Vaillant aroTHERM plus VWL 75/6 A Error F.733: High Pressure Switch Open Circuit
The inverter controller detected an open circuit condition on the high-pressure safety switch (HP switch) input circuit. This is a failsafe lock protecting the hermetic compressor from excessive discharge line pressures, typically caused by a disconnected harness, failed switch contact, or an actual overpressure event.
Hardware Diagnostic Matrix
Spec Ref #2c2e8b75Required 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 locks out compressor operation immediately or shortly after start-up
- F.733 fault code displayed on the outdoor unit display and indoor controller
- Auxiliary/backup immersion heater activates to satisfy heating demand
Root Causes & System Failure Modes
High-pressure switch contacts physically stuck open or tripped due to discharge pressures exceeding cut-out threshold (>41.5 bar on R290 circuit)
Wiring harness disconnected, corroded, or broken between the HP switch and the inverter PCB connector (typically plug X17/X20)
Defective high-pressure switch internal microswitch or diaphragm
Failure on the inverter main PCB input detection circuit
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
Isolate the aroTHERM outdoor unit and indoor unit at the external isolator switches. Verify zero potential across all live conductors and ground using an approved CAT III voltage tester before opening casing panels.
Access and Continuity Testing of the HP Switch
Remove the front maintenance cover to access the electrical box and refrigeration compartment. Locate the high-pressure switch on the compressor discharge pipe. Disconnect the wiring harness from the switch terminals. Use a digital multimeter set to continuity/resistance mode across the switch contacts; under normal resting conditions, the switch should be closed (< 1 Ohm). If open (infinite resistance) while refrigerant pressure is within normal static range, the switch is defective or tripped.
Harness and PCB Interface Inspection
Inspect the wiring loom between the HP switch and the inverter PCB for pinches, corrosion, or backing out of connector pins. Measure continuity of both harness conductors back to the inverter PCB terminal plug. Reseat firmly into the PCB header if intact.
Component Replacement or Refrigerant Check
If the switch remains open at rest, connect R290 gauges to verify actual static pressure. If pressure is normal, replace the high-pressure safety switch (requiring refrigerant recovery if non-Schrader mounted). If the switch is closed and wiring is continuous but F.733 persists, replace the inverter control board.
System Recommissioning & Clearance
Reassemble electrical covers and restore mains power. Clear stored fault codes via the technician menu (Live Monitor / Diagnostics D.094/D.095). Initiate a test run via the installer menu to ensure compressor modulates up without re-tripping.
Hardware Reset & Fault Clear Sequence
Controller ProcedurePrevention & Routine Maintenance Guidelines
- Ensure central heating system flow rates are maintained and secondary heat exchanger is descaled to prevent thermal rejection bottlenecks.
- Verify annual maintenance checks inspect wire terminal tension and vibration dampening on discharge line sensor clips.
Frequently Asked Diagnostic Questions
What does error code F.733 indicate on Vaillant aroTHERM plus VWL 75/6 A?
The inverter controller detected an open circuit condition on the high-pressure safety switch (HP switch) input circuit. This is a failsafe lock protecting the hermetic compressor from excessive discharge line pressures, typically caused by a disconnected harness, failed switch contact, or an actual overpressure event. In engineering telemetry, it indicates high pressure switch open circuit requiring circuit verification.
What are the primary triggers causing error F.733?
Diagnostic bench telemetry isolates the following common root causes: High-pressure switch contacts physically stuck open or tripped due to discharge pressures exceeding cut-out threshold (>41.5 bar on R290 circuit); Wiring harness disconnected, corroded, or broken between the HP switch and the inverter PCB connector (typically plug X17/X20); Defective high-pressure switch internal microswitch or diaphragm; Failure on the inverter main PCB input detection circuit.
How do I reset and clear Vaillant error F.733?
Press and hold the Fault Reset button on the outdoor inverter interface for 3 seconds, or navigate to Installer Level -> Fault List on the sensoCOMFORT / VWZ controller and select 'Reset Faults'. Cycle mains power if lock state persists. 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.