Vaillant Heat Pump Problems & Fault Codes

At Green Central, we’re dedicated to helping homeowners maintain their eco-friendly Vaillant heat pumps for reliable, sustainable heating. When your Vaillant aroTHERM heat pump displays an error code, it’s alerting you to a specific issue that needs attention, whether it’s a minor glitch or a more complex problem.

Understanding these Vaillant aroTHERM fault codes can save you time, reduce downtime, and keep your home comfortable.

In this blog post, we’ll dive into common Vaillant heat pump error codes, explain their causes, and provide practical troubleshooting steps to get your system back on track.

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What Do Vaillant Heat Pump Fault Codes Mean?

Vaillant heat pumps use diagnostic error codes to identify malfunctions, making it easier for homeowners and technicians to pinpoint issues. These codes cover a wide range of problems, from low water pressure to electrical faults and refrigerant circuit issues.

Below, we outline five common fault codes on Vaillant aroTHERM heat pumps, their meanings, and what might be causing them to appear.

Common Vaillant Heat Pump Error Codes

F.823 Error: Hot Gas Temperature Switch Open
This fault indicates an issue with the hot gas temperature sensor or a blockage in the refrigerant circuit. Such problems can disrupt the heat pump’s ability to transfer heat effectively, leading to reduced heating performance or system shutdown. Common causes include a faulty sensor, restricted refrigerant flow, or debris in the system.
F.825 Error: Capacitor Inlet Temperature Sensor Fault
This code signals a problem with the inlet temperature sensor, which may be defective or have a loose connection. A malfunctioning sensor can prevent the heat pump from accurately regulating temperature, resulting in inefficient operation or inconsistent heating.
F.1117 Error: Compressor Phase Failure
This fault points to an electrical issue with the compressor, such as a blown fuse, faulty wiring, or an unstable power supply. Since the compressor is critical to the heat pump’s operation, this error can halt the system entirely until resolved.
F.9998 Error: Communication Fault
This code indicates a communication breakdown between the heat pump and its control board, often due to a faulty eBUS connection, damaged wiring, or a malfunctioning control unit. This issue can prevent the system from responding to commands, stopping heating or hot water production.
F.22 Error: Low Water Pressure
This fault occurs when the system’s water pressure is too low, often caused by leaks, air trapped in the system, or improper filling during installation or maintenance. Low pressure can prevent the heat pump from operating, leaving you without heating or hot water.

How to Troubleshoot Vaillant Heat Pump Faults

When a fault code appears, you can take some initial steps to diagnose and potentially resolve the issue. However, safety is paramount, and complex problems should always be handled by a professional. Here’s a step-by-step guide to troubleshooting common Vaillant heat pump errors:

Reset Your Heat Pump
Start by resetting the system to clear temporary glitches. Locate the reset button, usually found near the control panel or display, and press it for 3–5 seconds. Allow up to 5 minutes for the system to restart fully. This step is particularly effective for resolving F.9998 communication errors or other minor issues.
Check Water Pressure for F.22 Errors
If an F.22 code appears, check the pressure gauge on your heat pump. Vaillant systems typically operate best between 1.0 and 2.0 bar. If the pressure is below this range, use the filling loop to add water, following the instructions in your Vaillant manual. Be cautious not to overpressurize the system, as this can cause other issues.
Inspect for Leaks (F.22)
Low water pressure is often caused by leaks. Carefully inspect the heat pump unit, pipes, radiators, and valves for signs of water loss. Small leaks may be manageable with basic repairs, but significant leaks or recurring pressure drops require a technician’s expertise.
Check Electrical Connections (F.1117, F.9998)
For errors like F.1117 (compressor phase failure) or F.9998 (communication fault), verify that all electrical connections, including the eBUS, are secure and undamaged. Never handle live electrical components yourself—contact a qualified technician to diagnose and fix electrical issues safely.
Consult Your Vaillant Manual
Your heat pump’s user manual provides model-specific guidance on fault codes and troubleshooting. Refer to it for detailed instructions tailored to your system, including recommended pressure levels and reset procedures.
Contact a Vaillant-Certified Engineer
If resetting the system or checking pressure and connections doesn’t resolve the issue, it’s time to call a professional. Faults like F.823 (hot gas sensor or refrigerant issues) and F.825 (inlet sensor problems) often require specialised tools and expertise to diagnose and repair. A Vaillant-certified engineer can ensure accurate, safe repairs.

