Ariston Boiler Fault Codes — What They Mean & What to Do
If your Ariston boiler is displaying a fault code, this guide explains what each code means in plain English and whether it is something you can resolve yourself or whether you need a Gas Safe engineer. Ariston boilers display fault codes as a number or alphanumeric code on the display panel when a problem is detected.
Some faults — such as low pressure — can be fixed by the homeowner in a few minutes. Others indicate a component failure that requires an engineer. If in doubt, do not attempt to repair gas components yourself. Only a Gas Safe registered engineer is legally permitted to work on gas appliances.
Dorset Plumbing carries out Ariston boiler repairs and servicing across Bournemouth, Poole, Christchurch, and Dorset. Gas Safe registered no. 943146 · Ariston boiler service from £69.99 + VAT
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01202 668822Before You Call an Engineer — Try This First
For many Ariston fault codes, the first step is to attempt a manual reset. Press and hold the reset button on your boiler for around 5 seconds until the display clears, then allow the boiler to restart. If the same fault code reappears immediately or shortly after, there is an underlying issue that a reset will not fix.
Also check your system pressure gauge. Most Ariston boilers require a pressure of between 1.0 and 1.5 bar when cold. If the needle is below 1 bar, re-pressurise via the filling loop before resetting. If you are not sure how to do this, call us and we will talk you through it.
Never attempt to reset more than once or twice for the same fault code. Repeated resets can mask a serious problem and cause further damage. If the fault returns, call an engineer.
Ariston Fault Codes 100–199 — Heating Circuit & Temperature Faults
The 100 series codes relate to temperature sensor faults, circulation problems, and system pressure issues.
101 — Boiler Overheating (All Models)
The boiler has shut down because it reached an unsafe temperature. Common causes include a scaled or blocked heat exchanger, a failing circulation pump, a dirty system filter causing poor flow, or high gas valve outlet pressure. Try resetting once. If the fault returns, call an engineer — running an overheating boiler risks serious damage to the heat exchanger.
102 — Pressure Sensor Error (E-Combi Evo, E-System Evo, Clas He Evo)
The pressure sensor has an open or short circuit fault. Check the pressure gauge manually — if pressure looks normal but the code persists, the sensor has failed. An engineer will check the sensor voltage and replace it if necessary.
103 — Insufficient Circulation (All Models)
The flow temperature is rising too quickly — more than 7°C per second — indicating poor water circulation. Usually caused by a failing pump, closed valve, or blocked system filter. Check that all radiator valves are open and system pressure is adequate.
104 — Insufficient Circulation (All Models)
The temperature difference between flow and return is rising too quickly — more than 20°C per second. Indicates a serious circulation blockage or pump failure. An engineer is required.
105 — Insufficient Circulation (All Models)
The temperature difference between flow and return has exceeded 55°C on three occasions. Poor pump performance or a severely blocked system. Engineer required.
106 — Insufficient Circulation (All Models)
The return pipe temperature is reading 10°C higher than the flow temperature on three occasions — this should not be physically possible and indicates a sensor or circulation fault. Engineer required.
107 — Insufficient Circulation (All Models)
Return pipe temperature has exceeded flow temperature by 30°C on a single occasion. Similar to 106 — indicates a sensor reading error or a circulation problem. Engineer required.
108 — Low System Pressure / Fill Required (All Combi Models)
System pressure has dropped below the required level. Re-pressurise using the filling loop until the pressure gauge reads 1.0–1.5 bar, then reset. If pressure drops again within a few days, there is a leak in the system that needs finding. This is one of the most common Ariston fault codes and is often straightforward to resolve.
109 — System Pressure Over 3 Bar (Clas Net One / Clas One Combi / Clas System One)
Pressure is dangerously high. Do not reset or use the boiler. Check whether the pressure relief valve is discharging water outside the property. Usually caused by an over-filled system or a failed expansion vessel. An engineer should attend promptly.
110 — Flow Temperature Probe Fault (All Models)
The sensor measuring water temperature leaving the boiler has an open or short circuit. The boiler cannot operate safely without this. An engineer is needed to test and replace the probe.
111 — Low Pressure / Auto Fill Fault (Models with Automatic Replenishment)
Similar to fault 108 but applies to Ariston models with an automatic filling function. The automatic replenishment system has failed to maintain pressure. Engineer required to check the auto-fill valve and system.
112 — Return Temperature Probe Fault (All Models)
The sensor measuring water temperature returning to the boiler has failed. Requires probe replacement by a Gas Safe engineer.
114 — External / Outdoor Sensor Fault (All Models)
If a weather compensation sensor is fitted, it has lost contact or developed a circuit fault. On some models this can also affect the electronic board firmware. The boiler may continue operating in a default mode. An engineer can check and replace the sensor.
