disconnect PV, grid, and battery, wait 15 minutes, then reconnect. If the relay remains stuck open after a power cycle, the relay component inside the inverter has likely failed and requires professional replacement.
Bat Relay Short Circuit Battery relay stuck closed
The battery relay is welded closed and cannot open to disconnect the battery circuit. This is a hardware fault — the relay contacts have fused together, typically from an overcurrent event. Disconnect all sources and contact a qualified engineer. The inverter's relay module will need replacing. Do not continue operating as the inverter cannot safely disconnect the battery in a fault condition.
Bat Buck Fault / Bat Boost Fault Battery DC-DC converter MOSFET failure
The battery buck circuit MOSFET (charge direction) or boost circuit MOSFET (discharge direction) has failed. These are internal power electronics components that control DC-DC conversion between the battery and the DC bus. Disconnect all sources and wait 15 minutes. If the fault persists after a power cycle, the inverter's battery converter board requires professional repair. Both faults indicate a hardware failure.
DC bus & inverter hardware

Internal inverter faults related to the DC bus voltage, AC output current, power electronics, temperature, and firmware. Fox ESS fault codes distinguish between SW (software-detected) and HW (hardware-detected) variants — HW faults are generally more severe.

SW Bus Vol Fault Software — DC bus voltage out of range
The inverter's software has detected the internal DC bus voltage outside the expected range. Often triggered by conflicting charge schedules — for example, running both the mode scheduler and a grid charge schedule simultaneously. Check your Fox Cloud settings for duplicate or overlapping schedules and disable any conflicts. On older firmware, this fault can also appear during EPS mode transitions. Disconnect all sources, wait 15 minutes, then reconnect.
HW Bus Vol Fault Hardware — DC bus voltage out of range
The inverter's hardware protection has detected the DC bus voltage outside the safe threshold. This is more severe than SW Bus Vol Fault and may indicate a hardware issue with the internal capacitor bank or power stage. Disconnect all sources and wait 15 minutes. If the fault returns immediately, the inverter requires professional inspection. Do not continue operating with a persistent HW bus voltage fault.
SW Inv Cur Fault / HW Inv Cur Fault AC output overcurrent — software / hardware
The inverter has detected its AC output current exceeding the rated limit. The SW variant is software-detected; the HW variant is hardware-detected and more severe, causing an immediate shutdown. Disconnect PV, grid, and battery, wait 15 minutes. If the HW variant persists after restart, the output MOSFETs or current sensors may be damaged — contact a qualified engineer.
Temp Fault Inverter over temperature
The inverter's internal temperature has exceeded its safe operating threshold. Check ventilation clearances — minimum 20cm from walls, not enclosed in a sealed cabinet, no direct sunlight. Clear any dust from heatsink vents. Wait for the inverter to cool and check if the fault clears. If ambient conditions are within range (up to 45°C) and vents are clear but the fault persists, the internal cooling fan may have failed and requires professional service.
Inv EEPROM Fault / ARM EEPROM Fault Internal memory fault — inverter / manager
The inverter or manager board's EEPROM memory has failed a read/write operation. An occasional occurrence after a firmware update is typically transient — disconnect all sources, wait 15 minutes, then reconnect. If persistent, the EEPROM chip may be failing and the board requires a firmware reflash or replacement. Firmware can be updated via USB using a FAT32-formatted U-disk (max 32GB, USB 2.0 only) inserted into the port on the inverter's underside.
MDSP Smpl Fault / RDSP Smpl Fault Master/slave sample detection circuit failed
The master (MDSP) or slave (RDSP) sample detection circuit has failed. These circuits monitor voltage and current waveforms within the inverter. Disconnect all sources, wait 15 minutes, then reconnect. If persistent, the monitoring circuitry on the affected control board is faulty and requires professional repair.
Relay & EPS faults

Fault codes related to the inverter's internal grid relays and the Emergency Power Supply (EPS / backup) circuit. Relay faults are typically hardware failures requiring professional repair. EPS faults relate to the backup power function during grid outages.

Main Relay Open Grid relay stuck open
The main grid relay is stuck in the open position, preventing the inverter from connecting to the grid. Disconnect all sources, wait 15 minutes, then reconnect. If the relay remains stuck open, the relay component has likely failed mechanically and requires professional replacement.
S1 / S2 / M1 / M2 Close Fault Grid relay stuck closed
One of the grid relays (S1, S2, M1, or M2) is welded closed and cannot open to disconnect the inverter from the grid. This is a safety-critical hardware fault — the relay contacts have fused together. The inverter cannot safely island (disconnect from grid) in this state. Disconnect all sources and contact a qualified engineer. The affected relay module requires replacement.
Eps Relay Fault EPS transfer relay failed
The EPS (Emergency Power Supply) transfer relay has failed. This relay switches the backup circuit between grid and inverter power during an outage. Disconnect all sources, wait 15 minutes, then reconnect. If persistent, the EPS relay requires professional replacement. EPS backup will not function until this is resolved. EPS not working guide →
Eps Over Load EPS backup circuit overloaded
The load on the EPS (backup) circuit exceeds the inverter's off-grid output rating. During a power cut, the inverter can only supply a limited amount of power to backed-up circuits — typically 3.7–6kW depending on model. Check which appliances are connected to the EPS circuit and reduce the load. High-draw appliances like kettles, ovens, or immersion heaters should not be on the EPS circuit. EPS not working guide →
Communication & internal faults

Faults related to communication between the inverter's internal control boards, the external electricity meter, and consistency checks between master and slave processors. These are mostly internal hardware faults that require a power cycle followed by professional diagnosis if persistent.