Full List of Vaillant aroTHERM Heat Pump Fault Codes

Fault Code	Meaning	Cause	Solution
F.022	Water pressure too low	Pressure loss in the building circuit due to leakages or air pockets; Building circuit pressure sensor defective	Check the building circuit for leaks; Top up with water, purge; Check the quality of the plug contact on the PCB and on the cable harness; Check that the pressure sensor is working correctly; Replace the pressure sensor
F.042	Fault: Coding resistor	Coding resistor damaged or not set	Check that the coding resistor is positioned correctly and
F.073	Sensor fault: Build. circ. pressure	The sensor is not connected or the sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.094	Fault: Vortex	Volume flow sensor not connected or sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.103	Fault: Spare part ident.	Incorrect control PCB installed on the outdoor unit	Installing the correct PCB
F.514	Sensor fault: Compr. inlet temp.	The sensor is not connected or the sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.517	Sensor fault: Compr. outlet temp.	The sensor is not connected or the sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.519	Sensor fault: Building circuit return temp.	The sensor is not connected or the sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.520	Sensor fault: Building circuit flow temp.	The sensor is not connected or the sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.526	Sensor fault: EEV outlet temp.	The sensor is not connected or the sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.546	Sensor fault: High pressure	The sensor is not connected or the sensor input has short-circuited	Check the sensor (e.g. using an installation aid), and replace if necessary; Replace the cable harness
F.582	Fault: EEV	EEV is not connected correctly or there is a cable break to the coil	Check the plug connections and, if required, replace the coil from the EEV
F.585	Sensor fault: Capac. outlet temp.	The sensor is not connected or the sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.703	Sensor fault: Low pressure	The sensor is not connected or the sensor input has short-circuited	Checking the sensor (e.g. using an installation aid), and replacing it if necessary; Replace the cable harness
F.718	Fan unit 1: Fan blocked	There is no confirmation signal stating that the fan is rotating	Check the air route and, if required, remove any blockages
F.729	Compressor outlet temperature too low	The compressor outlet temperature is lower than 0 °C for more than 10 minutes or the compressor outlet temperature is lower than -10 °C even though the heat pump is in the operating characteristics.	Checking the high-pressure sensor; Checking that the EEV is working correctly; Checking for condensate leaking from the temperature sensor (undercooling); Check whether the 4-port diverter valve is in the intermediate position; Checking the refrigerant volume for overfilling
F.731	High-pressure switch open	Refrigerant pressure too high. The integrated high-pressure switch in the outdoor unit has tripped at 41.5 bar (g) or 42.5 bar (abs); Insufficient energy output via the condenser	Purging the building circuit; Too low a volume flow as a result of closing single room controls in an underfloor heating system; Check that the dirt filter that is in place is permeable; Refrigerant flow rate too low (e.g. electronic expansion valve defective, 4-port diverter valve is mechanically blocked, filter is blocked). Inform customer service.; Cooling mode: Check the fan unit for dirt; Check the high-pressure switch and high-pressure sensor; Reset the high-pressure switch and carry out a manual reset on the product.
F.732	Compressor outlet temperature too high	The compressor outlet temperature is above 110 °C: Application limits exceeded; EEV does not work or does not open correctly; Refrigerant volume too low (frequent defrosting due to extremely low evaporation temperatures)	Check the compressor inlet sensor and outlet sensor; Check the compressor outlet temperature sensor (TT135); Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the refrigerant volume (see Technical data); Carry out a leak-tightness test; Check whether the service valves on the outdoor unit are open.
F.733	Evaporation temperature too low	Insufficient air volume flow through the outdoor unit’s heat exchanger (heating mode) leads to an insufficient energy input in the environment circuit (heating mode) or building circuit (cooling mode); Refrigerant volume too low	If the building circuit contains thermostatic valves, check that they are suitable for cooling mode (check volume flow in cooling mode); Check the fan unit for dirt; Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor inlet sensor; Check the refrigerant volume
F.734	Condensation temperature too low	The temperature in the heating circuit is too low, outside of the operating characteristics; Refrigerant volume too low	Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor inlet sensor; Check the refrigerant filling volume (see technical data); Check whether the 4-port diverter valve is in an intermediate position and is not correctly switched; Checking the high-pressure sensor; Check the pressure sensor in the heating circuit
F.735	Evaporation temperature too high	Temperature in the environment circuit (heating mode) or building circuit (cooling mode) too high for compressor operation; Feed-in of external heat into the environment circuit due to increased fan speed	Check the system temperatures; Check the refrigerant filling volume for overfilling; Check the EEV (does the EEV move to the limit stop? Use sensor/actuator test); Check the sensor for the evaporation temperature (depending on the position of the 4-port diverter valve); Check the volume flow in cooling mode; Check the air volume flow in heating mode