116 — Room Temperature Sensor Fault (All Models)
The room temperature sensor has lost contact or developed a circuit fault. Similar to fault 114 but applies to the room sensor rather than the outdoor sensor. Check sensor wiring connections. If the fault persists an engineer will need to replace the sensor.
118 — Heating Delivery Probe Fault (All Models)
A fault has been detected with the heating delivery temperature probe. Requires engineer attendance to test and replace the probe.
1P1, 1P2, 1P3, 1P4 — Insufficient Circulation Warning
These are early warning indicators rather than hard lockout codes. The boiler is flagging poor circulation before it escalates into a full fault. Worth having an engineer check the pump and system at this stage to prevent a 103–107 lockout.
Ariston Fault Codes 200–299 — Hot Water Circuit & Solar Faults
The 200 series codes relate to domestic hot water (DHW) circuit sensors and solar system faults. These apply primarily to system boilers with a separate hot water cylinder or solar thermal input.
201 — DHW Flow Sensor No Contact
The temperature sensor on the domestic hot water flow circuit has lost contact. Hot water may be unavailable. An engineer is needed to check and replace the sensor.
202 — DHW Flow Sensor Short Circuit
Short circuit on the DHW flow sensor. Engineer required to replace the sensor.
203 / 204 — Cylinder Temperature Sensor Fault
The temperature sensor in the hot water storage cylinder has either lost contact (203) or short-circuited (204). An engineer will need to replace the cylinder sensor.
205 / 206 — Solar Input Sensor Fault
Applies to systems with a solar thermal input. The sensor at the solar inlet to the DHW circuit has lost contact (205) or short-circuited (206). Engineer required.
207 — Solar Collector Overheating
The solar collector pipework has overheated. This can occur during very hot weather with low hot water demand. The system will protect itself automatically in most cases, but if the fault persists an engineer should check the solar circuit.
208 — Solar Collector Low Temperature
Low temperature detected in the solar collector pipeline. May indicate a fault with the solar pump or a loss of fluid in the solar circuit. Engineer required.
209 — Cylinder Overheating
The hot water storage cylinder has overheated. This is a safety shutdown. Do not reset without first investigating the cause — a faulty thermostat or motorised valve is the usual culprit. An engineer should attend.
Ariston fault you can’t clear? Our engineers cover Bournemouth, Poole & Dorset.
Call 01202 668822Ariston Fault Codes 300–399 — PCB & Electronic Faults
The 300 series codes relate to the boiler’s main printed circuit board (PCB) and electronic control systems. These are generally more serious faults. PCB issues are not homeowner-serviceable — always call a Gas Safe engineer.
301 — EEPROM / Memory Error (All Models)
The boiler’s internal memory chip has developed a fault. This stores the boiler’s settings and operational data. A single reset may clear it temporarily. If the code returns, the PCB will likely need replacing.
302 — Communication Error (All Models)
The main PCB is not communicating correctly with another board or component. First check all wiring connections are secure. If connections are fine, the PCB or a connected component has failed and requires an engineer.
303 — Internal PCB Fault (All Models)
An internal fault has been detected on the main PCB. Try switching the boiler off at the mains, waiting a few minutes, and restarting. If the fault returns, the PCB needs replacing.
304 — Too Many Resets (All Models)
The boiler has been reset more than five times in a 15-minute period and has locked out as a safety measure. To clear fault 304, switch the boiler off at the mains, wait a few minutes, and switch back on. However, the boiler is telling you that resetting is not fixing the underlying problem — an engineer needs to diagnose and repair the root cause.
305, 306, 307 — Internal PCB Errors (All Models)
Further internal control unit faults. These all indicate PCB failure. On an older Ariston boiler, PCB replacement can be costly — your engineer may advise whether a new boiler represents better value.
3P9 — Service Due (E-Combi One, E-System One, Clas Net One, Clas One Combi, Clas System One)
Not a fault — this is an annual service reminder. Your boiler will continue to operate but is overdue a service. An unserviced boiler runs less efficiently and is more likely to develop costly faults. Call us to book an Ariston boiler service from £69.99 + VAT.
Ariston Fault Codes 500–599 — Ignition & Flame Faults
The 500 series codes relate to ignition failures and flame detection problems. These are among the most common Ariston faults and can sometimes be caused by a gas supply issue rather than a component failure.
501 — No Flame / Ignition Failure (All Models)
The boiler has attempted to ignite and failed. First check that your gas supply is on — check other gas appliances such as a hob to confirm. If gas is present but the boiler won’t light, the ionisation electrode (flame sensor) may be dirty, bent, or have a broken wire. An engineer can clean or replace the electrode and check gas pressure at the burner.
502 — Flame Detected When Gas Valve Closed (All Models)
The boiler is detecting a flame signal when no flame should be present. If no flame actually exists, the ionisation electrode is giving a false reading — possibly because it is wet or damaged. A faulty gas valve can also cause this. An engineer is required — do not continue using the boiler.
