Mitsubishi Red LED Error Pulse Decoding
When you encounter a 5-blink error, the outdoor board has attempted to start the compressor but detected an abnormal surge in current or a loss of phase feedback. You must isolate the three-phase DC inverted power supply (U, V, W) from the physical compressor windings to determine if the $2,000 board or the $1,500 compressor has failed.
The UVW Phase Testing Protocol
A 5-blink code stops the system entirely. Before condemning expensive parts, you must perform the UVW disconnect test. Turn off all power. Disconnect the U, V, and W compressor wires directly from the outdoor inverter board.
| Testing Phase | Meter Setting | Expected Good Result |
|---|---|---|
| Compressor Windings | Ohms (Ω) | U-V, V-W, U-W should all read identically (usually 0.5 to 3.0 Ω). |
| Compressor Ground | Mega-Ohms / Continuity | OL (Open Loop) from any winding to the copper pipe/ground. |
| IPM Board Output | Diode Check Mode | Uniform voltage drop (~0.4V – 0.7V) from (+/-) bus to U, V, W terminals. |
Diagnostic Gateway Challenge
You arrive at a Mitsubishi unit flashing 5 Red blinks. You test the compressor windings and read: U-V = 1.2Ω, V-W = 1.2Ω, U-W = 1.2Ω. None are shorted to ground. You use a multi-meter in Diode mode to test the board, and find the W terminal is reading ‘OL’ (Open) from the DC bus. What is the repair?