These three codes come from the same underlying inverter firmware family, commonly called Voltronic or Axpert-style firmware, which powers a large share of the hybrid and off-grid inverters sold in Pakistan under brands like Must, Inverex, PowMr and several white-label units. If your inverter’s display just flashed one of these, here is the short version before the detail:
| Code | Meaning in one line |
|---|---|
| F01 | Fan locked, the cooling fan did not spin when it should have |
| F07 | Overload time out, the inverter carried too much load for too long |
| F22 | Bus voltage too low, the internal DC bus dropped below a safe operating level |
None of these three is an emergency by itself. All three are protection trips, meaning the inverter deliberately stopped itself to avoid damage rather than something breaking unexpectedly. That is actually good news: it means the fault is usually traceable and fixable rather than a sign the unit is dying.
One caution before anything else: exact code numbering can differ slightly between brands and firmware versions, even within inverters that share the same underlying Voltronic-style design. What is described here matches the codes as documented across Must, Inverex and similar Pakistani-market inverters. If your specific model shows a different meaning for the same number on its printed manual, always trust the manual over a general guide, or WhatsApp us the model and code and we will confirm it for you.
F01: fan locked
What it means. During a self-check, usually when the inverter is idle or first powering on, it tests whether the internal cooling fan spins as expected. If the fan does not respond, it throws F01 rather than letting the unit run without cooling.
Why it happens. The most common cause by far is years of dust accumulation gumming up the fan bearing until it can no longer spin freely. This becomes more likely as an inverter ages, which is one reason inverters typically need attention well before their panels do. Component lifespan differences across a solar system, including why the inverter usually needs servicing years before the panels, are covered in our solar panel kitne saal chalta hai guide. Less commonly, the fan motor itself has failed electrically, or a wire connecting the fan to the control board has come loose.
How to fix it. If the inverter is still under warranty, do not open the casing yourself, since that can void the warranty even if the actual repair is trivial. Contact your seller or installer first. If warranty is not a concern, powering down, opening the vent panel and clearing visible dust from the fan blades with a soft brush or compressed air often clears the fault immediately. If the fan still does not spin freely by hand when the unit is off, the fan itself likely needs replacing, which is a low-cost part on most of these models.
F07: overload time out
What it means. Every inverter has a rated continuous output and a short-term overload tolerance above that rating, commonly allowing something like 110 to 150 percent of rated load for a limited number of seconds before it must shut down to protect itself. F07 means the connected load exceeded that tolerance for longer than the allowed window.
Why it happens. This is almost always a sizing or usage issue rather than a hardware fault. A common Pakistani household scenario is an inverter rated for a certain continuous load suddenly facing an air conditioner’s startup surge stacked on top of whatever else was already running, an iron, a water motor, kitchen appliances. The startup surge on an AC compressor alone can run two to three times its normal running draw for a few seconds, which is enough to trip an inverter that was sized close to the edge in the first place. If this fault shows up specifically when the AC starts, the underlying issue is almost certainly an inverter sized without enough headroom for that surge, a sizing mistake covered in detail in our solar panels for 1 ton AC guide.
How to fix it. In the short term, avoid running your heaviest appliances simultaneously, stagger the AC, iron and motor rather than starting them together. In the longer term, if this fault recurs regularly rather than as a one-off, the honest fix is upgrading to an inverter with a higher continuous and surge rating rather than continuing to work around an undersized unit. Getting the inverter rating matched properly to your real appliance list from the start, including surge loads and not just running loads, is covered in our hybrid inverter price in Pakistan guide.
F22: bus voltage too low
What it means. Inside the inverter, incoming power from the battery or PV input passes through an internal DC bus before being converted to the output waveform. F22 fires when that internal bus voltage drops below the minimum level the inverter needs to keep operating safely.
Why it happens. Several distinct things can cause this, and separating them matters because the fixes are very different. The cheapest and most common cause is a wiring problem: loose DC connectors, undersized cable gauge for the run length, or corroded terminals all cause voltage to sag under load even when the battery itself is healthy. Poor connection quality at installation time is exactly the kind of thing a properly done earthing and cabling job avoids, which is covered in our solar panel installation in Karachi guide, and the same principles apply anywhere in Pakistan. A second cause is a battery bank that genuinely cannot sustain bus voltage under a sudden load spike, either because it is undersized for the load or because individual cells or a BMS issue are limiting how much current it can safely deliver, a distinction that matters when choosing a lithium battery sized correctly for your actual peak loads rather than just your average ones. A third and less common cause is an internal fault in the inverter’s own boost converter stage, which is not something to diagnose or repair yourself.