F.737	Condensation temperature too high	Temperature in the environment circuit (cooling mode) or building circuit (heating mode) too high for compressor operation; Feed-in of external heat into the building circuit; Refrigerant circuit overfilled; Insufficient flow rate in the building circuit	Reduce or stop the external heat that is entering; Check the back-up heater (heats up even though it is off in the sensor/actuator test?); Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor outlet sensor, compressor outlet temperature sensor (TT135) and high-pressure sensor; Check the refrigerant filling volume for overfilling; Check whether the service valves on the outdoor unit are open.; Check the air volume flow in cooling mode for sufficient flow rate; Checking the heating pump; Check the building circuit flow rate
F.741	Building circuit: Return temp. too low	During the de-icing, the return temperature falls below 13 °C	Ensure the minimum installation volume is maintained; if required, install a series return cylinder; The fault message is displayed until the return temperature increases to above 20 °C.; Activate the electric back-up heater in the product’s control panel and in the system control in order to increase the return temperature. The compressor is blocked while the fault message is present.
F.752	Fault: Frequency converter	Internal electronics fault on the inverter PCB; Mains voltage outside of 70 V–282 V	Check the integrity of the power supply cables and compressor connection cables. The plugs must audibly click into place.; Check the cable; Check the mains voltage. The mains voltage must be between 195 V and 253 V.; Check the phases; Replace the frequency converter, if required
F.753	Connection fault: Freq. conv. n.recogn.	Missing communication between the frequency converter and the outdoor unit’s control PCB	Check the integrity of the cable harness and plug connection and, if required, replace them; Check the frequency converter by actuating the compressor safety relay; Read the assigned parameters for the frequency converter and check whether values are displayed
F.755	Fault: 4-port valve position not correct	Incorrect position of the 4-port diverter valve. If, in heating mode, the flow temperature is lower than the return temperature in the building circuit.; The temperature sensor in the electronic expansion valve environment circuit displays an incorrect temperature.	Check the 4-port diverter valve (is an audible switching available? Use the sensor/actuator test); Check that the coil is positioned correctly on the 4-port valve; Check the cable harness and plug connections; Check the temperature sensor in the electronic expansion valve environment circuit
F.774	Sensor fault: Air inlet temp.	The sensor is not connected or the sensor input has short-circuited	Check and, if required, replace the sensor; Replace the cable harness
F.785	Fan unit 2: Fan blocked	There is no confirmation signal stating that the fan is rotating	Check the air route and, if required, remove any blockages
F.788	Building circuit: Pump fault	The electronics system of the high-efficiency pump has detected a fault (e.g. dry running, blockage, overvoltage, undervoltage) and has switched off and locked the pump.	Switch the heat pump off for at least 30 seconds (no current); Check the quality of the plug contact on the PCB; Check that the pump functions correctly; Purging the building circuit; Check that the dirt filter that is in place is permeable
F.817	Freq. converter fault: Compressor	Defect in the compressor (e.g. short circuit); Defect in the frequency converter; Connection cable to the compressor is defective or loose	Measure the winding resistance in the compressor; Measure the frequency converter output between the three phases, (it must be > 1 kΩ); Check the cable harness and plug connections
F.818	Freq. converter fault: Mains voltage	Incorrect mains voltage for operating the frequency converter; Shutdown via the energy supply company	Measure and, if required, correct the mains voltage. The mains voltage must be between 195 V and 253 V.
F.819	Freq. converter fault: Overheating	Internal overheating of the frequency converter	Allow the frequency converter to cool and restart the product; Check the frequency converter’s air route; Check that the fan is working correctly; The maximum environmental temperature of the outdoor unit (46 °C) has been exceeded.
F.820	Connection fault: Building circuit pump	Pump does not report any signal back to the heat pump	Check the cable to the pump for defects and, if required, replace it; Replace the pump
F.823	Hot gas temperature switch open	The hot gas thermostat shuts down the heat pump if the temperature in the refrigerant circuit is too high. Following a waiting period, the heat pump attempts to start once more. After three failed start attempts in succession, a fault message is displayed.; Max. refrigerant circuit temperature: 110 °C; Waiting period: 5 mins (after it first occurs); Waiting period: 30 mins (after it occurs a second time and any other time after that); The fault counter is reset if both of the following conditions are met: Heat requirement without switching off prematurely; 60 mins uninterrupted operation	Check the EEV; Replace the dirt filter in the refrigeration circuit, if required
F.825	Sensor fault: Capac. inlet temp.	Refrigerant circuit temperature sensor (vapour-forming) not connected or the sensor input has short-circuited	Check and, if required, replace the sensor and cable
F.1117	Compressor: Phase failure	Defective fuse; Incorrect electrical connections; Mains voltage too low; Compressor/low tariff power supply not connected; Blocked by energy supply company for over three hours	Check the fuse; Checking the electrical connections; Check the voltage at the heat pump electrical connection; Reduce energy supply company anti-cycling time to under three hours
F.9998	Connection fault: Heat pump	eBUS cable not connected or connected incorrectly; Outdoor unit without supply voltage	Check the connection pipes between the power supply PCB and control PCB on the indoor and outdoor unit