5P1 — First Ignition Attempt Failed
The boiler failed on its first ignition attempt. Try resetting once. If the fault recurs, check your gas supply is on and call an engineer. Common causes include a faulty ignition electrode, low gas pressure, or a flue blockage preventing proper combustion airflow.
5P2 — Second Ignition Attempt Failed
The boiler has failed on its second ignition attempt and locked out. Do not keep resetting. Check gas supply and call an engineer. In winter, condensate from the flue can freeze and block the flue outlet, preventing ignition — this is worth checking if temperatures are below freezing.
5P3 — Flame Loss During Operation
The boiler lit successfully but the flame went out during operation. Possible causes include fluctuating gas pressure, a faulty gas valve, or a problem with combustion air flow. An engineer is required to diagnose the root cause.
5P4 — Three Flame Failures in One Cycle
The flame has failed three times in a single working cycle. The boiler has locked out. An engineer is needed to investigate the ignition and gas valve system.
Ariston Fault Codes 600–699 — Fan, Flue & Air Pressure Faults
The 600 series codes relate to the flue, fan, and air pressure switch — components that ensure combustion gases are safely expelled from the boiler. These faults always require a Gas Safe engineer.
601 — Flue Gas Overheat Sensor Triggered (Open-Flue Models)
The flue gas temperature has exceeded safe limits. Applies to open-flue (non-room-sealed) boilers. Usually caused by a blocked or poorly fitted flue. Do not use the boiler until an engineer has inspected the flue.
604 — Fan Speed Too Low / Hall Sensor Fault (All Models)
The fan is running below the required speed, or the Hall sensor that monitors fan speed has failed. Without the correct fan speed the boiler cannot safely expel combustion gases. An engineer is needed to test the fan and Hall sensor.
607 — Air Pressure Switch Tripped Before Ignition (All Models)
The air pressure switch detected a problem before the ignition sequence even started. This means the fan is not creating the correct air pressure in the flue. Possible causes include a blocked flue terminal, a faulty fan, or a damaged pressure switch tube. An engineer is required.
608 — Air Pressure Switch Did Not Operate (All Models)
The fan started but the air pressure switch failed to close, meaning the boiler cannot confirm the flue is clear. Check the rubber pressure switch tube for splits or blockages. If the tube is intact, the pressure switch itself has likely failed and needs replacing.
6P1 / 6P2 — Air Pressure Switch Delay Faults (All Models)
These codes indicate a delay in the air pressure switch closing. 6P1 occurs when the boiler attempts to ignite with the fault already present; 6P2 occurs when the fault develops during normal operation. A known cause is overheating that has melted the Venturi tube inside the boiler, creating a small air leak that prevents the pressure switch from operating correctly. This is an engineer job — the Venturi tube or pressure switch will need replacing.
Ariston Boiler Repairs & Servicing — Bournemouth, Poole & Dorset
Dorset Plumbing’s Gas Safe registered engineers (no. 943146) carry out Ariston boiler repairs and annual servicing across Bournemouth, Poole, Christchurch, and the wider Dorset area. Our vans carry a wide range of common Ariston spare parts, meaning most repairs are completed on the first visit. We provide a written diagnosis and quote before carrying out any repair work — no hidden charges. Annual Ariston boiler servicing starts from £69.99 + VAT, with your service record issued on the day.
Frequently Asked Questions
Can I reset my Ariston boiler myself?
Yes — press and hold the reset button for around 5 seconds. One reset attempt is reasonable for most fault codes. If the same code reappears, do not keep resetting — call an engineer to diagnose the underlying problem.
What does fault 108 mean on an Ariston boiler?
Fault 108 means system pressure is too low. Re-pressurise using the filling loop until the gauge reads 1.0–1.5 bar, then reset. If pressure drops repeatedly, there is a leak in the system that needs finding.
What does fault 501 mean on an Ariston boiler?
Fault 501 means the boiler failed to ignite. Check your gas supply is on by testing another gas appliance. If gas is available but the boiler won’t light, call an engineer — the ignition electrode or gas valve may need attention.
What does fault code 304 mean on an Ariston boiler?
Fault 304 means the boiler has been reset more than five times in 15 minutes and has locked out. Switch the boiler off at the mains, wait a few minutes, and switch back on. Then call an engineer — the underlying fault needs diagnosing and fixing.
Is a 3P9 code serious?
No — 3P9 is simply a service reminder. Your boiler will continue to operate but is due its annual service. Book a service to keep the warranty valid and prevent faults developing.
How much does an Ariston boiler service cost?
Dorset Plumbing charges £69.99 + VAT for a full annual boiler service on all makes and models including Ariston. Your service record is issued on the day. Call 01202 668822 to book.
Do you carry Ariston spare parts?
We carry a range of common Ariston parts on our vans including pumps, pressure sensors, temperature probes, ignition electrodes, and ionisation sensors. Most standard repairs are completed on the first visit. For less common parts we will source and return as quickly as possible.