October 8, 2025

Mitsubishi Error Code U4: Communication Loss Between Indoor and Outdoor Units

Mitsubishi error code U4 indicates a communication failure between indoor and outdoor units on the CITY MULTI system bus. The most common causes are loose or reversed S1/S2 wiring (45 %), faulty indoor control board (25 %), or electrical noise interference (20 %). This fault stops all indoor units from responding to the outdoor unit’s signals until communication is restored. Average repair cost: $350 – $800; main board replacement may reach $1 400 – $1 600. Fixair’s technicians restore data synchronization and re-establish control signal integrity within hours.

Quick Answer — Mitsubishi Error Code U4

Error code U4 on Mitsubishi CITY MULTI systems means the indoor and outdoor units have lost communication through the S1/S2 network.

Frequent causes: loose or crossed wiring (45 %), faulty control board (25 %), and electrical noise or grounding issues (20 %).

The system halts to avoid mismatched commands that could damage compressor logic or valves.

Turn off power, inspect all S1/S2 connections for correct polarity and tight fit, and verify 15 – 30 V DC signal voltage.

Average repair cost: $350 – $800; board replacement may reach $1 400 – $1 600.

Fixair’s signal technicians diagnose and resolve most U4 errors the same day.

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What U4 Means

The U4 fault is logged when the indoor unit fails to receive a valid data frame from the outdoor controller for over two seconds.

This digital signal carries commands for valve position, fan speed, and temperature feedback.

Loss of signal forces the system to stop all communication to prevent desynchronization.

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Main Causes of U4

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  1. Loose or Reversed Wiring (≈ 45 %) — Misconnected S1/S2 wires between boards.
  2. Electrical Noise or Grounding Issue (≈ 20 %) — Interference from unshielded cables near power lines.
  3. Faulty Indoor Control Board (≈ 25 %) — Corrupted microprocessor or EEPROM error.
  4. Shared Neutral Voltage Drop (≈ 10 %) — Improper line isolation on multi-unit systems.

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Immediate Safety Steps

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  1. Turn off all unit breakers.
  2. Confirm correct polarity and tightness of S1/S2 connections.
  3. Use a multimeter to verify 15–30 V DC across S1/S2.
  4. Ensure signal wires are not routed along high-voltage conduits.
  5. If voltage is normal but error persists, replace the indoor control PCB.

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Repair Costs and Timeframes (2025)

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  • S1/S2 rewiring or terminal tightening — $350 – $500 (1–2 h)
  • Signal filter installation — $500 – $700 (2–3 h)
  • Control board replacement — $1 400 – $1 600 (3–4 h)
  • Fixair technicians complete communication and voltage tests in a single visit.

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Prevention Tips

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  • Use twisted-pair shielded cable for S1/S2 lines.
  • Maintain minimum 12-inch separation from power cables.
  • Ground shield at one end only.
  • Verify S1/S2 voltage and polarity during routine maintenance.
  • These practices eliminate ≈ 85 % of U4 errors.

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Frequently Asked Questions

Is error U4 dangerous?

Not physically, but it stops all units and can cause comfort loss or system shutdowns.

Can I fix it myself?

You can inspect and tighten wires, but board diagnostics should be done by a technician.

How much does it cost to repair?

$350 – $800 for wiring repairs; $1 400 – $1 600 for control board replacement.

How long does it take?

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

Will it come back?

Not if proper shielding, grounding, and maintenance are performed.

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Summary

Error code U4 means the indoor and outdoor Mitsubishi units have lost communication. It’s a common signal fault caused by wiring issues or interference.

Shut off power, check S1/S2 wiring and voltage, and schedule Fixair for diagnostics.

Most U4 errors are resolved in hours for under $800.

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