Mitsubishi error code 6600 indicates a transmission circuit fault between indoor and outdoor control boards. It occurs when the inverter’s data-communication link is interrupted by wiring defects (40 %), board noise (25 %), or failed signal driver IC (20 %). The system shuts down communication to protect compressor timing and prevent erratic operation. Average repair cost: $350 – $750; main PCB replacement can reach $1 200 – $1 600. Fixair’s technicians restore transmission stability and re-sync control signals within hours of arrival.
Quick Answer — Mitsubishi Error Code 6600
Error code 6600 on Mitsubishi CITY MULTI systems means the transmission circuit link between indoor and outdoor boards has failed.
Frequent causes are wiring open or short (40 %), electrical noise (25 %), and driver IC failure (20 %).
The system stops signal exchange to protect compressor commands and temperature control.
Power off the unit, check continuity between S1/S2 terminals, and inspect for damaged or unshielded cable.
Typical repair cost: $350 – $750, main board replacement $1 200 – $1 600.
Fixair technicians complete most 6600 repairs same day with signal testing equipment.
What 6600 Means
Error 6600 indicates the A-B transmission line for control data is not operating within voltage specifications. When communication pulses stop for more than two seconds, the system logs the fault and shuts down to avoid timing conflicts between indoor and outdoor processors.
Main Causes of 6600
- Open or Shorted Wiring (≈ 40 %) — Damaged signal cable or reversed polarity between S1/S2.
- Electrical Noise (≈ 25 %) — High voltage lines running too close to communication cable.
- Failed Signal Driver IC (≈ 20 %) — Faulty chip on main board interrupts signal.
- Connector Corrosion (≈ 10 %) — Moisture or oxidation on terminal blocks.
- Unstable Power Supply (< 5 %) — Voltage fluctuation corrupts data transfer.
Immediate Safety Steps
- Switch off main power to the system.
- Check S1/S2 continuity and correct polarity.
- Inspect connectors for oxidation and clean if necessary.
- Separate signal cables from power conductors by at least 12 inches.
- If error returns after reset, call Fixair for oscilloscope testing and PCB evaluation.
Repair Costs and Timeframes (2025)
- Signal rewiring / terminal repair — $350 – $500 (1–2 h)
- Noise filter or ferrite installation — $450 – $650 (2–3 h)
- Main board replacement — $1 200 – $1 600 (3–4 h)
- Fixair technicians restore communication and verify line integrity the same day.
Prevention Tips
- Use twisted-pair shielded cable for S1/S2 and ground one end only.
- Keep signal cables clear of power wiring and transformers.
- Add surge protection to main power feed.
- Check terminal torque annually.
- Preventive installation standards eliminate ≈ 90 % of 6600 errors.
Frequently Asked Questions
Is error 6600 dangerous?
No immediate hazard, but it disables communication and stops system operation until resolved.
Can I fix it myself?
You can tighten terminals and check wiring layout, but PCB or IC diagnostics require a professional.
How much does it cost to repair?
$350 – $750 for wiring or filtering; $1 200 – $1 600 for board replacement.
How long does it take?
1–3 hours for wiring repairs, up to 4 hours for board replacement.
Will it come back?
Not if shielded wiring, proper grounding, and noise filters are installed.
Summary
Error code 6600 indicates a transmission circuit failure in Mitsubishi CITY MULTI systems.
It halts indoor/outdoor communication to prevent timing errors. Check S1/S2 continuity, cable routing, and connectors.
Most repairs cost under $750 and are completed the same day.
