If your Mitsubishi City Multi system is showing error code 4220, the DC voltage inside your inverter moved outside its safe range.
This is one of the few VRF codes that frequently points outside the equipment — at your building’s electrical supply rather than the air conditioning system.
Read the detail code first
4220 covers four distinct conditions, and they lead different directions:
| Detail | What it indicates |
|---|---|
| 108 | Bus voltage drop — DC voltage fell below the operating range |
| 109 | Bus voltage rise — DC voltage climbed above the operating range |
| 111 | Logic error within the inverter control |
| 131 | Low bus voltage at startup — not enough voltage to begin |
A drop and a rise are opposite problems. A drop often points at supply sag or a failing rectifier. A rise points at regeneration issues or supply overvoltage. Treating them as the same fault wastes a visit.
Record the bracketed number before resetting.
What this means for your building
The inverter converts incoming AC power to DC, then uses that DC to drive the compressor at variable speed. The DC bus is the intermediate stage, and it has to stay within a defined window.
When it moves outside that window, the system stops — because running a compressor from unstable DC damages both.
The useful insight here: the inverter may be reporting accurately about a problem it did not cause. If your building’s supply is sagging under load, the inverter sees it and stops. Nothing is wrong with the air conditioning system at all.
The preliminary code
4320 is the preliminary code for 4220. Its presence in your fault history means the system saw the condition developing before it locked out.
Supply problems in particular tend to announce themselves this way — a series of preliminary codes over weeks as a connection degrades or building load grows.
How urgent is it?
Urgent, and possibly broader than your HVAC.
If the cause is unstable building power, your VRF system is not the only equipment being stressed. Elevators, lighting controls, IT equipment and anything else with power electronics is exposed to the same condition. The air conditioning may simply be the first thing sensitive enough to complain.
If the cause is the inverter itself, running it in that state shortens its life and risks the compressor it drives.
What it typically costs
Medium to high, with a wide range.
If the cause is a loose connection or a supply issue in the building, the fix may sit with an electrician rather than an HVAC contractor — sometimes an inexpensive repair, occasionally a significant electrical remediation.
If the inverter itself has failed, that is a board-level replacement: substantial but well short of a compressor.
Diagnosis here genuinely saves money, because replacing an inverter that was accurately reporting a supply fault leaves you with a new board and the same problem.
What causes it
- Unstable incoming supply — voltage sag under building load, or utility-side variation
- Loose or corroded connections upstream of the outdoor unit
- Undersized or degraded supply wiring to the unit
- Failed rectifier or capacitor within the inverter — detail codes 108 or 131
- Inverter control fault — detail code 111
- Phase problems on the incoming supply, which may also raise 4102
What happens if you leave it
Two distinct risks depending on cause.
If it is the equipment, running an inverter with unstable DC is hard on both the inverter and the compressor. A board fault left running can become a board and compressor fault.
If it is the building supply, that is a problem you want to know about for reasons well beyond air conditioning. Supply instability that trips a VRF inverter is affecting everything else on that supply, usually invisibly, until something else fails.
What we check
- Record the detail code — drop, rise, logic or startup determines the whole approach.
- Measure incoming supply voltage at the unit, under load and at rest.
- Check supply stability over time, particularly if the fault correlates with building activity.
- Inspect connections from the disconnect through to the unit for looseness or heat damage.
- Verify supply wiring adequacy for the unit’s actual draw.
- Test inverter components — rectifier and capacitors — if supply proves clean.
- Review fault history for preliminary 4320 events to establish the timeline.
What you can tell us before we arrive
- The detail code in brackets. Drop and rise are different problems.
- Does it correlate with time of day or building activity? Strong indicator of a supply cause.
- Have you had other electrical issues? Lights dimming, other equipment faulting, breakers tripping.
- Was there recent electrical work? In the building or by the utility.
Related codes
- 4250 — Overcurrent. Current rather than voltage, more often equipment than supply.
- 4240 — Overload protection. Sustained high load.
- 4102 — Open phase. A supply fault that may appear alongside voltage problems.
Get it diagnosed properly
We are certified on Mitsubishi, Daikin and LG VRF platforms, and we service systems we did not install. Where a fault turns out to be building supply rather than equipment, we will tell you that plainly rather than sell you a board.
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 4240 is overload protection. The compressor circuit drew high current too long — usually fighting a condition it could not overcome.
Error 4102 means the system lost a phase of its power supply. Usually electrical rather than mechanical, and it can damage the compressor if forced.
Frequently asked questions
What does error code 4220 mean on a Mitsubishi City Multi system?
The DC bus voltage inside the inverter moved outside its acceptable range and the system stopped. The inverter converts incoming AC to DC before driving the compressor, and that DC level has to stay stable for the compressor to run safely.
What do the numbers in brackets after 4220 mean?
They are detail codes identifying the specific condition. 108 is a voltage drop, 109 is a voltage rise, 111 is an internal logic error, and 131 is insufficient voltage at startup. A drop and a rise point in different directions, so the detail code matters.
Can building power problems cause a 4220 error?
Yes, and it is one of the most common causes. Unstable incoming supply, voltage sag under load, loose connections upstream, or utility-side issues all show up here. If your building has other electrical oddities, they are likely related.
Is 4220 an inverter failure?
Not necessarily. The inverter is reporting a voltage condition, which may originate in your building's supply rather than in the equipment. Diagnosis has to establish whether the inverter is faulty or accurately reporting a supply problem it did not cause.
Why does 4220 appear only at certain times of day?
That pattern usually points at supply rather than equipment. If voltage sags when other large building loads start, the inverter sees it. Timing that correlates with building occupancy or other equipment is a strong diagnostic clue worth recording.
What is the difference between 4220 and 4250?
4220 concerns voltage on the inverter DC bus, often originating in supply. 4250 concerns current, and includes short circuits and power module failures. 4220 more often points outward at the building supply; 4250 more often points inward at the equipment.