Error only after hours of running: corroded joint in ceiling

The E6 error came and went, showing up after the unit ran for a while and clearing after a rest. An error that follows running time rarely means a dead board. The fault was hiding along the cable route, inside the ceiling.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Mitsubishi Electric Wall-mounted7-12 years oldHDBYishun, Singapore

Concern: A control board with an intermittent fault was the assumption, and replacement the expected outcome.
Found: Signal wire splice corroded inside an unsealed junction box in the ceiling void. Intermittent contact under thermal expansion during load
Key check: Traced the cable route from the indoor to the outdoor unit and found a junction box in the ceiling void with a visibly corroded signal-wire splice. The connection became unstable when the wire was gently flexed
Result: The intermittent E6 error stopped completely once the splice was replaced. The unit has run under load without communication faults since. No control board or other parts were replaced.

What we were told

The aircon throws an error code after it has been running for a while. Switching it off and waiting lets it start again and work fine for a bit. It has been happening more often over the past few weeks. The unit is about 10 years old.

What we checked

An error that builds with running time points to a connection that fails as it heats up. We checked the terminals at both units first, then traced the full cable route to find the weak joint.

The unit logged E6, an indoor-to-outdoor communication fault.
The terminals at both the indoor and outdoor units were clean and tight.
Continuity along the interconnect wiring at both ends read correct.
Tracing the cable through the ceiling void uncovered a junction box where the signal wire had been spliced during the original install.
That splice was visibly corroded, with green oxidation on both wire ends. The connection went unstable when the wire was gently flexed.

What we found

The signal wire had been spliced inside a ceiling void junction box during the original install. The box was not sealed, so moisture from the humid ceiling space reached the splice over the years. Corrosion built up on the wire ends and weakened the contact. Under sustained cooling load, the wire warmed and expanded just enough to open the corroded joint, dropping the communication signal and triggering E6. When the unit idled and cooled, the wire contracted and contact returned. That is why the error cleared after a rest.

What fixed it

We cut out the corroded splice and re-joined the signal wire with weatherproof sealed connectors and heat-shrink insulation. We then sealed the junction box to keep moisture out. After the repair, we ran the unit under sustained load for a long stretch and watched for any E6 return. Communication stayed stable throughout.

Outcome

The intermittent E6 error stopped completely once the splice was replaced. The unit has run under load without communication faults since. No control board or other parts were replaced.

What this case teaches us

An error that follows running time points to a connection, not a board

E6 only under load, clearing at idle, is the signature of a loose or corroded joint that opens as the wire warms and expands.
Before quoting a control board, the cable route deserves a full trace. Hidden splices in ceiling junction boxes are a common culprit on older flats.
When the unit is about ten years old, the original wiring joints matter. Note when the error shows up and how long it takes to appear.

Questions this case answers

Aircon error only appears after it has run for hours: why?

Heat is opening a weak joint. A corroded or loose connection can hold contact while cool and separate as the wire warms and expands. In this Yishun flat the code arrived under sustained cooling load and cleared after a rest, which is the signature of that expansion. The joint sat in a ceiling junction box.

Where do hidden wire joints usually sit in an older flat?

In junction boxes inside the ceiling void, left there when the original install spliced the signal wire instead of running one continuous length. That box was not sealed here, so humid ceiling air reached the splice over the years. Both wire ends carried green oxidation and contact went unstable when the wire was flexed.

Does an error that clears after a rest rule out a board fault?

It makes one far less likely. Boards do not usually recover on cooling down, but a metal joint does contract back into contact. Terminals at both units were clean here and continuity read correct at each end, so the weak point had to be somewhere along the route between them. Tracing the cable found it.

Is it worth tracing the whole cable route before replacing a board?

Yes, especially in an older flat where the original wiring joints are still in service. A trace takes time and finds faults a terminal check cannot. Here it turned up a corroded splice, which was re-joined with sealed weatherproof connectors and heat shrink. The box was sealed afterwards and no board was touched.

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