How to fix it. Start with the cheap checks. Power down, inspect every DC connection between battery and inverter for looseness or corrosion, and confirm the cable gauge matches the run length and current involved rather than being undersized to save cost. If connections and cabling check out, look at battery health and sizing next. If the fault persists after both of those checks, treat it as a job for a technician, since it may point to an internal inverter fault rather than anything on the wiring side.
Why the same number can mean different things on different brands
Voltronic-style firmware is licensed and rebranded across many manufacturers, and each one sometimes adjusts the numbering slightly, adds brand-specific codes, or uses a completely different scheme entirely. Growatt, Solis and Sungrow, for instance, run their own proprietary fault-code systems that do not map onto the F01 to F2x range described here at all. If you own one of those and see a code that looks similar, check that brand’s own manual rather than assuming this table applies.
Even among Must, Inverex and similar Pakistani-market units built on the same underlying architecture, firmware revisions between production years have occasionally shifted what a specific number means. The safest practice, whenever you are not certain, is to read the code alongside your unit’s printed manual or its manufacturer’s app if it has one, and to treat a general guide like this one as a strong starting point rather than the final word for your exact model.
How to reset an inverter after one of these faults
For all three codes, the standard first step is the same: power the inverter off completely at its isolator or main switch, wait roughly thirty seconds to a full minute, then power it back on. This clears a one-off trip and lets you see whether the underlying condition has actually resolved.
What you should not do is keep force-restarting an inverter that immediately re-trips the same fault. If F07 reappears the instant you restart with the same load connected, that confirms a genuine overload rather than a glitch, and repeated forced restarts under a real fault condition risk stressing components rather than fixing anything. The same logic applies to F22: if it returns immediately on restart, the underlying wiring or battery issue needs addressing before the inverter will run reliably again.
A quick check order if you are not sure which cause applies
Rather than guessing, work through the same short sequence a technician typically follows, in this order, since it moves from cheapest and fastest to most involved.
First, read the exact code again and note whether it appeared once or keeps repeating, since a one-off trip and a recurring one point to very different urgency levels. Second, check whether the fault lines up with a specific action, load spike right before F07, a cold start right before F01, a heavy appliance switching on right before F22, since that timing usually tells you which of the three explanations above actually applies. Third, do a visual check with the inverter powered off: dust on the fan and vents, looseness at any visible DC or AC terminal, and any discoloration or smell that suggests heat damage. Fourth, if nothing visual stands out, restart once and observe whether the fault returns immediately under the same conditions or only intermittently. An immediate repeat under identical conditions means a real, fixable cause. An intermittent one that comes and goes without an obvious trigger is more likely to need a technician’s meter and experience rather than a visual inspection alone.
This order matters because it avoids the two common mistakes people make with these codes: assuming the worst and calling for an expensive repair before checking anything simple, or the opposite, restarting the unit repeatedly without ever looking at what actually triggered the fault in the first place.
Preventing all three from recurring
A system sized and wired correctly from day one runs into these three faults far less often. That means matching the inverter’s continuous and surge rating to your real appliance list rather than the bare minimum, using correctly gauged DC cable for the actual run length rather than whatever was cheapest, and building an annual maintenance check into the system’s upkeep that includes cleaning the fan and vents, checking DC connection tightness, and confirming battery health before the peak summer load season arrives.
Getting help through BuySolars
If you have gone through the checks above and the fault persists, or if you are simply not confident opening the casing yourself, send us the exact code, your inverter’s brand and model, and a photo of the nameplate over WhatsApp. Because the same code can mean slightly different things across firmware versions, having the model in hand lets us confirm the precise cause rather than working from the code number alone.
WhatsApp 03192070022 with your inverter’s model and fault code for a fast, accurate diagnosis.
Fan locked. The cooling fan did not spin when the inverter checked during self-test, usually because of dust buildup, a worn bearing, or a loose fan connection.
Overload time out. The connected load exceeded the inverter’s short-term overload tolerance for longer than its allowed time window, almost always a sizing or usage issue rather than a hardware fault.
Bus voltage too low. The internal DC bus voltage dropped below the safe operating level, most often due to loose or undersized wiring, an undersized battery bank, or occasionally an internal inverter fault.
No. This numbering matches Voltronic-style firmware used by Must, Inverex and similar Pakistani-market inverters. Brands like Growatt, Solis and Sungrow use entirely different, proprietary code systems.
Yes, if it does not recur under normal use. If it trips again as soon as the same load is applied, the inverter is genuinely undersized for that load and the fix is reducing simultaneous load or upgrading the inverter rather than repeatedly restarting it.