Random error with no pattern: drain pump cable interference

An E6 error that appears at random feels like a board slowly failing. This one had a pattern hiding in plain sight. The error landed every time the drain pump switched on, because two cables had been run side by side inside the ceiling.

By Team Snowflake | Updated 28 Jul 2026

Case summary

Mitsubishi Electric Ceiling-concealed3-7 years oldCondoBishan, Singapore

Concern: An intermittent fault developing in the indoor control board, and a full replacement to follow.
Found: Electromagnetic interference from drain pump disrupting communication signal
Key check: E6 timing correlated exactly with drain pump activation cycles. Separating cables eliminated the error
Result: The E6 stopped for good once the cables were separated and the noise filter was in place. The unit has run without a single communication fault since. No control board, wiring, or pump parts were replaced, so the owner avoided a costly board swap.

What we were told

The aircon kept showing an error code at random. Some days it appeared twice. Other times it stayed quiet for days. The owner could find no pattern. It did not seem tied to the temperature setting or how long the unit had been running. The unit sat hidden above the ceiling, out of easy reach.

What we checked

An on-and-off communication error with no link to cooling load or run time points away from an internal fault. Something outside the unit was likely disrupting the signal. We started by checking how the cables were routed inside the ceiling.

The E6 code points to a fault in the signal between the indoor and outdoor units.
Wire connections at both the indoor and outdoor units were clean and tight.
The wiring tested fine end to end, so a broken or loose wire was not the cause.
The communication cable ran beside the drain pump power cable inside the ceiling, side by side for about 2 metres.
The E6 appeared within seconds of the drain pump switching on, then cleared shortly after the pump stopped.

What we found

At the first installation, the communication cable was run beside the drain pump power cable. Both ran side by side for about 2 metres inside the ceiling. The signal between the units is very low power. When the pump motor switched on, it threw off enough electrical noise to scramble that weak signal, and the unit read it as a fault. The pump runs on a water-level switch, not on cooling demand. It turns on whenever water collects, so the error looked random. The control board, wiring, and pump were all healthy. The only problem was how the two cables had been laid.

What fixed it

We moved the communication cable away from the pump power cable inside the ceiling, so the two no longer ran together. We also fitted a small noise filter on the signal cable near the indoor unit to soak up any leftover interference. We then ran the unit through several pump cycles and watched for the E6. None came up.

Outcome

The E6 stopped for good once the cables were separated and the noise filter was in place. The unit has run without a single communication fault since. No control board, wiring, or pump parts were replaced, so the owner avoided a costly board swap.

What this case teaches us

A random error code often has a hidden trigger

An E6 that comes and goes is usually a communication problem, not a dying control board. Healthy wiring and clean connections rule the board out quickly.
When an error has no link to temperature or run time, look for another device that switches on nearby. Here the drain pump was the trigger every time.
Note the exact moment the error shows and what runs at that moment. That timing points us to the cable or part to test first.

Questions this case answers

E6 error showing up at random times: what could trigger it?

Look for another device switching on nearby. In this Bishan condo the code landed within seconds of the drain pump starting and cleared shortly after it stopped. The pump runs on a water level switch rather than on cooling demand, so it fires whenever water collects. That made the pattern look like none.

Can cables run side by side inside a ceiling cause an aircon fault?

They can. The signal between indoor and outdoor units is very low power. Here the communication cable had been laid alongside the drain pump power cable for a stretch inside the ceiling. Each time the pump motor started it threw off enough electrical noise to scramble that weak signal, and the unit read a fault.

Is a random error code a sign the control board is dying?

Not usually. A board on its way out tends to fault consistently, not in bursts tied to nothing you can see. On this unit the wiring tested sound end to end and every terminal was clean and tight. Separating the two cables and adding a small noise filter ended it with no parts replaced.

What should I record when an aircon error comes and goes?

The exact moment it shows and what else runs at that moment. That timing is what narrowed this job down. An error with no link to temperature setting or run time is rarely internal, so a note about what switched on nearby points straight at the cable or component worth testing first.

Ready to get started?

Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.

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