Vaillant aroTHERM plus VWL 125/6 A Error F.733: Compressor Motor Phase Disconnection
The inverter PCB detects a loss of current feedback or an open circuit across one of the three motor drive phases (U, V, W) supplying the hermetic scroll compressor, triggering an immediate shutdown to prevent IPM driver destruction and unbalance stress.
Hardware Diagnostic Matrix
Spec Ref #5ad242a1Required 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
- Heat pump enters lockout mode with fault F.733 displayed on the interface
- Outdoor unit compressor attempts to start but cuts out after 2 to 5 seconds
- Auxiliary back-up heater activates continuously to meet thermal demand
Root Causes & System Failure Modes
Loose, corroded, or detached terminal spade/screw connector on the compressor terminal block (U, V, W)
Open circuit or burnt winding inside the inverter scroll compressor motor (winding resistance deviation > 0.5 Ohms)
Failed Intelligent Power Module (IPM) or blown gate drive circuitry on the outdoor unit inverter PCB
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 Electrical Isolation & DC Bus Discharge
Isolate the aroTHERM outdoor unit and indoor hydraulic station via the dual-pole rotary isolators. Wait at least 10 minutes for the inverter smoothing DC bus capacitors to discharge. Verify 0.0V DC across positive (+) and negative (-) inverter DC link terminals before touching wiring.
Wiring Harness & Terminal Integrity Inspection
Remove the electrical box cover and inspect the heavy-gauge 3-phase wiring harness from the inverter board terminals (U, V, W) down to the compressor sound jacket. Remove the terminal cover on the compressor and inspect for thermal damage, loose push-on terminals, or signs of arcing.
Compressor Winding Resistance & Insulation Test
Disconnect the harness from the compressor terminals. Measure phase-to-phase resistance across U-V, V-W, and W-U using a calibrated multimeter (normal range is approximately 0.4 to 1.2 Ohms balanced within 5%). Test each phase to chassis earth using an insulation tester at 500V DC; readings must exceed 10 Megohms. If any phase is open-circuit or shorted to ground, the compressor must be replaced.
Inverter Output Stage Diagnostic & Module Replacement
If compressor windings and harness are intact, perform a diode-check on the inverter board IPM across DC(+) to U/V/W and DC(-) to U/V/W. An open or shorted diode drop confirms blown IGBTs. Replace the defective inverter PCB assembly, re-terminate wiring to correct torque specifications, and refit weather-tight covers.
Hardware Reset & Fault Clear Sequence
Controller ProcedurePrevention & Routine Maintenance Guidelines
- Ensure outdoor unit compressor terminal spade connections are properly crimped and torqued during commissioning or maintenance.
- Verify mains supply stability and ensure neutral-earth loop impedance adheres to Vaillant specifications to prevent harmonic distortion on the inverter.
Frequently Asked Diagnostic Questions
What does error code F.733 indicate on Vaillant aroTHERM plus VWL 125/6 A?
The inverter PCB detects a loss of current feedback or an open circuit across one of the three motor drive phases (U, V, W) supplying the hermetic scroll compressor, triggering an immediate shutdown to prevent IPM driver destruction and unbalance stress. In engineering telemetry, it indicates compressor motor phase disconnection requiring circuit verification.
What are the primary triggers causing error F.733?
Diagnostic bench telemetry isolates the following common root causes: Loose, corroded, or detached terminal spade/screw connector on the compressor terminal block (U, V, W); Open circuit or burnt winding inside the inverter scroll compressor motor (winding resistance deviation > 0.5 Ohms); Failed Intelligent Power Module (IPM) or blown gate drive circuitry on the outdoor unit inverter PCB.
How do I reset and clear Vaillant error F.733?
Clear the error via the VWZ AI or sensoCOMFORT control interface: enter the Installer level (code 17), navigate to Fault History, and confirm fault reset. Alternatively, perform a full 5-minute cold power cycle of the outdoor unit isolator. If the error status repeats immediately after clearing, discontinue clearing and test circuit continuity.
Can I safely resolve error F.733 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.