Intergas F031 Fault Code: Causes, Fixes & Repair Costs
What does the Intergas F031 fault code mean?
The F031 code appears when your Intergas boiler's PCB detects a problem with the S1 sensor — the component responsible for monitoring the temperature of water leaving the heat exchanger (the flow). The PCB continuously reads data from this sensor to confirm the boiler is heating water correctly. When that signal is absent, erratic, or outside expected parameters, the boiler locks out and displays F031 as a safety measure. Until the underlying issue is resolved, the boiler will not restart. Note that F031 is distinct from F015, which relates to an S1 sensor mounting fault; both involve the same sensor but point to different root causes, so your engineer will use the code to narrow down their starting point.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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.
Common causes
- Failed or degrading S1 flow temperature sensor Common
The S1 sensor itself is the most frequent culprit. Like all electronic sensors, it can deteriorate over time, begin producing inaccurate readings, or stop sending a signal to the PCB altogether. When the PCB cannot get a reliable flow temperature reading, it shuts the boiler down to prevent overheating or unsafe operation.
- Loose, damaged, or corroded wiring and connections Sometimes
The wiring harness connecting the S1 sensor to the PCB can suffer from loose terminals, corrosion (particularly in older or damp installations), or physical damage. A poor connection produces intermittent or absent signals that the PCB interprets as a sensor fault, even if the sensor itself is in good working order.
- Faulty PCB Rare
The PCB is the boiler's control centre, receiving and interpreting signals from all sensors including the S1. If the PCB develops a fault on the relevant input channel, it may misread or completely fail to register the S1 signal, triggering F031 despite the sensor and wiring being perfectly serviceable.
How to fix it
- Check your gas supply is working DIY safe
Before doing anything else, confirm other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply to the property, contact your gas supplier rather than attempting any boiler work.
- Reset the boiler once — no more DIY safe
Press and hold the reset button on your Intergas boiler for around three seconds. If the boiler fires up and runs normally, monitor it closely. If F031 returns, do not keep resetting — repeated resets without fixing the underlying fault will not help and may mask useful diagnostic information for your engineer.
- Have a Gas Safe engineer test the S1 sensor Gas Safe engineer
An engineer will use a multimeter to check the S1 sensor's resistance against the manufacturer's temperature-resistance curve. A reading that falls outside the expected range confirms the sensor has failed and needs replacing. This involves opening the boiler casing and handling internal components — Gas Safe work only.
- Have the engineer inspect all related wiring and terminal connections Gas Safe engineer
The engineer will trace the wiring from the S1 sensor back to the PCB, checking for loose terminals, signs of heat damage, corrosion, or chafing. Any damaged sections will be repaired or replaced, and all connectors reseated securely before retesting.
- Have the engineer test the PCB if the sensor and wiring check out Gas Safe engineer
If the S1 sensor and its wiring are confirmed to be in good condition, attention turns to the PCB. The engineer can test the relevant PCB input using a multimeter. A confirmed PCB fault means the board will need to be replaced — this is a more involved and costly repair.
- Reset the boiler once the fault has been rectified DIY safe
After the engineer has completed the repair and confirmed the fix, a final reset allows the boiler's control system to re-sync and return to normal operation. At this stage a reset is appropriate; it is not a substitute for the repair itself.
- If you have not already done so, contact a Gas Safe registered engineer Gas Safe engineer
F031 is not a fault a homeowner can safely resolve without professional intervention. You can verify any engineer's Gas Safe registration at gassaferegister.co.uk before they begin work.
Parts you may need
- Intergas S1 flow temperature sensor · from £50
- Sensor wiring harness / loom · from £35
- Intergas PCB (model-dependent) · from £220
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 £150–£320, depending on the underlying cause.
Frequently asked questions
Can I reset my Intergas boiler to clear F031 myself?
You can try a single reset — press and hold the reset button for around three seconds. Occasionally a transient signal glitch can trigger F031, and one reset will clear it. However, if the code comes back, further resets will not fix the problem. The S1 sensor, its wiring, or the PCB will need professional diagnosis. Repeatedly resetting a locked-out boiler is not recommended.
How much does it cost to fix an Intergas F031 fault?
If the S1 sensor itself has failed, parts typically cost around £50 and total repair costs including a Gas Safe engineer's labour usually land between £150 and £250. If the wiring needs attention, costs are similar. A PCB replacement is the most expensive outcome — parts alone can be £200 or more depending on your specific Intergas model, pushing the total bill to £400–£500. Most people with an F031 fault pay towards the lower end of the range because the sensor is the most common cause.
What is the difference between Intergas fault codes F031 and F015?
Both codes involve the S1 flow temperature sensor, but they point to different problems. F015 indicates a sensor mounting fault — meaning the sensor may be poorly positioned or not making proper thermal contact, causing misleading temperature readings. F031, by contrast, points to an electrical fault with the sensor itself, its wiring, or the PCB's ability to read the signal. An engineer familiar with Intergas boilers will use the specific code to guide their initial checks.
How long does it take to repair an F031 fault?
A straightforward S1 sensor replacement is usually completed within an hour once the engineer is on site, assuming they carry the part. Wiring repairs are similarly quick. A PCB replacement may take longer depending on parts availability and your boiler model, and may require a return visit if the board needs ordering. Getting an Intergas-experienced engineer is worthwhile as they are more likely to carry compatible parts.