Ariston Boiler Error Codes: Common Faults & Reset Procedures

Quick Answer: Ariston boilers display fault codes as alphanumeric codes (e.g. A01, C6, F01) on the display panel, with LED indicator lights on some models. The most common faults are A01 (ignition failure), C6 (fan speed fault), and F01/F02 (temperature sensor faults). Reset by pressing the reset button (marked with a flame symbol on most models) for 3 seconds; persistent faults require a Gas Safe registered engineer.

Summary

Ariston is an Italian manufacturer with a long UK heritage — the Ariston brand is well established in the UK boiler market through the Clas HE, Clas One, E-Combi, Alteas One, and System ranges. Ariston is part of the Ariston Thermo Group, which also includes Chaffoteaux and ELCO brands. Ariston boilers are known for compact dimensions and good hot water performance.

Ariston fault codes vary across model generations — the older Clas and E-Combi ranges use a different code format from the newer Clas One and Alteas One ranges. This article covers the main patterns across both generations. The key to Ariston diagnosis is that many fault codes come with a temperature readout alongside — the temperature display can help distinguish between a sensor fault and an actual overtemperature condition.

Ariston boilers have been sold under multiple brand names in the UK over the years including Saunier Duval (sister brand). Parts availability is generally good through Ariston's UK distribution network and general plumbing merchants.

All gas-side fault diagnosis and repair requires a Gas Safe registered engineer.

Key Facts

• Reset method: Press and hold the reset button (flame symbol) for 3 seconds; release; boiler restarts
• Hard reset: Switch off at fused spur, wait 30 seconds, switch on — clears volatile memory on PCB
• Operating pressure: 1.0–1.5 bar (cold); E407-equivalent appears if below 0.5 bar
• Service interval: Annual service; Ariston recommends registered Ariston engineers or Gas Safe qualified engineers
• Condensate: Standard discharge system; winter freeze-up is a common cause of lockout
• Display type: LCD with numeric/alpha codes on newer models; LED fault light combinations on older models
• Inhibitor requirement: BS 7593 — system inhibitor required; lack of inhibitor can cause heat exchanger corrosion and related faults
• Gas type: Natural Gas standard; LPG conversion kit available from Ariston (requires recommissioning)
• Benchmark: Commissioning checklist must be completed and signed at installation and service

Ariston Error Code Table (Current Models — Clas One, Alteas One, E-Combi Evo)

Older Model Codes (Clas HE, E-Combi, Genesis ranges)

Detailed Guidance

A01 — Ignition Failure

A01 (or A1 on older models) is the most common Ariston fault code. The boiler attempted ignition but failed to establish a stable flame. The boiler will attempt ignition 3–5 times before locking out.

Systematic diagnosis for Gas Safe engineer:

1. Gas supply present: Verify gas pressure at meter — test point on gas valve. If gas pressure is low or absent, suspect supply issue.

2. Electrode and lead condition: The ignition electrode assembly (spark electrode + flame sense) should be visually inspected. Carbon tracking, cracked ceramic, or an incorrect gap (typically 3–4mm) will prevent reliable ignition or flame detection.

3. Gas valve operation: The gas valve should open when the PCB sends the signal. A failed solenoid means gas does not reach the burner. Check gas valve with a multimeter for solenoid continuity.

4. Combustion air supply: Insufficient combustion air (blocked flue, inadequate room ventilation on open-flued models) can prevent stable ignition.

5. PCB output to gas valve: The PCB may fail to send the correct signal to the gas valve even when the gas valve is functional. Oscilloscope testing by an experienced Ariston engineer.

Consumer action: single A01 after a gas interruption or power cut — reset the boiler. If A01 returns within a few firing cycles, call a Gas Safe engineer.

F56 — Low System Pressure

F56 indicates the system pressure has dropped below the minimum threshold. The pressure transducer measures system pressure continuously — when it falls below approximately 0.5 bar, F56 is triggered.

Repressurise procedure:

1. Allow the boiler to cool if it has been running.
2. Locate the filling loop — usually a silver braided hose with two quarter-turn ball valves, positioned under or alongside the boiler.
3. Slowly open both filling loop valves. Water enters the system — watch the pressure gauge. Target 1.2–1.4 bar.
4. Close both filling loop valves when target pressure is reached. Ensure both are fully closed.
5. Press and hold the reset button for 3 seconds. The boiler should restart.

If F56 recurs within days: system leak. Common sources: radiator valve packings (look for damp around radiator valve bodies), pump seals (check pump housing for drip), TRV bodies, compression fitting under floor. Trace and repair. If no leak found, expansion vessel may be failed — check PRV discharge pipe for evidence of dripping.

