October 8, 2025

Mitsubishi Error Code 6840: A-Control Communication Failure

Mitsubishi error code 6840 indicates a communication loss in the A-Control signal network between indoor and outdoor boards. The most frequent causes are loose S1/S2 connections (40 %), signal interference (30 %), or faulty A-Control PCB (20 %). The system halts data exchange to prevent mis-sequencing of compressor or expansion-valve commands. Average repair cost: $300 – $650; board replacement can reach $1 000 – $1 400. Fixair’s field technicians resolve A-Control communication faults within hours using specialized diagnostic tools.

Quick Answer — Mitsubishi Error Code 6840

Error code 6840 appears when the indoor and outdoor A-Control boards lose signal communication through the S1/S2 network.

Typical causes: loose or reversed wiring (40 %), electrical noise (30 %), and defective A-Control board (20 %).

When the signal breaks, the system stops compressor and fan commands to avoid sync errors.

Power off the unit, inspect S1/S2 connections for polarity, and ensure voltage between S1–S2 is 15–30 V DC.

Average repair cost: $300 – $650, up to $1 400 for board replacement.

Fixair technicians diagnose and stabilize communication faults within a single service call.

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What 6840 Means

Error 6840 signals that the outdoor unit is not receiving a valid communication frame from the indoor controller. This digital signal manages timing for compressor, expansion valve, and temperature feedback.

A break or interference can cause complete shutdown of connected units until the network is restored.

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Main Causes of 6840

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  1. Loose or Reversed Wiring (≈ 40 %) — Poor terminal contact at S1/S2 on either board.
  2. Signal Interference (≈ 30 %) — Unshielded wires near power lines creating noise.
  3. Defective A-Control Board (≈ 20 %) — Failed microcontroller or EEPROM.
  4. Ground Loop or Voltage Spike (≈ 10 %) — Improper grounding or surge events.

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Immediate Safety Steps

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  1. Shut off power to both indoor and outdoor units.
  2. Verify S1/S2 polarity and tighten connectors.
  3. Ensure signal cables are separated from 230 V lines.
  4. Check voltage between S1–S2 (15–30 V DC).
  5. If voltage is normal but error persists, replace the A-Control board.

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Repair Costs and Timeframes (2025)

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  • Re-terminate or rewire S1/S2 — $300 – $450 (1–2 h)
  • Noise filter installation — $450 – $650 (2–3 h)
  • A-Control board replacement — $1 000 – $1 400 (3–4 h)
  • Fixair technicians complete signal stabilization and board testing the same day.

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Prevention Tips

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  • Use shielded communication wire for all A-Control loops.
  • Ground one end of the shield only.
  • Keep signal cables > 12 inches from power lines.
  • Install surge protection at outdoor control board.
  • Verify terminal torque during each maintenance visit.

Preventive measures eliminate ≈ 85 % of communication faults.

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Frequently Asked Questions

Is error 6840 dangerous?

Not immediately, but it shuts down heating and cooling until communication is restored.

Can I fix it myself?

You can check and tighten wiring, but electronic diagnostics require a technician.

How much does it cost to repair?

$300 – $650 for wiring correction; $1 000 – $1 400 for board replacement.

How long does it take?

1–3 hours for most repairs, 4 hours for board replacement.

Will it come back?

Not if shielded wiring and grounding are maintained properly.

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Summary

Error code 6840 means the Mitsubishi CITY MULTI A-Control network has lost communication between boards. This stops the system to protect timing and safety logic.

Check S1/S2 connections and signal voltage, then call Fixair for professional testing.

Most repairs cost under $650 and are resolved same day.

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