Fixing Your Vaillant Heat Pump

While simple troubleshooting steps like resetting or repressurising can resolve some issues, many Vaillant heat pump faults—especially those involving refrigerants, sensors, or electrical components—require professional intervention.

Attempting to fix complex problems yourself could damage the system, pose safety risks, or void your warranty. A certified Vaillant technician has the training and tools to diagnose issues accurately and restore your heat pump’s efficiency, ensuring long-term reliability.

Maintenance Tips to Prevent Fault Codes

Preventing fault codes starts with regular maintenance. Here are a few tips to keep your Vaillant heat pump in top condition:

Schedule Annual Servicing: A professional service checks for potential issues like leaks, pressure problems, or worn components before they trigger fault codes.
Monitor System Pressure: Regularly check the pressure gauge to ensure it stays within the recommended 1.0–2.0 bar range.
Keep the System Clean: Ensure the heat pump and surrounding area are free of debris, which can affect sensors or refrigerant flow.
Stay Proactive: Address minor issues, like small leaks or unusual noises, promptly to prevent them from escalating into major faults.

Why Choose Green Central for Vaillant Heat Pump Support?

At Green Central, we’re passionate about sustainable heating solutions. Vaillant heat pumps are a reliable choice for eco-friendly homes, but they require proper care to perform at their best. Our expert guides and resources empower you to understand and address common fault codes, while our network of Vaillant-certified technicians is ready to tackle complex repairs. Whether you’re dealing with an F.22 low pressure error or an F.1117 compressor issue, we’re here to help you keep your home warm and energy-efficient.

Ready to resolve your heat pump issues? Start by checking your Vaillant manual for specific guidance, and don’t hesitate to contact a certified technician for persistent faults. Visit Green Central for more tips, troubleshooting advice, and professional support to maintain your Vaillant heat pump.

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