Intergas Fault Code 11: S1 Sensor Fault or Circulation Problem
Fault code 11 on an Intergas boiler indicates that the boiler's control board has detected either a problem with the S1 temperature sensor or a circulation fault. The S1 sensor sits on the heat exchanger and monitors the flow water temperature. When you call for heat, the PCB expects to see the flow temperature rise within a set time. If the S1 sensor isn't reporting the expected temperature increase — whether because the sensor itself is faulty or because water isn't circulating properly around the heat exchanger — the boiler locks out to protect itself from damage. Note that fault codes 10 through 14 all relate to heat exchanger sensor issues, with codes differing by which sensor or condition has triggered the lockout.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Intergas 11 fault code mean?
Fault code 11 on an Intergas boiler indicates that the boiler's control board has detected either a problem with the S1 temperature sensor or a circulation fault. The S1 sensor sits on the heat exchanger and monitors the flow water temperature. When you call for heat, the PCB expects to see the flow temperature rise within a set time. If the S1 sensor isn't reporting the expected temperature increase — whether because the sensor itself is faulty or because water isn't circulating properly around the heat exchanger — the boiler locks out to protect itself from damage. Note that fault codes 10 through 14 all relate to heat exchanger sensor issues, with codes differing by which sensor or condition has triggered the lockout.
Common causes
- Faulty or damaged S1 flow temperature sensor Common
The S1 sensor itself may have developed a fault — either due to component failure, damaged wiring, or a poor electrical connection at the sensor or PCB. When the sensor sends a reading outside the expected range, the boiler interprets this as a safety risk and locks out. This is the most straightforward diagnosis and the most cost-effective fix if confirmed.
- Limescale build-up blocking the heat exchanger Common
In hard water areas, dissolved minerals gradually deposit inside the heat exchanger. A restricted or partially blocked heat exchanger reduces water flow to the point where the S1 sensor detects an abnormal temperature rise, triggering the fault. This is particularly common in properties that have never had a powerflush or inhibitor treatment.
- Airlock or circulation pump problem Sometimes
An airlock in the system or a pump that is running at the wrong speed, partially seized, or blocked by debris can prevent adequate water movement through the heat exchanger. Without sufficient flow, temperatures climb rapidly and the boiler cuts out. An engineer will bleed the pump, check pump speed settings, and inspect for blockages.
- Cracked or otherwise damaged heat exchanger Rare
Physical damage to the heat exchanger — such as stress cracks caused by repeated overheating or corrosion — can disrupt water flow and cause erratic temperature readings at the S1 sensor. This is the most serious and costly diagnosis and is more likely on older boilers with a history of circulation problems.
How to fix it
- Check your gas supply is on DIY safe
Confirm that other gas appliances in the property are working normally. If there is no gas supply, contact your supplier rather than attempting any work on the boiler.
- Check the system pressure DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, top it up via the filling loop following your boiler's user guide. Low pressure can contribute to poor circulation and sensor faults.
- Attempt a single reset — no more than once or twice DIY safe
Press and hold the reset button as described in your Intergas user manual. If the boiler fires up and runs normally, monitor it closely. If the fault code 11 returns promptly, do not keep resetting — repeated resets without fixing the underlying cause can cause further internal damage. Call an engineer instead.
- Do not attempt to access internal components Gas Safe engineer
Diagnosing whether the S1 sensor, its wiring, the pump, or the heat exchanger is at fault requires opening the boiler casing and using specialist diagnostic tools. This must only be carried out by a Gas Safe registered engineer.
- Engineer to test the S1 sensor and wiring Gas Safe engineer
A Gas Safe engineer will check the resistance of the S1 sensor against the manufacturer's specification and inspect the wiring harness for signs of damage, corrosion, or loose connections. If the sensor is out of range, it will be replaced.
- Engineer to assess circulation and heat exchanger condition Gas Safe engineer
If the sensor tests within spec, the engineer will investigate the pump (speed, operation, blockages) and check for limescale or debris restricting the heat exchanger. A powerflush may be recommended, along with installation of a magnetic filter and inhibitor to prevent future build-up.
- Call a Gas Safe registered engineer Gas Safe engineer
Fault code 11 cannot be safely or reliably resolved without professional diagnosis. Contact a Gas Safe registered engineer to inspect the boiler. You can verify an engineer's registration at gassaferegister.co.uk.
Parts you may need
- Intergas S1 flow temperature sensor · from £50
- Circulation pump (Grundfos or equivalent) · from £80
- Magnetic system filter (e.g. Adey MagnaClean) · from £70
- Heat exchanger (Intergas compatible) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£400, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my Intergas boiler to clear fault code 11?
It is best to avoid repeatedly resetting the boiler. One or two resets are reasonable to see whether the fault clears, but if code 11 keeps returning, the boiler is locking out for a reason. Persistent resets without fixing the root cause risk causing further damage to the heat exchanger or other internal components. Get a Gas Safe engineer to diagnose the fault properly.
How much does it cost to fix Intergas fault code 11 in the UK?
The most common repair is replacement of the S1 sensor, which typically costs around £50 for the part plus one to two hours of labour — expect to pay roughly £120–£200 all in. If a powerflush is needed to clear a blocked heat exchanger, costs generally run from £300–£500 depending on system size. A heat exchanger replacement is significantly more expensive, often £450–£700 including labour, and on an older boiler it may be worth comparing that against the cost of a new boiler installation.
Is fault code 11 dangerous?
The boiler has locked out as a safety measure, so you are not in immediate danger. The lockout prevents the heat exchanger from overheating or operating in an unsafe condition. Do not attempt to bypass the lockout. Leave the boiler switched off if you are unable to get an engineer promptly, and make sure you have carbon monoxide detectors fitted and working in your home.
Do I live in a hard water area and does it affect fault code 11?
If you are in a hard water region — much of southern and eastern England in particular — limescale accumulation inside the heat exchanger is a very plausible cause of this fault code. A Gas Safe engineer can confirm this and arrange a powerflush if needed. Going forward, fitting a limescale inhibitor filter and dosing the system with a quality inhibitor annually (ideally at your boiler service) will greatly reduce the risk of recurrence.