Mitsubishi Error Code P8 (4220): DC Bus Overvoltage Fault

Quick Answer — Mitsubishi Error Code P8 (4220)

Error code P8 (4220) on Mitsubishi CITY MULTI systems means the inverter’s DC bus voltage has exceeded safe limits.

When the voltage spikes above 410 V DC, the controller stops the compressor to prevent IGBT failure or electrical fire.

Frequent causes include incoming power surge (40 %), degraded DC capacitors (30 %), and compressor braking overvoltage (20 %).

Turn off the unit immediately, check incoming voltage balance, and inspect the inverter for heat discoloration.

Typical repair cost: $400 – $900, rising to $1 600 if the inverter board requires replacement.

Most P8 faults are resolved within a day by Fixair HVAC diagnostic technicians.

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What P8 Means

P8 represents a DC bus overvoltage error detected by the outdoor inverter’s main board.

During normal operation, the inverter rectifies AC power into approximately 340 V DC for compressor drive.

When this value rises beyond the safety threshold (~410 V), the system halts immediately to avoid electronic component breakdown.

The fault often appears after power restoration or compressor ramp-down events.

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Main Causes of P8

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1. Incoming Voltage Spikes (≈ 40 %) — Utility fluctuations or unprotected power feeds raise DC bus voltage.
2. Degraded Inverter Capacitors (≈ 30 %) — Aging electrolytic capacitors lose capacity and fail to absorb surge energy.
3. Compressor Braking Energy (≈ 20 %) — Fast compressor deceleration sends energy back to the DC bus.
4. Faulty Snubber Circuit (≈ 10 %) — Defective resistors or IGBT gate drive cause instantaneous voltage rise.

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Immediate Safety Steps

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1. Disconnect main power to the outdoor unit.
2. Inspect for burnt smell or swollen capacitors on inverter PCB.
3. Verify line-to-line voltage does not exceed specified range (±10 %).
4. Wait 10 minutes for capacitors to discharge before handling components.
5. Do not restart until voltage stability is confirmed by a technician.
6. Contact Fixair for DC bus testing and capacitor evaluation.

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Repair Costs and Timeframes (2025)

Minor repairs such as surge protection or wiring tightening cost $400 – $600 (2–3 hours).

Replacing degraded capacitors or snubber components costs $700 – $900 (3–4 hours).

If the main inverter board is damaged, expect $1 200 – $1 600 with full functional testing.

Fixair technicians complete most jobs within the same day after power verification.

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Prevention Tips

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• Install surge protectors on main power feed to outdoor unit.
• Schedule annual inverter inspections to test capacitor health.
• Ensure proper grounding to discharge static energy.
• Avoid abrupt power cycling after outages; wait 5 minutes before restart.
• Verify voltage balance at every maintenance visit.

Preventive measures eliminate ≈ 85 % of P8 faults.

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Frequently Asked Questions

Is P8 dangerous?

Yes. High DC bus voltage can destroy inverter IGBTs and lead to electrical fire. Power down immediately.

Can I reset it myself?

No. The voltage must be verified with a meter and discharged safely by a technician.

How much does it cost to repair?

Between $400 and $900 for most cases, up to $1 600 for inverter PCB replacement.

How long does it take?

2–4 hours for minor repairs; same-day for board replacement.

Will it return?

Not if voltage stabilization and surge protection are implemented.

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Summary

Error code P8 (4220) means the DC bus voltage in the Mitsubishi inverter has risen above safe limits.

This protective shutdown prevents IGBT and capacitor failure.

Turn off power, check voltage stability, and have a professional inspect the inverter board.

Most repairs cost under $900 and are completed within a day.

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