Meter Lost Fault Fox Cloud Error 242 — meter comms lost
Communication between the inverter and the external electricity meter has been interrupted. Check the RS485 cable between meter and inverter — verify pin connections (pins 3 and 4 for A and B, matched at both ends). Confirm the meter is set to 9600 baud, address 1, and is powered on. Cable runs exceeding 50m can cause signal degradation. A power cycle of the meter itself can resolve lockups. Fox Cloud offline guide →
SCI Fault Fox Cloud Error 11 — master/manager comms
Communication has failed between the inverter's master and manager control boards. This is an internal hardware fault. Disconnect all sources, wait 15 minutes, then reconnect. If the fault persists, an internal ribbon cable may have come loose or one of the control boards is faulty. Requires professional inspection.
MDSP SPI Fault / RDSP SPI Fault Master/slave SPI communication failed
Communication between the master and slave DSP processors has failed over the internal SPI bus. Disconnect all sources, wait 15 minutes, then reconnect. If persistent, the internal communication bus or one of the DSP processors is faulty and requires professional board-level repair.
GridV / GridF / Dci / Rc Cons Fault Master/slave measurement consistency
The sample values measured by the master and slave processors are not consistent — this covers grid voltage (GridV Cons), grid frequency (GridF Cons), DCI (Dci Cons), and residual current (Rc Cons). This is a safety check to ensure both processors agree on the system state. Disconnect all sources, wait 15 minutes, then reconnect. Persistent consistency faults indicate a hardware issue with one of the measurement circuits and require professional diagnosis.
Code not listed here?

Fox ESS add new fault codes with firmware updates, and some codes vary between H1, H3, KH, and T-series models. If your code isn't listed, share the exact text from the inverter LCD or Fox Cloud alarm log and we'll identify it. The Fox ESS community forum at foxesscommunity.com is also a useful resource.

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Full diagnostic guides by fault type
BMS Lost · Bms Int · Error 14 · Error 222
BMS communication fault

Full diagnostic — DIP switch, RS485 cable, V1/V2 battery compatibility, firmware updates, and power cycle.
Grid Lost · Grid Volt · Grid Freq · 10min Volt
Grid voltage & frequency fault

MCB checks, grid voltage readings, DNO voltage issues, and export limit configuration.
Bat Power Low · SOC stuck · charge schedule
Battery not charging

System mode, charge schedules, temperature limits, and CT clamp direction.
Eps Over Load · Eps Relay · EPS disabled
EPS not working

EPS enable, wiring, backed-up circuits, SOC limits, and overload protection.
Meter Lost · dongle · WiFi lost
Fox Cloud offline

Dongle LED, WiFi configuration, data gaps, and meter communication loss.

Fox ESS fault code questions

Open Fox Cloud (app or web portal), select your device, and check the alarm history. Each fault has a numeric code and an LCD name — for example, Error 14 appears as BMS Lost on the inverter display. The red indicator LED lights up when the inverter is in fault mode. On the LCD, navigate to History → Error log for a timestamped record of all past faults.

Many codes clear after a full power cycle. Turn off the AC isolator below the inverter, switch off the DC isolator on the underside (rotary switch to OFF), and if batteries are fitted, switch them off at the battery isolator. Wait 10–15 minutes for capacitors to discharge, then restore power in reverse order — batteries first, then DC, then AC. Grid codes like Grid Volt Fault and Grid Freq Fault often self-clear once mains conditions normalise. If the same code returns, the underlying fault needs professional diagnosis.

BMS Lost (Error 14) means the inverter has lost communication with the battery BMS over RS485. The most common causes are an incorrect DIP switch setting on the battery master, a loose or damaged RS485 cable, the cable plugged into the wrong port on the inverter, or a firmware mismatch between V1 and V2 ECS battery generations. Check the DIP switch first — if set to the wrong slave count, communication fails immediately. See our full BMS comm fault guide for step-by-step diagnosis.

STS offers remote diagnostic assessments from £75. Our engineers access your Fox Cloud monitoring data remotely to analyse fault history, system performance, and inverter settings without needing to visit site. The diagnostic identifies the root cause and recommends whether a settings change, firmware update, or on-site repair is needed — saving you the cost of an unnecessary callout.

Iso Fault (Error 3) means the inverter has detected low insulation resistance between the DC circuit and earth. This is commonly caused by moisture on PV panel cables or junction boxes — it often appears during or after rain and clears when conditions dry out. To test, isolate the PV switch on the underside of the inverter. If the fault clears with PV off, the issue is on the DC string side. Persistent Iso Faults in dry weather require professional testing with a megohmmeter to identify the affected string.

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