Solar Inverter Fault Codes: What the Owner Can Check and What Needs a Technician

Read the message before you reset anything

The call usually starts like this: "The inverter has a red light and a code on the screen. Should we switch it off and on?" Sometimes that is all it needs. Often the code is telling you about a wiring problem that will come back, or a safety function doing its job. Each brand uses its own codes and wording, but the underlying fault families are the same across makers. This guide groups them, explains what each means in plain terms, and draws a clear line between what an owner can safely check and what needs a technician.

Your inverter's manual has the exact code list for your model. Use this guide to understand the family the code belongs to.

Before anything else: record the fault

The pattern of a fault is often the best diagnostic clue. Before touching anything, write down or photograph:

• The exact code and text on the display or in the app, with the date and time
• The inverter brand, model and serial number (from the label on the side)
• When it happens: early morning, after rain, at midday, in the evening, every day or once
• The weather at the time
• Whether it cleared by itself
• Any recent work: panel cleaning, roof repairs, new air-conditioners or machines, changes to the Wi-Fi router

A fault that appears every morning after rain and clears by 10:00 points to moisture. A fault that appears at 13:00 on sunny days points to grid voltage or heat. A technician who gets this information can often narrow the cause before arriving.

What an owner can safely do

• Read the display and the monitoring app.
• Check that the inverter's AC breaker in the switchboard is on and has not tripped.
• Check whether the building has grid power at all, and whether neighbours do.
• Look for smoke, a burnt smell, melted plastic, water inside the display window, or insects nesting in the vents. If you see any of these, switch off as below and do not restart.
• If the manual describes a restart, follow it exactly. Many makers say: switch off the AC side first, then the DC switch on the inverter, wait the time stated in the manual so internal capacitors can discharge, then switch on in reverse order. One restart is reasonable. Repeated resets to hide a recurring fault are not.
• With the inverter off, brush dust from the cooling fins and vents.

What an owner should never do

• Open the inverter cover. Capacitors inside hold a dangerous charge after switch-off.
• Pull MC4 connectors apart while current flows. DC current draws an arc that can burn the connector and the person holding it.
• Touch damaged cables or panels with broken glass.
• Open or rewire battery terminals.
• Change grid or protection settings with an installer password found online.
• Go onto the roof in rain or alone.

The fault families

Insulation resistance low

Every morning, before connecting to the grid, a transformerless inverter measures the resistance between the PV conductors and earth. If it is below the maker's threshold, the inverter refuses to start. The minimum depends on the maker and on the array voltage, so do not compare numbers between brands.

If the fault appears in the morning or after rain and clears once things dry, look for moisture: a connector lying in water, a junction box with a failed seal, a cable rubbed through on a sharp sheet edge, or rodent bites. If it is constant, the insulation is damaged somewhere. A technician splits the array and tests each string with an insulation tester to find the bad section. Do not ignore this fault. A second insulation failure elsewhere on the array can create a path for current that starts a fire.

Ground fault and residual current

While running, the inverter watches leakage current to earth and disconnects on a sudden jump. Causes overlap with insulation faults. One more cause is common on new systems: an AC-side RCD whose type or rating does not match what the inverter maker specifies, which trips on normal leakage. The fix is the correct device, never removing protection.

Grid voltage and frequency

The inverter must disconnect when the grid is outside its set window. This is how it avoids feeding a dead line that a lineman is working on. On many sites in Laos we see two patterns:

• Over-voltage at midday: at the end of a long feeder, voltage rises when the inverter exports. Undersized AC cable between the inverter and the main board adds to it. Check cable size, tighten terminals, and consider an export-limit setting.
• Under-voltage or frequency trips in the evening: around 17:00 to 19:00 when everyone switches on air-conditioners and rice cookers.

Protection windows are safety functions. Changing them is a decision for the installer together with the power utility, not a quick fix. On a hybrid system, "grid lost" during an outage is normal: the inverter should switch to backup.

Overtemperature and derating

If output flattens around midday or the app shows derating, the inverter is protecting itself from heat. Typical causes: afternoon sun on the inverter, a closed store room, dust-blocked fins, a failed fan. Shade and airflow solve most cases.

PV input errors

Reverse polarity usually appears at commissioning or after someone reworked connectors. A good inverter refuses to start; do not try again until it is fixed. PV over-voltage means too many panels in series for the coldest morning. In the northern highlands, morning temperatures can fall below 10 °C, raising panel voltage above what a design based on lowland weather expected. That is a design issue. "String abnormal" or "string lost" usually means a blown string fuse, a burnt connector or a failed panel.

Arc fault

Inverters with arc fault detection (AFCI) shut down when they detect an electric arc on the DC side, usually from a loose, badly crimped or mixed-brand connector. Several makers lock the inverter after repeated arcs. Do not keep resetting it. Arcs start roof fires, and the cause must be found.

Communication loss

The most common alarm we see and usually the least serious. The inverter is often still producing; only the data is missing. Check whether the router was replaced or its Wi-Fi password changed, whether the logger or dongle has power, and whether a SIM has expired. If the display itself shows an internal communication error between boards, that is a hardware fault.

Battery and BMS faults

On hybrid and off-grid systems, the inverter and the battery's BMS talk to each other. "BMS communication fail" often comes from a wrong cable, protocol setting or a battery that is not on the inverter maker's compatibility list. Over- and under-temperature alarms come from hot battery rooms. Low SOC shutdowns may simply mean the load is bigger than the system was designed for. Never connect a different battery brand in parallel to "add capacity". See the LiFePO4 battery guide.

Internal hardware faults

Relay self-check failures, bus voltage errors, DC injection, fan faults and "permanent fault" messages come from inside the box. After one restart following the manual, call a technician. This is repair or a warranty case.

When is it a warranty case?

Standard inverter warranties are commonly 5 to 10 years depending on maker and model, with optional extensions. Internal hardware faults within that period are usually warranty claims. Faults caused from outside the inverter usually are not: insulation damage in wiring, grid voltage, overheating because of where the unit was installed, lightning, water through a missing gland.

Makers typically ask for the serial number, photos of the label and the installation, fault screenshots or logs, the commissioning date and proof of purchase. Some ask for remote access to the monitoring account. Keep these from day one.

Do not let anyone open an inverter under warranty without the maker's approval; it can void the claim. In Laos, replacement units and boards usually come through Thailand, so allow for lead time and ask your installer whether a loan unit is possible while the claim runs. Out of warranty, board-level repair is often possible where parts are available.

Send us a photo

We repair systems installed by anyone. Photograph the inverter display, the label with model and serial, and the switchboard, and send them on WhatsApp with a note of when the fault happens. Our inverter repair service covers diagnosis, board repair and warranty claims. For routine checks, see the O&M checklist.