Ariston Boiler Error Codes: Causes & Fixes

Ariston boiler error codes are the helpful diagnostic messages your boiler displays when something needs attention. The most common ones UK homeowners and landlords see are 101 (overheat), 108 (low pressure or insufficient water), 501 (no flame detected), and circulation issues like 103–107 or 1P1–1P3. 

These usually point to straightforward problems such as a pressure drop, closed valves, sensor faults, or ignition hiccups that can often be reset safely at home. 

This guide gives you in-depth explanations of the most frequent Ariston boiler error codes, their causes, and step-by-step fixes so you can get your heating and hot water back quickly and confidently.

Ariston Boiler Error Codes: Quick Overview

Below is a summary of the most common Ariston boiler error codes, grouped by category, with what they mean and the typical models affected.

These codes help you act fast. Many can be cleared with a simple reset, while others need a quick check or professional help.

Ariston Boiler 101 Error Code

The 101 error code on an Ariston boiler means the system has detected overheating, usually because the water temperature has risen too high and tripped the safety thermostat to protect the boiler.

What Causes the 101 Error Code

Common triggers include all radiator valves being closed (no water flow), a failed or blocked pump, airlocks in the system, heavy limescale in hard-water areas, or the boiler running without enough water circulation.

How to Fix

1. Check every radiator valve is fully open and bleed any air from the system.
2. Verify the boiler pressure is between 1 and 1.5 bar and top up if low using the filling loop.
3. Power off the boiler at the isolator for 5 minutes, then restart and press the reset button.
4. If the code returns quickly, call a Gas Safe engineer. Repeated overheating can damage components and needs professional diagnosis.

Ariston Boiler 108 Error Code

The 108 error code on an Ariston boiler signals low water pressure or insufficient water in combi models, so the boiler safely shuts down to avoid damage.

What Causes the 108 Error Code

This is often caused by a small leak, bleeding radiators recently, natural pressure loss over time, or a faulty pressure sensor. It’s one of the simplest and most frequent Ariston boiler error codes in UK homes.

How to Fix

1. Look at the pressure gauge on the front of the boiler. It should read 1–1.5 bar when cold.
2. Locate the filling loop (flexible silver hose underneath) and slowly top up to 1.5 bar while watching the gauge.
3. Close the valves, restart the boiler and test heating/hot water.
4. If pressure drops again within a day or two, stop topping up and arrange an engineer to find the leak.

If your Ariston boiler keeps showing circulation or pressure-related codes, a professional boiler service can diagnose and resolve it before it becomes a costly breakdown. 

Ariston Boiler 110 Error Code (and 112)

The 110 (flow) or 112 (return) error code means the temperature sensor (probe) on the heating flow or return pipe has an open or short circuit, so the boiler cannot read the water temperature accurately.

What Causes the 110 Error Code

Wiring damage, corrosion, a loose connection, or the sensor itself failing are the main causes. These often develop gradually in older Ariston boilers or after nearby plumbing work.

How to Fix

1. Power off the boiler completely for 5 minutes and restart to see if the code clears.
2. Check visible wiring to the sensor for damage (do not remove the boiler casing yourself).
3. If the code persists, the sensor usually needs replacing. This is a quick job for a Gas Safe engineer.
4. Ask the engineer to test both flow and return sensors while they’re there.

Ariston 501 Error Code

The 501 error code on an Ariston boiler indicates no flame has been detected during ignition, so the boiler locks out for safety.

What Causes the 501 Error Code

Typical causes are no gas supply (meter valve off or supplier issue), dirty or faulty ignition electrodes, a blocked flue, low gas pressure, or a failing gas valve.

How to Fix

1. Check other gas appliances (hob/cooker) work. If not, contact your gas supplier.
2. Ensure the gas valve at the meter is fully open.
3. Reset the boiler (hold the button for 5–10 seconds or follow your manual).
4. If the code returns, do not keep resetting. Call a Gas Safe engineer immediately for gas-related checks.

Ariston Boiler 5P1 Error Code (and 5P2 / SP1 / SP2)

The 5P1 (or SP1) error code means the first ignition attempt failed; 5P2/SP2 means the second also failed, triggering a lockout.

What Causes the 5P1 Error Code

Causes include low gas pressure, dirty electrodes, condensate pipe issues in winter, or a temporary flue blockage. These are very common after power cuts or cold snaps.

How to Fix

1. Confirm gas supply is on and other appliances light.
2. Gently thaw any frozen condensate pipe with warm water if it’s winter.
3. Reset the boiler and allow it to attempt ignition again.
4. Persistent codes require an engineer to clean electrodes or check gas pressure safely.

Ariston Boiler 604 Error Code

The 604 error code means the boiler fan is not reaching the correct speed, preventing safe combustion.

What Causes the 604 Error Code

A dirty fan, worn motor, blocked flue, or electrical fault. The fan is a critical safety component on Ariston Evo and Clas models.

How to Fix

1. Reset the boiler and listen for the fan whirring when it tries to start.
2. Power cycle fully (off at isolator for 5 minutes).
3. If the code returns, only a Gas Safe engineer can test fan speed and replace the unit.

Ariston Boiler 3P9 Error Code

The 3P9 error code is a helpful reminder that your Ariston boiler is due a service. It’s not a fault but a proactive warning.

What Causes the 3P9 Error Code

It simply counts operating hours or time since the last service on newer Ariston One series boilers.

How to Fix

1. Note the code and book an annual service (this often clears it automatically).
2. Do not ignore it. Regular servicing prevents more serious Ariston boiler error codes later.
3. A Gas Safe engineer will perform safety checks, clean components, and reset the reminder.

Getting your boiler serviced annually catches small issues like sensor wear or fan problems long before they turn into expensive repairs and keeps your warranty valid.

Protect Your Boiler Year-Round with Service and Cover Options

At PlusHeat, we make it simple to keep your boiler safe, efficient, and reliable across the UK. If you’re due for a routine check-up, tune-up, or want to prevent future Ariston boiler error codes, start with our flexible boiler service plans that focus on proactive maintenance and early fault detection.

If you’re looking for full protection against unexpected breakdowns, emergency repairs, and peace of mind all year, our boiler cover plans have you covered with repairs, parts, labour, and 24/7 support so you’re never left with surprise bills.

Frequently Asked Questions 

Do Ariston error codes vary between older and newer boiler models?

Yes. Older Ariston models may show SP1/SP2 for ignition issues while newer Evo and One series use 5P1 or 501. Always check your exact model number (on the front or data plate) and refer to the manual for precise guidance.

Can I keep resetting an Ariston boiler error code without calling an engineer?

You can safely reset most codes once or twice to see if the problem clears, but repeated resets (especially 304 or ignition codes) can strain components and may void your warranty if damage occurs.

Will my Ariston boiler still work if I ignore a 3P9 service reminder code?

The boiler will continue running for a while, but ignoring the reminder risks reduced efficiency, higher bills, and potential safety issues, plus you may lose manufacturer warranty cover.

Are Ariston boiler error codes covered under landlord gas safety regulations?

Yes. Landlords must ensure any fault (including error codes) is repaired promptly to maintain a valid Gas Safe certificate. A boiler cover plan makes this straightforward and cost-effective.

How do I find my exact Ariston boiler model to check the right error code meaning?

Look for the model name and serial number on the white data badge inside the front casing or on the side panel, or take a clear photo and show it to your engineer for instant identification.