Mitsubishi Electric Ecodan R290 Monobloc (PUZ-WZ / PUHA Series) Error U6: Compressor Overcurrent Lockout
Error U6 designates an outdoor unit compressor overcurrent trip. The inverter driver circuit detected current exceeding safe operational thresholds (typically >20-28A peak depending on chassis size) across the DC bus or U/V/W output phases, tripping the Intelligent Power Module (IPM) protection lockout.
Hardware Diagnostic Matrix
Spec Ref #f28428a5Required Diagnostic Tools & Instrumentation
Multimeter Probing Targets & Circuit Tolerances
Bench Calibrated ±5%| Test Point / Circuit | Target Nominal Spec | Expected State | Fault Condition |
|---|---|---|---|
| Main Logic DC Bus | 3.3V / 5.0V DC (±0.1V) | Stable DC Potential | <2.9V (Brownout / Sag) |
| Thermistor / NTC Sensor | 10kΩ - 100kΩ @ 25°C | Smooth Thermal Curve | 0Ω (Short) or ∞ (Open) |
| Safety Ground / PE Continuity | < 0.5Ω Resistance | Solid Earth Bond | > 1.0Ω (Floating Chassis) |
Common Symptoms & Fault Manifestations
- Outdoor compressor attempts startup 3 times before locking out permanently
- Flashing LED indication (LD1/LD2) on the outdoor control and power circuit boards
- No domestic hot water or space heating production; system defaults to auxiliary immersion heaters
Root Causes & System Failure Modes
Failed or short-circuit Intelligent Power Module (IPM) / Inverter Power Board
Compressor motor winding breakdown (inter-turn short or short-to-ground with winding resistance deviating from balanced 0.5-2.0 Ohm baseline)
Mechanical compressor lock-up or liquid slugging caused by poor expansion valve (LEV) metering
DC reactor short/open or sudden utility supply voltage dip (<198V AC under load)
High-Voltage & Mechanical Safety Isolation
Prior to opening access panels, disconnecting terminals, or probing wiring, perform complete lock-out / tag-out (LOTO). Disconnect all AC mains, DC string isolators, and lithium battery banks. Allow internal smoothing capacitors at least 3 to 5 minutes to dissipate stored residual energy below safe threshold (<30VDC).
Step-by-Step Diagnostic & Resolution Procedure
Safe Isolation and DC Bus Depletion
Isolate the heat pump at the dedicated double-pole isolator. Wait a minimum of 10 minutes for the high-voltage smoothing capacitors to discharge. Remove the outdoor service panels. Verify zero AC voltage at terminals L and N, then measure DC bus voltage across IPM terminals (P and N) with a multimeter to ensure it is below 10V DC before touching connections.
Inverter Power Module (IPM) Diode & Resistance Test
Disconnect the compressor output terminal connector (U, V, W) from the power board. Set the multimeter to Diode Check mode. Place the negative probe on DC Bus positive (P) and test U, V, and W pins in sequence (nominal drop: 0.4V to 0.7V). Reverse probes and confirm an open circuit (O.L.). Repeat by placing the positive probe on DC Bus negative (N) and check U, V, and W (nominal drop: 0.4V to 0.7V). Any 0.00V reading indicates a punctured IGBT/IPM; the power board must be replaced.
Compressor Motor Stator Resistance & Insulation Checks
Measure coil resistance across terminals U-V, V-W, and W-U directly at the compressor harness. All three phase-to-phase readings must be precisely balanced within ±2% (typically between 0.6 and 1.8 Ohms at 20°C). Use the 500V DC Megohmmeter to test each terminal against the chassis earth point; resistance must be >10 Megohms. If winding values are unequal or insulation breakdown occurs (<2 Megohms), the compressor has suffered a mechanical or electrical failure and requires replacement by an R290-certified technician.
DC Reactor, LEV, and Line Verification
Inspect the DC reactor (DCL) terminals for continuity, heat deformation, or loose lugs. Verify the linear expansion valve (LEV) stepper coil resistance (~46 Ohms across phase pins) to rule out overfeeding liquid R290 into the compressor cylinder. Tighten all electrical spade and screw terminals on the terminal block and noise filter board to factory torque specifications.
Hardware Reset & Fault Clear Sequence
Controller ProcedurePrevention & Routine Maintenance Guidelines
- Verify mains line voltage stability under heavy household load (must stay within 207V-253V AC range).
- Ensure annual inspection of heat exchanger fin cleanliness to prevent high discharge pressures and excessive amp draw.
Frequently Asked Diagnostic Questions
What does error code U6 indicate on Mitsubishi Electric Ecodan R290 Monobloc (PUZ-WZ / PUHA Series)?
Error U6 designates an outdoor unit compressor overcurrent trip. The inverter driver circuit detected current exceeding safe operational thresholds (typically >20-28A peak depending on chassis size) across the DC bus or U/V/W output phases, tripping the Intelligent Power Module (IPM) protection lockout. In engineering telemetry, it indicates compressor overcurrent lockout requiring circuit verification.
What are the primary triggers causing error U6?
Diagnostic bench telemetry isolates the following common root causes: Failed or short-circuit Intelligent Power Module (IPM) / Inverter Power Board; Compressor motor winding breakdown (inter-turn short or short-to-ground with winding resistance deviating from balanced 0.5-2.0 Ohm baseline); Mechanical compressor lock-up or liquid slugging caused by poor expansion valve (LEV) metering; DC reactor short/open or sudden utility supply voltage dip (<198V AC under load).
How do I reset and clear Mitsubishi Electric error U6?
Turn off the outdoor unit mains electrical isolator and indoor hydrobox controller breaker simultaneously. Wait for 5 minutes until all power board discharge LEDs extinguish completely. Power on the indoor unit first, then the outdoor isolator. Alternatively, navigate the FTC control interface to Service Menu > Fault History and press Clear Faults. If the error status repeats immediately after clearing, discontinue clearing and test circuit continuity.
Can I safely resolve error U6 without specialized shop tooling?
This fault is graded as Advanced difficulty with Critical severity. Standard diagnostics require an insulated digital multimeter and appropriate hand tools. Always observe high-voltage isolation procedures.