Mitsubishi Electric E6 Corroded Junction Box Splice

Case Details

What We Checked

• E6 error code logged — indoor-outdoor communication fault.
• Terminal blocks at both indoor and outdoor units were clean and tight.
• Interconnect wiring at both ends showed correct continuity.
• Cable route traced through the ceiling void revealed a junction box where the signal wire had been spliced during the original installation.
• The splice inside the junction box was visibly corroded — green oxidation on both conductor ends. Resistance across the splice fluctuated when the wire was gently flexed.

The Diagnosis

The signal wire between the indoor and outdoor units had been spliced inside a junction box in the ceiling void during the original installation. The junction box was not sealed, allowing moisture from the humid ceiling space to reach the splice over the years. Corrosion built up gradually on the conductor surfaces, increasing resistance at the joint. Under sustained cooling load, slight thermal expansion in the wire was enough to open the corroded contact intermittently, dropping the communication signal and triggering E6. When the unit idled and cooled down, the wire contracted and contact was restored — which is why the error cleared after a rest period.

What Fixed It

We cut out the corroded splice and re-joined the signal wire using weatherproof crimp connectors with heat-shrink insulation. The junction box was sealed to prevent future moisture ingress. After the repair, we ran the unit under sustained load for an extended period while monitoring for any E6 recurrence. Communication remained stable throughout.

The intermittent E6 error stopped completely after the splice was replaced. The unit has been running under load without communication faults since. No PCB or other parts were replaced.

Why This Happens

Intermittent communication errors and hidden splices.

• An intermittent E6 that appears under load and clears at idle suggests a connection that holds when cool but breaks under thermal expansion. The signal wire heats up slightly during sustained operation, and a corroded splice with marginal contact can open up just enough to drop the communication signal.
• Junction boxes in ceiling voids are often installed during the original fit-out and forgotten. If the box was not sealed properly, moisture from condensation or the surrounding environment corrodes the splices over years.
• Tracing the full cable route — not just checking the terminal blocks at each end — is essential for intermittent communication faults. The fault can sit anywhere along the path, and a junction box splice is easy to miss if the route is not followed physically.