Ariston heat pump fault codes: Nimbus and Nuos explained

A code on an Ariston heat pump identifies a fault or a protective response, but it does not always identify a failed part. On Nimbus systems, 923 indicates a pressure problem in the heating circuit; 937 concerns water circulation, while 931 indicates an inverter fault that needs further information. Before resetting, identify the model and refrigerant: some meanings change between R32 and R410A. Nuos hot water heat pumps use a separate table.

If you have a problem with your heat pump, you can use our free error code finder. It helps you identify errors and find straightforward, effective ways to resolve them.

The equipment label determines which table applies

Identification starts with the rating plate, which should be checked without dismantling the appliance. Note the full model name, reference and refrigerant. The distinction between R32 and R410A separates two Nimbus generations, although it is not enough on its own: whether the unit is a Plus, Pocket or Compact and its specific documentation version also matter.

Copy the entire code, including any letters. Codes 923, 2P8 and 9E28 are not interchangeable, even though all three may concern pressure: the first two relate to the water circuit, while the third is an outdoor unit high-pressure protection. Confusing water and refrigerant leads to the wrong checks and, sometimes, unnecessary work.

The tables below distinguish the applications identified in the available information. Where Nimbus R32/R410A appears, the description is recorded for both generations, but the installed unit’s manual remains the definitive reference. Instructions for an Ariston boiler or a Chaffoteaux appliance must not be transferred automatically: sharing a manufacturer or platform does not make their codes universal.

Nimbus sensors, pressure and water circulation

Nimbus controls monitor temperatures, water movement, storage and communication between modules. A temperature sensor measures heat; the circulating pump moves water around the system. When a reading is invalid, the system may stop a particular function or halt production to avoid operating without a reliable measurement.

The cause column distinguishes the detected problem from its possible origins. A sensor fault, for example, may involve the sensor itself or its connection. Similarly, insufficient circulation does not prove that the pump has failed. These distinctions matter before buying a replacement part: the code guides diagnosis but does not replace a technician’s checks.

Configuration faults deserve particular attention if they appear after installation, the addition of heating zones or a circuit board replacement. That provides diagnostic context, but it does not authorise changes in the installer menu. Parameters describe how the system is built; changing them without checking the installation may hide the warning while leaving an incorrect configuration.

The outdoor unit and differences between R32 and R410A

The outdoor unit houses the compressor, fan and refrigerant circuit components. Its protections may respond to temperature, pressure, power supply or communication problems. The inverter is the electronics that regulates the compressor; an associated error requires interpretation of the recorded details, not an immediate conclusion that the board needs replacing.

Codes 910, 922, 950 and 951 show why generation identification matters. Differences also exist between models using the same refrigerant. Where a description changes, the table preserves that distinction rather than presenting a potentially false equivalence. Sensor abbreviations are retained to make comparison with the unit’s manual easier.

A high or low refrigerant pressure code cannot be resolved by adjusting heating water pressure. Nor does a low-charge warning establish, without testing, how much refrigerant is missing or where the problem lies. Refrigerant circuit work, electrical measurements and access to internal components belong to a qualified technician.

2P warnings also need context

The 2P family covers different situations: domestic hot water production, backup heaters, precirculation and configuration. The whole group must not be treated as a simple reminder. Code 2P2, for example, means that thermal disinfection has not reached the required temperature within six hours, not that the treatment has been completed successfully.

Using hot water during disinfection or BOOST operation may prevent the cylinder from reaching its target. Avoiding consumption during the cycle is an operating check, not a repair. If the warning recurs, hot water production and electric backup need inspection; permanently disabling the anti-legionella function is not an appropriate workaround.

Codes 2P4 and 2P5 require particularly careful reading: the link between code and reset type changes between generations. A manual reset mentioned in the documentation does not necessarily mean there is a user-accessible button. If it requires opening a casing or accessing a heater, it is outside the scope of household checks.

923 and 2P8 allow inspection, not blind topping up

With these warnings, the safe check is to read the pressure gauge or the pressure shown on the interface and look for visible drips. You can visually inspect the area around the indoor unit, radiators and underfloor heating manifold without loosening fittings. A photograph of the reading, noting whether the system was cold or running, provides useful diagnostic information.

