Mitsubishi Electric E6 Drain Pump Electromagnetic Interference

Case Details

What We Checked

• E6 error code logged — indoor-outdoor communication fault.
• Terminal connections at both indoor and outdoor units were clean and secure.
• Wiring continuity checked normal end to end.
• Communication cable was routed alongside the condensate drain pump power cable inside the ceiling void — running parallel for approximately 2 metres.
• E6 timing correlated exactly with drain pump activation — error appeared within seconds of the pump switching on and cleared shortly after it stopped.

The Diagnosis

The communication signal cable between the indoor and outdoor units had been routed alongside the condensate drain pump power cable during the original installation. Both cables ran parallel inside the ceiling void for approximately 2 metres. When the drain pump motor activated, it generated electromagnetic interference that was strong enough to disrupt the low-voltage communication signal. The pump runs on a float switch that triggers based on condensate water level — which is why the error timing appeared random and did not correlate with cooling load or temperature settings. The PCB, wiring, and pump were all individually healthy. The fault was purely a cable routing issue.

What Fixed It

We separated the communication cable from the drain pump power cable, creating approximately 30cm of clearance between them inside the ceiling void. We also fitted a ferrite core on the signal cable near the indoor unit to suppress any residual electromagnetic noise. After the rerouting, we ran the unit through multiple drain pump activation cycles while monitoring for E6 recurrence. No errors appeared.

The E6 error stopped completely after separating the cables and adding the ferrite core. The unit has been running without communication faults since. No PCB, wiring, or pump components were replaced.

Why This Happens

Electromagnetic interference from drain pumps on communication signals.

• Condensate drain pump motors generate electromagnetic noise when they switch on and off. If the communication signal cable runs alongside the pump power cable, the noise can be strong enough to corrupt the communication signal and trigger an E6 error.
• The key diagnostic clue is timing correlation. If the E6 appears at irregular intervals that do not match cooling load changes, check whether a drain pump is installed and whether its activation coincides with the error. The pump runs when the float switch triggers — which depends on condensate volume, not cooling demand.
• Separating the communication cable from the pump power cable by at least 30cm and adding a ferrite core on the signal cable are standard fixes. No parts need to be replaced — the PCB, wiring, and pump are all functioning correctly individually.