If your Mitsubishi City Multi system is showing error code 4240, the compressor circuit drew high current for long enough that overload protection intervened.
The distinction from other current-related codes matters: this is not a sudden spike. It is sustained high load — the compressor working hard, for a while, against something.
What this means for your building
Overload protection watches how much current the compressor draws over time. A brief surge on startup is normal. Sustained elevated draw is not, because it means the compressor is doing more work than the system was designed for.
Two broad explanations:
The system is fighting a condition. Something is making the compressor work harder — high pressure from a dirty coil, a failing fan, restricted airflow, or extreme ambient load.
The compressor itself is degrading. A compressor with internal mechanical wear draws more current to do the same work. In that case the overload is a symptom of the compressor, not a cause.
Establishing which of those you have is the central question, and it determines whether this is a moderate repair or a major one.
The preliminary code
4340 is the preliminary code for 4240. It appears when elevated current is detected before it becomes a lockout.
This family is where preliminary codes are most useful. A compressor gradually degrading, or a coil gradually fouling, produces preliminary codes for weeks or months before the system finally stops. Almost nobody looks, which is why these faults so often present as sudden failures that were nothing of the sort.
How urgent is it?
Urgent. Sustained overload is one of the clearest warnings you get before a compressor failure.
Compressors are rarely killed by single events. They are worn out by running outside their envelope repeatedly. A system throwing 4240 is doing exactly that, and each reset buys operating hours at the cost of compressor life.
No safety concern. Substantial equipment risk.
What it typically costs
High, with meaningful range.
If the driver is external — dirty coil, failing outdoor fan, obstructed airflow — the repair is moderate and the compressor is likely recoverable.
If the compressor is drawing excess current because of internal wear, you are approaching the most expensive repair on a VRF system.
The gap between those outcomes is large, which is why diagnosis matters more here than on almost any other code. And the earlier it is caught, the more likely you are still in the cheaper category.
What causes it
- Poor heat rejection — dirty outdoor coil, obstructed airflow, or a failing fan raising system pressure
- High pressure conditions generally, which is why 4240 and 1302 often appear together
- Refrigerant overcharge, increasing the work the compressor does
- Supply voltage problems — low voltage means higher current for the same power
- Compressor mechanical wear — bearings, internal clearances
- Extreme ambient conditions compounding any marginal fault
What happens if you leave it
The compressor keeps running against a load it cannot sustain, until it does not.
The pattern is consistent: a coil fouls gradually over a season, the compressor works progressively harder, 4340 preliminary codes accumulate unnoticed, 4240 starts tripping on the hottest days, someone resets it through summer, and the compressor fails in the following year.
Nearly every external cause of 4240 is inexpensive relative to the compressor it threatens. Coil cleaning is not a large job. Compressor replacement is.
What we check
- Measure actual compressor current draw against specification — establishes how far outside the envelope it is running.
- Inspect the outdoor coil and airflow path for the most common external cause.
- Verify outdoor fan operation — speed, obstruction, motor condition.
- Measure system pressures to determine what the compressor is working against.
- Assess refrigerant charge — overcharge increases load.
- Check supply voltage under load, since low voltage raises current.
- Electrical testing on the compressor — winding resistance and insulation, to assess whether it is cause or victim.
- Review fault history for preliminary 4340 events to establish how long this has been building.
What you can tell us before we arrive
- Does it trip only in hot weather? Suggests a marginal condition operating near its limit.
- How long has this been happening? Even roughly — it changes what we expect to find.
- Has the outdoor unit been cleaned recently? And is anything stored or built near it.
- Any change in noise from the outdoor unit? Mechanical compressor wear is often audible before it is measurable.
Related codes
- 4250 — Overcurrent. Acute electrical faults rather than sustained load.
- 4220 — Bus voltage abnormality. Supply problems that can drive higher current draw.
- 1302 — High pressure fault. Frequently the underlying condition causing the overload.
Catch it while it is still the cheap repair
We are certified on Mitsubishi, Daikin and LG VRF platforms, and we service systems we did not install.
Next business day service for commercial VRF.
Related codes
Error 4250 is the inverter overcurrent family. Its detail code tells you whether this is a board repair or a failed compressor — a big cost difference.
Error 4220 covers inverter DC bus voltage faults. The detail code tells you whether the problem is your building's power supply or the inverter itself.
Error 1302 means refrigerant pressure exceeded its safe limit and the system shut down to protect the compressor. Left unresolved, it damages equipment.
Frequently asked questions
What does error code 4240 mean on a Mitsubishi City Multi system?
Overload protection has activated. The compressor circuit drew high current for a sustained period rather than momentarily, and the system stopped to prevent damage. It usually means the compressor was working against a condition it could not overcome, such as high pressure from poor heat rejection.
What is the difference between 4240 and 4250?
4240 is overload protection responding to sustained high current, while 4250 is overcurrent covering acute faults including short circuits and power module failures. 4240 tends to indicate a system working too hard; 4250 tends to indicate something electrically broken.
Can a dirty condenser coil cause a 4240 error?
Yes, indirectly. A coil that cannot reject heat raises system pressure, and the compressor draws more current working against that pressure. Sustained high load then trips overload protection. This is why 4240 and high pressure faults often appear together.
Is 4240 a sign my compressor is failing?
It can be. A compressor with internal mechanical wear draws more current than a healthy one doing the same work. Whether the compressor is the cause or the victim is the central diagnostic question, and it determines whether this is a moderate or major repair.
How much does a 4240 error cost to fix?
It varies widely with cause. If the driver is a dirty coil or a failing fan, the repair is moderate. If the compressor itself is drawing excess current because of internal wear, the cost rises substantially toward compressor replacement.
Why does 4240 only happen on hot days?
Heat increases the load the system is working against. A marginal fault that the compressor can overcome in mild weather becomes an overload once ambient temperature rises. Faults that appear only in heat are real faults operating close to their limit year-round.