Error only after running for hours: corroded wire joint in the ceiling
The E6 error came and went. It showed up after the unit ran for a while, then cleared 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 | Reviewed 24 Mar 2026
Case summary
Mitsubishi Electric Wall-mounted10 years oldHDBYishun, Singapore
- Concern
- The owner worried that a control board had developed an intermittent fault and would need replacing.
- Found
- Signal wire splice corroded inside an unsealed junction box in the ceiling void. Intermittent contact under thermal expansion during load
- Key check
- Traced cable route from indoor to outdoor unit and found a junction box in the ceiling void with 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.
Related reading
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