There is no universal filling pressure for every Nimbus installation. The correct pressure depends on the design and the manufacturer’s and installer’s instructions. The 1 to 1.5 bar range found in many boiler guides must not automatically be applied to a heat pump. Without a specific reference, it is safer to check the intended pressure than add water by guesswork.

An apparently normal pressure does not rule out the fault: the pressure switch or its wiring may send an incorrect signal. If the reading repeatedly falls, the loss needs investigation; if it stays stable but the warning persists, pressure detection needs checking. Neither situation justifies bypassing the protection, removing hydraulic filters or working on pressurised water.

If there is water near electrical connections, do not touch the wet area or open the appliance to locate the source. The leak and the installation’s safety need professional assessment. Repeatedly topping up to keep the unit running should also be avoided: it restores a reading but may leave the cause of the loss unresolved.

An authorised reset does not remove the cause

The documentation distinguishes between an interface reset, a reset involving interruption of power and a reset reserved for a technician. These are not interchangeable options. Permission to reset one code does not authorise applying the same method to another, opening electrical panels, removing covers or handling protective devices in the electrical installation.

Users may operate the normal controls or the interface reset function where their model’s manual permits it. For cases described as HP Power OFF, the manufacturer’s specific instruction must be followed; no procedure for manipulating the power supply is provided here. Unless it is clearly defined as an accessible user operation, it belongs to authorised personnel.

If the same code returns after the permitted reset, repeating it does not provide a diagnosis. Before clearing a warning, photograph the display and note whether it appeared during hot water heating, space heating, cooling or after a power cut. Recording simultaneous codes also matters: a protective response may be the consequence of an earlier fault.

A layer of frost or a paused fan does not, by itself, establish a fault. However, code 962 identifies a defrost problem and needs attention if persistent. You can look for objects externally blocking airflow without inserting hands, tools or water into the unit. Ice must not be removed by striking the heat exchanger fins.

Nuos has its own fault table

Nuos produces hot water using a heat pump and a storage tank; it must not be confused with a Nimbus space-conditioning system. The following table applies specifically to Nuos Plus S2 Wi-Fi with R290. It must not be extended to every Nuos generation, which may use different references and diagnostic systems.

On this range, some faults stop the heat pump while allowing the electric heater to continue. That explains why hot water may still be available even though the appliance has not recovered. Continued service does not guarantee unchanged energy consumption: resistance heating is not equivalent to normal heat pump operation. Whether that mode can remain in use must be checked against the manual and the relevant diagnosis.

For code 042, household checks end at accessible grilles and ducts: spotting a blocked inlet does not require opening the refrigerant circuit. R290 is a flammable refrigerant, and any work involving it belongs to a qualified professional. Sensors, flanges and tank heaters must not be removed in an attempt to confirm an internal fault.

A diagnosis that avoids replacing parts by elimination

Deciding what to do next requires the full model, exact code, recurrence and operating behaviour. A 931 without a subcode or a 939 without outdoor unit details leaves the diagnosis incomplete. In these cases, the technician needs the additional information before attributing the problem to the compressor or a circuit board. An error display is not a repair estimate.

There is no verified tariff that assigns a price to each code in these tables. Cost depends on the confirmed cause, access and required components. Before authorising work, establish the scope of diagnosis, the call-out charge and whether that amount will be deducted from any subsequent repair. Do not assume that the visit is included.

If the appliance may be covered by a warranty, keep photographs and maintenance records before authorising dismantling or buying parts. The applicable conditions should be checked with the party managing that coverage. Opening the unit unnecessarily does not improve the available information and may complicate assessment of what happened; an external, documented inspection provides a clearer starting point.

Whether the system can remain in use depends on the warning and what its manual expressly permits, not on the code disappearing for a few minutes. Recurring protection, a leak or a refrigerant circuit fault requires diagnosis before normal operation resumes. The aim is not simply to clear the screen, but to confirm that the installation is back within its operating conditions.