Intergas 21 Fault Code: Causes, Fixes & Repair Costs
What does the Intergas 21 fault code mean?
Fault code 21 on Intergas boilers indicates a problem with the S2 temperature sensor — the return-water sensor built into Intergas's distinctive two-in-one heat exchanger. This sensor continuously monitors the temperature of water coming back from the heating circuit into the boiler, and the control board uses that reading to manage burner output safely. When the signal from S2 falls outside the expected range — or disappears entirely — the boiler locks out and shows code 21 as a protective measure. Codes 20, 21, 22, 23, and 24 all relate to the same S2 sensor circuit; they represent slightly different failure modes (for example, an open-circuit reading, a short-circuit reading, or a value that is implausibly high or low) but all point to the same root cause. If your display shows any code in the 20–24 range, the guidance on this page applies.
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 S2 NTC temperature sensor Common
The S2 sensor is a negative temperature coefficient (NTC) thermistor whose electrical resistance changes predictably with water temperature. Over time — or following a sudden overheat event — the thermistor element can degrade or fail completely, sending a resistance value that is out of range. This is the single most common reason for codes 20–24 and is usually confirmed by a resistance measurement that does not match the expected value for the current water temperature.
- Broken, corroded, or loose sensor wiring Common
The wiring loom that runs between the S2 sensor and the control board can develop a break, a loose pin connection, or corrosion on a terminal over time — particularly in boilers installed in damp utility rooms or garages. The control board then sees either no signal or an erratic one, triggering the same lockout as a failed sensor. The Intergas service manual specifically lists wiring inspection as the first diagnostic step for codes 20–24.
- Incorrectly seated or disturbed sensor Sometimes
If the boiler has recently been serviced or had other work carried out, the S2 sensor may not have been refitted fully into its pocket in the heat exchanger. Even a small gap between the sensor and the heat exchanger wall can cause inaccurate readings. Intergas's own documentation warns that failure to fit the S1 or S2 sensors correctly can cause serious damage, so proper seating is critical.
- PCB (control board) fault Rare
In a smaller number of cases, the control board itself is unable to process the sensor signal correctly even though the sensor and its wiring are both intact. This is typically only confirmed after the sensor and wiring have already been tested and found to be within specification. PCB failure can also affect multiple sensor readings simultaneously.
How to fix it
- Reset the boiler once DIY safe
Press the reset button on the boiler's operating panel. A single reset is reasonable — if the boiler fires up and runs normally, monitor it over the next few hours. However, if code 21 returns promptly, do not keep resetting. Repeated resets without fixing the underlying fault will not help and may mask a developing problem. Two to three attempts is the safe upper limit before calling an engineer.
- Check that no recent work has disturbed the boiler DIY safe
If a service or repair was carried out recently, note that for the engineer — a sensor that was removed and not fully reseated is a straightforward fix compared to a failed component. This background information will speed up the diagnosis.
- Inspect the S2 sensor wiring loom Gas Safe engineer
A Gas Safe registered engineer will trace the wiring from the S2 sensor back to the control board, checking for any visible breaks, burned or corroded terminals, and loose plug connections. This is often enough to identify the fault without replacing any parts.
- Test the S2 sensor resistance Gas Safe engineer
Using a multimeter, the engineer will measure the resistance of the S2 NTC thermistor and compare it against the expected value for the current water temperature. A reading that is open-circuit, short-circuit, or significantly out of the expected range confirms sensor failure. Intergas specifies the use of genuine Intergas replacement parts when a new sensor is needed.
- Replace the S2 sensor if out of specification Gas Safe engineer
If the resistance test confirms the sensor has failed, the engineer will fit a new genuine Intergas S2 sensor, ensuring it is correctly seated in the heat exchanger pocket. An incorrectly fitted sensor can cause further damage, so proper installation is essential. The part typically costs around £50.
- Test the control board if sensor and wiring are both sound Gas Safe engineer
If the wiring and sensor both test within specification but the fault code persists, the engineer will assess the control board. A faulty PCB will need either specialist repair or replacement and is a less common but more expensive outcome.
- Call a Gas Safe registered engineer if the fault persists after a reset Gas Safe engineer
Fault code 21 requires hands-on electrical and component testing that must be carried out by a Gas Safe registered engineer. Do not attempt to access the sensor, wiring, or control board yourself. Book a qualified engineer to diagnose and resolve the fault.
Parts you may need
- Intergas S2 return-water NTC temperature sensor · from £50
- Sensor wiring loom / harness · from £35
- Intergas control PCB · 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 £150–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Intergas fault code 21 myself?
No. While you can safely try a single boiler reset, diagnosing and repairing a sensor fault requires electrical testing and component replacement inside the boiler — work that must be carried out by a Gas Safe registered engineer. Accessing the S2 sensor or its wiring yourself would also risk voiding any remaining warranty on your boiler.
What is the difference between Intergas fault codes 20, 21, 22, 23, and 24?
All five codes relate to the same S2 return-water temperature sensor circuit. The different numbers indicate slightly different failure modes — for example, one may represent a completely open circuit while another indicates a short circuit or an implausibly high reading — but all are diagnosed and repaired in the same way. Whatever code in the 20–24 range your display shows, the investigation starts with the S2 sensor wiring and the sensor itself.
How much does it cost to repair Intergas fault code 21 in the UK?
For most homeowners, the repair costs between £150 and £320 including parts and labour. Replacing the S2 sensor itself is the most common fix — the part is around £50 and most engineers will complete the job within a single visit. If the control board (PCB) turns out to be the cause, costs can rise to £350–£450 or more, but this is a less frequent outcome. Check whether your boiler is still within its warranty period (Intergas offers 3–10 years depending on the model) before paying for any repair.
Is my Intergas boiler still under warranty if it shows fault code 21?
Intergas boilers come with a standard 12-month warranty, which can extend to between 3 and 10 years depending on the model and registration. If your boiler is within that period, contact Intergas or your installer before arranging an independent repair, as the work may be covered. Note that Intergas specifies that only genuine Intergas parts should be used for replacements, so always confirm this with whoever carries out the repair.