F65 — Overheating (High Limit Thermostat)

F65 indicates the primary heat exchanger reached the high temperature limit (typically 105–110°C). This is a safety shutdown — the boiler will not restart until it cools and the fault is reset.

The high limit thermostat is a safety device and should not operate during normal use. If F65 occurs once after an unusual event (power cut, pump isolation), a single reset may be appropriate. Recurring F65 requires investigation.

Causes: pump failure or seizure (most common — check pump is audible when boiler fires), air lock in primary circuit (bleed radiators and boiler drain point), partially closed isolation valve on CH circuit, dirty heat exchanger restricting flow.

A Gas Safe engineer should perform pump current check (compare to nameplate), verify system flow rate, and inspect heat exchanger for blockage.

C6 — Fan Fault

C6 appears when the fan speed signal does not match the expected value at startup. The fan speed is monitored by a Hall-effect sensor on the fan spindle. Faults can be:

• Fan motor failed (capacitor, windings, bearings): fan runs slowly or not at all
• Fan speed sensor (Hall-effect) failed: fan runs but signal not detected
• Loose wiring between fan and PCB
• Debris/obstruction on fan impeller

Diagnosis: at startup, the fan should audibly spin up before the ignition sequence begins. If no fan noise — fan motor likely failed. If fan noise but C6 fault — fan speed sensor or wiring.

Fan assembly replacement is a standard Ariston service job. The fan is an FU-rated component for CE compliance.

Service and Maintenance

Ariston boilers require an annual service which includes:

• Visual inspection of gas connections, flue, and combustion air supply
• Combustion analysis (CO/CO₂ measurement at combustion chamber) with calibrated flue gas analyser
• Removal and inspection of burner assembly
• Check and clean heat exchanger (if accessible)
• Flow and return temperature differential check
• Safety device test (overheat thermostat, gas valve shut-off)
• System pressure check and inhibitor concentration check
• Benchmark service record completed

The Benchmark service record should be in the boiler's logbook. Check this at each service visit for previous fault history.

Condensate Blockage in Cold Weather

Ariston condensing boilers generate significant condensate (a litre per hour is not unusual). In sub-zero temperatures, the external section of the condensate pipe can freeze, blocking the drainage path. The boiler detects back-pressure (via F74 — pressure switch closed) or simply shuts down on a temperature fault.

To thaw: pour warm water (not boiling — this can damage polypropylene pipe) over the external section of the condensate pipe. Reset the boiler. To prevent recurrence: insulate the external section of the condensate pipe with waterproof pipe lagging. Better still, extend the condensate discharge to an internal drain where possible.

Frequently Asked Questions

My Ariston shows A01 but only in the morning — why?

Morning lockouts on A01 often indicate low gas pressure at peak demand. The gas network pressure is highest off-peak and lower during morning peak demand. Report to your gas supplier if you suspect low gas pressure — they can fit a data logger on the meter to record pressure over 24 hours. Alternatively, an upstream isolation valve partially closed, or debris on the gas valve seat, can cause intermittent A01.

What does the flashing flame light mean on older Ariston models?

On Ariston models with LED indicator lights rather than an LCD display, the flame/burner LED flashing indicates a lockout fault. The number of flashes before a pause identifies the fault code (e.g. 2 flashes = A2, 6 flashes = C6). Count carefully — the sequence repeats continuously until reset.

Do Ariston boilers come with a warranty?

Standard Ariston warranty is typically 2 years parts and labour. Some ranges offer extended warranties (up to 5 or 7 years) if registered with Ariston and serviced annually by a Gas Safe registered engineer. Check the specific model's warranty documentation and register online.

My Ariston boiler loses pressure every few weeks — is this normal?

No. A correctly functioning system should hold pressure for months with minimal loss. Small losses (less than 0.1 bar per week) in very hot weather can result from air in the system being purged over time, but this should stabilise after the first season. Significant weekly pressure loss indicates a leak that must be found and repaired.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — Gas Safe registration required for all work on gas boilers

• Gas Safe Register — mandatory registration scheme for all UK gas engineers

• Benchmark Commissioning Checklist — required at installation and each service

• BS 7593 (Treatment of water in domestic hot water central heating systems) — system inhibitor requirements

• Ariston UK Technical Documentation — official manuals, error code guides, and spare parts

• Gas Safe Register — find a registered engineer

• HHIC Technical Guidance — boiler service standards

• Benchmark Scheme — commissioning checklist requirements

• vaillant — Vaillant error codes

• baxi — Baxi error codes

• low pressure — boiler pressure loss diagnosis

• boiler selection — boiler selection and sizing guide