October 7, 2025

Mitsubishi Error Code U4 (0403): Communication Failure Between Units

Mitsubishi error code U4 (0403) appears when indoor and outdoor units stop communicating through the control wiring network. The most frequent causes are loose or reversed signal wiring (50 %), ground-loop or power interference (25 %), and faulty control boards (15 %). The system locks out to prevent erratic compressor operation. Average repair cost: $250–$600; control-board replacement may reach $900–$1 400. Fixair’s diagnostic specialists identify wiring and signal issues with same-day on-site testing.

Quick Answer — Mitsubishi Error Code U4 (0403)

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Error code U4 on Mitsubishi CITY MULTI systems means the indoor and outdoor units cannot communicate through the transmission line.

Typical causes include loose or reversed polarity wires (50 %), electrical noise or ground loop interference (25 %), and faulty control board (15 %).

When the signal fails, the entire system stops to protect compressor timing and inverter control.

Check for secure, correctly polarized connections between indoor and outdoor units and verify voltage between TB3 terminals S1–S2 (DC 15–30 V).

Average repair cost: $250–$600; board replacement can reach $1 400.

If the fault returns after reset, book Fixair signal diagnostic service for same-day resolution.

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What U4 Means

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U4 (0403) is a transmission error between the indoor control board and the outdoor unit’s inverter PCB. The system requires continuous data exchange for compressor speed, temperature feedback, and safety control.

When communication is lost for more than 90 seconds, the controller logs U4 and halts operation to prevent sync errors.

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Main Causes of U4

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  1. Loose or Reversed Wiring (≈ 50 %) — S1/S2 polarity reversed or terminal loosened during service.
  2. Ground Loop or Electrical Noise (≈ 25 %) — Shared power lines or improper shielding distort signal.
  3. Faulty Control Board (≈ 15 %) — Damaged indoor/outdoor PCB causes communication loss.
  4. Disconnected Indoor Unit (≈ 10 %) — One unit offline can trigger system-wide U4.

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Immediate Safety Steps

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  1. Turn off power to the entire system.
  2. Check S1/S2 terminal connections for correct polarity and tightness.
  3. Inspect cables for cuts or splices — use twisted-pair shielded wire only.
  4. Measure voltage between S1–S2 (15–30 V DC).
  5. Restore power and observe if U4 clears.
  6. If code returns, call Fixair for transmission line and PCB diagnostic.

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Repair Costs and Timeframes (2025)

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  • Simple wiring tightening or polarity correction — $250–$350 (1–2 hours)
  • Cable replacement or rewiring — $350–$600 (2–3 hours)
  • PCB replacement — $900–$1 400 (3–5 hours)
  • Most U4 faults are resolved the same day after signal stabilization and ground verification.

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Prevention Tips

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  • Use shielded communication cable and ground one end only.
  • Keep S1/S2 lines away from power wiring to reduce noise.
  • Verify terminal torque every maintenance visit.
  • Install surge protection on main power supply.
  • Proper installation and annual checks prevent over 90 % of U4 occurrences.

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Frequently Asked Questions

Is error U4 dangerous?

Not immediately, but it stops all heating and cooling until communication is restored.

Can I fix it myself?

You can check for loose or reversed wires, but electrical noise and board issues require professional testing.

How much does it cost to repair?

Typically $250–$600 for wiring repairs; up to $1 400 if a control board is replaced.

How long does it take?

1–3 hours for most repairs, 5 hours for board replacement.

Will the error return?

Not if wiring is secured and noise filters are installed during maintenance.

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Summary

Error code U4 (0403) means the Mitsubishi CITY MULTI system lost communication between indoor and outdoor units.

Check S1/S2 connections, verify voltage, and ensure shielded wiring is used.

Most cases are fixed within a few hours for under $600.

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