Why did all three units fail at the same time?

They did not fail individually. The outdoor unit is the common point that all three indoor units communicate with. When the outdoor PCB receiving circuit stopped working, all three zones lost their communication link simultaneously.

Is a 10-year-old Starmex system worth repairing?

If the compressor and refrigerant circuit are healthy, a PCB replacement is a practical repair that extends the system's life. The board is one component — everything else in the system can continue serving for years. A full replacement discussion makes more sense when multiple major components are failing at once.

Can I replace just one indoor unit on a multi-split system?

In some cases, yes — if a compatible replacement indoor unit is available for the existing outdoor unit. But in this case, the indoor units were all working correctly. The fault was entirely on the outdoor board.

Mitsubishi Electric E7 Outdoor Board Failure on Starmex System

Aircon case in Punggol, Singapore: electrical/control traced to outdoor PCB receiving circuit failed — board not processing incoming communication signals from any indoor unit after targeted diagnosis checks.

Case Details

Reported: All three aircon units stopped working at the same time and are all showing the same error code. Nothing happened that I know of — no power trip, no servicing recently. The system is about 10 years old and I am wondering if it is time to replace everything.
Unit: Mitsubishi Electric · Multi-split · 7–12 years
Location: HDB · Punggol, Singapore

What We Checked

E7 displayed on all three indoor units — outdoor unit communication error on the receiving side.
Terminal connections at the outdoor unit were clean and tight across all three zone terminals.
Interconnect wiring for all three zones tested normal — correct continuity, no damage.
Signal voltage measured at the outdoor terminal block confirmed all three indoor units were transmitting correctly.
Outdoor PCB showed no processing activity on the receive side despite valid signals arriving at the terminal block.

The Diagnosis

The outdoor PCB receiving circuit had failed. All three indoor units were sending their communication signals correctly, and the wiring delivered those signals to the outdoor terminal block without loss. But the outdoor board was not processing any incoming data. The receiving circuit handles signals from all connected indoor units through a shared input stage — when that stage fails, every zone loses communication at once. The failure was age-related component degradation on the outdoor board, consistent with a unit that had been running for approximately 10 years.

What Fixed It

We explained that the compressor, refrigerant circuit, and all three indoor units were healthy — the fault was isolated to the outdoor PCB receiving circuit. We sourced a matching replacement board for the Starmex model and fitted it to the outdoor unit. After installation, we powered up each zone individually and confirmed E7 had cleared on all three indoor units. A full cooling cycle was run on each zone to verify stable two-way communication and normal compressor operation across all three rooms.

E7 cleared on all three indoor units after the outdoor PCB replacement. The system has been running normally across all zones since. No full system replacement was needed — the existing indoor units, piping, and refrigerant charge were all retained.

Why This Happens

Simultaneous errors across all zones — finding the common point.

When every indoor unit on a multi-split system throws the same communication error at the same time, the fault almost certainly sits at the outdoor unit. Each indoor unit has its own independent connection to the outdoor board — the only thing they share is the outdoor unit itself.
E7 on Mitsubishi Electric systems indicates the outdoor unit is not receiving communication signals. The receiving circuit on the outdoor PCB processes incoming signals from all connected indoor units. If that circuit fails, every zone loses communication simultaneously.
On a system that is 10 or more years old, a full replacement discussion is understandable. But if the compressor and refrigerant circuit are healthy, a PCB replacement restores the entire system at a fraction of the cost — and preserves the existing piping and gas charge.

Same situation with your aircon?

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