Vaillant aroTHERM plus VWL 75/6 Error F.733: Compressor High Pressure Trip
Error F.733 indicates a high-pressure safety circuit activation on the refrigeration loop. The discharge pressure sensor or high-pressure switch (P_HD) detected refrigerant pressure exceeding 41.5 bar (or equivalent R290 design safety threshold), triggering an immediate inverter compressor lockout to prevent mechanical burst.
Hardware Diagnostic Matrix
Spec Ref #1add0035Required 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 unit shuts down abruptly with an audible click or pressure equalization hiss
- Heat pump controller displays fault code F.733 and switches to auxiliary/backup immersion mode
- Outdoor fan ramped to maximum speed shortly before lockout
- Continuous lockout preventing compressor restart attempts after automatic cool-down period
Root Causes & System Failure Modes
Severe hydraulic flow restriction or airlock in the primary water circuit preventing heat rejection across the plate heat exchanger (condenser)
Circulation pump failure, PWM control signal loss, or clogged magnetic/strainer filter causing volumetric flow below minimum 540 L/h
Defective high-pressure sensor/switch providing erratic or high DC millivolt feedback to the inverter control board
Thermal expansion valve (EEV) stuck closed or overcharged R290 refrigerant charge
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 & Hydraulic Flow Verification
Electrically isolate the heat pump via the external rotary isolator. Verify zero voltage on the main terminal blocks (L1, L2, L3/N, PE). Check primary system pressure on the external pressure gauge (must be between 1.5 and 2.0 bar). Inspect the internal Y-strainer and magnetic filter; clean any debris obstructing primary hydraulic flow.
Hydraulic Pump and Bypass Inspection
Power on the system control interface without demanding compressor start. Enter the installer menu (code 17) and run the actuator test for the internal circulation pump. Verify minimum flow rate reaches at least 900–1200 L/h on live diagnostic code D.029. Ensure motorized diverter valves are not stuck in mid-position and the differential bypass valve is correctly adjusted to prevent dead-heading.
Sensor and Pressure Switch Continuity Check
Isolate power again. Access the electrical interface enclosure. Locate the high-pressure switch wiring connector on the main PCB. Using a multimeter in continuity mode, check switch contacts across the harness; the switch must read closed circuit (<1 Ohm) at ambient pressure. Disconnect the high-pressure transducer and measure reference DC voltage (5.0V ± 0.1V) from the PCB. If the switch reads open circuit at ambient pressure or sensor calibration has drifted, replace the high-pressure safety assembly.
EEV & Inverter Operational Verification
Inspect the electronic expansion valve (EEV) stepper coil resistance (typically 46 Ohms ± 3 Ohms per phase). Re-energize the unit, initiate a diagnostic test run in heating mode, and monitor live refrigerant temperatures and pressures via Live Monitor (D.080–D.084). Confirm condensing temperature remains within 5K of primary water flow temperature under steady load.
Hardware Reset & Fault Clear Sequence
Controller ProcedurePrevention & Routine Maintenance Guidelines
- Inspect and clean in-line magnetic and particle filters annually during routine service.
- Maintain hydraulic system static pressure between 1.5 and 2.0 bar to prevent cavitation and air pockets.
Frequently Asked Diagnostic Questions
What does error code F.733 indicate on Vaillant aroTHERM plus VWL 75/6?
Error F.733 indicates a high-pressure safety circuit activation on the refrigeration loop. The discharge pressure sensor or high-pressure switch (P_HD) detected refrigerant pressure exceeding 41.5 bar (or equivalent R290 design safety threshold), triggering an immediate inverter compressor lockout to prevent mechanical burst. In engineering telemetry, it indicates compressor high pressure trip requiring circuit verification.
What are the primary triggers causing error F.733?
Diagnostic bench telemetry isolates the following common root causes: Severe hydraulic flow restriction or airlock in the primary water circuit preventing heat rejection across the plate heat exchanger (condenser); Circulation pump failure, PWM control signal loss, or clogged magnetic/strainer filter causing volumetric flow below minimum 540 L/h; Defective high-pressure sensor/switch providing erratic or high DC millivolt feedback to the inverter control board; Thermal expansion valve (EEV) stuck closed or overcharged R290 refrigerant charge.
How do I reset and clear Vaillant error F.733?
Clear the error code in the installer menu: Navigate to Menu > Installer Level (enter code 17) > Fault History > Reset Faults. Alternatively, toggle the external double-pole circuit breaker OFF for 60 seconds, then restore power to re-initialize inverter bus diagnostics. 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.