Intergas 20 Fault Code: Causes, Fixes & Repair Costs
What does the Intergas 20 fault code mean?
Fault code 20 on an Intergas boiler signals a problem with the S2 temperature sensor — one of two NTC (Negative Temperature Coefficient) sensors mounted on the heat exchanger. The S2 sensor sits at the bottom of the heat exchanger and continuously feeds water-temperature readings back to the PCB. When the PCB loses a valid signal from S2, it cannot safely manage combustion and shuts the boiler down in lockout. Codes 21 and 22 are closely related and also point to an S2 sensor fault (21 indicates the sensor is reading abnormally high; 22 indicates abnormally low), so if your display shows any of these three codes the diagnosis and repair path is essentially the same. The fix is usually straightforward — either a wiring issue or a failed sensor — and because the S2 sensor itself is an inexpensive part, this tends to be one of the cheaper boiler repairs.
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 sensor Common
The NTC sensor itself has degraded or failed outright. Over time heat cycling causes the sensor's resistance characteristics to drift outside acceptable limits, so the PCB receives a reading it cannot trust. This is the single most common root cause of codes 20, 21, and 22.
- Broken, loose, or corroded sensor wiring Common
The wiring loom between the S2 sensor and the PCB can develop a break, a loose push-fit connector, or corrosion — particularly in older boilers or those in damp plant-room conditions. This interrupts the signal just as effectively as a failed sensor.
- Incorrectly seated or displaced sensor Sometimes
If the boiler has been recently serviced or worked on, the S2 sensor may not be fully pressed into its pocket on the heat exchanger. Even a small gap affects the resistance reading the PCB sees. Intergas explicitly warns that incorrectly fitted sensors can cause serious damage.
- Poor circulation — airlock or pump fault Sometimes
If water flow through the heat exchanger is restricted (for example, an airlock, closed radiator valves, or a pump running at too low a speed), temperatures at the sensor can behave erratically. Codes 20–22 can appear as a secondary symptom in these cases, though a primary pump or airlock fault code will often appear alongside it.
How to fix it
- Press reset and observe whether the boiler restarts DIY safe
On most Intergas models the lockout is indicated by a flashing LED above the key symbol. Press and hold the key button briefly to attempt a restart. If the boiler fires and runs normally, monitor it over the next hour. If code 20, 21, or 22 returns promptly, a reset alone will not cure the underlying fault and you should move on to the steps below rather than repeatedly resetting.
- Check your gas supply is live DIY safe
Confirm other gas appliances in the property (hob, gas fire) are working. If there is no gas supply, contact your gas supplier rather than continuing to reset the boiler.
- Check boiler system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold. If it is below 1 bar, top up via the filling loop following the instructions in your boiler's user guide. Low pressure alone is unlikely to cause code 20, but it is worth ruling out before calling an engineer.
- Inspect the S2 sensor wiring visually (engineer task) Gas Safe engineer
A Gas Safe engineer will trace the wiring loom from the S2 sensor at the base of the heat exchanger back to the PCB, checking for any obvious breaks, pinched cables, burnt insulation, or connector pins that have backed out of their housing. Corroded connectors will be cleaned or replaced.
- Test S2 sensor resistance with a multimeter (engineer task) Gas Safe engineer
The engineer will disconnect the S2 sensor and measure its resistance at a known temperature. An NTC sensor has a predictable resistance-temperature curve; a reading well outside that curve confirms the sensor has failed and needs replacing.
- Replace the S2 NTC sensor — part number 200117 (engineer task) Gas Safe engineer
The Intergas heat exchanger NTC sensor (part no. 200117) is the same part for both the S1 and S2 positions, making it straightforward to source. Intergas strongly recommends using genuine Intergas spare parts. After fitting, the engineer must ensure the sensor is fully and correctly seated in its pocket before testing.
- Check for circulation issues if the fault persists (engineer task) Gas Safe engineer
If both wiring and the sensor test satisfactory, the engineer will check for reduced water flow: bleeding radiators for trapped air, verifying the pump runs at the correct speed setting, and confirming no zone valves or isolating valves are closed that should be open.
- Call a Gas Safe registered engineer Gas Safe engineer
Because all the diagnostic and repair work for fault code 20 involves internal boiler components, a Gas Safe registered engineer must carry out the work. If your boiler is still within its Intergas warranty period (standard 12 months, extendable up to 10 years), contact Intergas Heating directly on 01527 888000 or service@intergasheating.co.uk for a fixed-price or warranty repair — note that the warranty requires annual Gas Safe servicing to remain valid.
Parts you may need
- Intergas NTC Sensor S1/S2 — part no. 200117 · from £13
- Sensor wiring harness / loom · from £25
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 £100–£200, depending on the underlying cause.
Frequently asked questions
What is the difference between Intergas fault codes 20, 21, and 22?
All three relate to the same S2 temperature sensor on the heat exchanger. Code 20 is a general S2 sensor fault (no valid signal), code 21 means the sensor is returning an abnormally high reading, and code 22 means it is returning an abnormally low reading. In practice all three are diagnosed and repaired in the same way — check the wiring, test the sensor, replace it if faulty — so if you see any of these three codes on your Intergas display, this page applies to you.
Can I fix Intergas fault code 20 myself?
The only homeowner-safe steps are attempting a reset (once or twice), checking that the gas supply is on, and verifying that system pressure is between 1 and 1.5 bar. Everything beyond that — inspecting sensor wiring, testing resistance, replacing the sensor — must be done by a Gas Safe registered engineer. The sensor itself is very cheap (around £12–£15), but incorrect fitting can cause serious damage to the heat exchanger, so DIY replacement is not advisable.
How much will it cost to repair Intergas fault code 20?
Because the S2 sensor (part no. 200117) costs roughly £12–£15, most of the bill is the engineer's callout and labour, typically £70–£100 for an independent Gas Safe engineer. All in, most homeowners pay £100–£200 for a code-20 repair. If the fault turns out to be a PCB issue rather than the sensor itself, costs can be significantly higher — a PCB replacement on an Intergas boiler typically runs to £300–£500 including labour — but this is an uncommon outcome for this specific fault.
My Intergas boiler keeps showing fault code 20 after reset — why?
A reset clears the lockout and allows the boiler to attempt a restart, but it does not fix the underlying fault. If code 20 returns every time, the S2 sensor or its wiring is genuinely faulty and needs attention from a Gas Safe engineer. Repeatedly resetting without fixing the root cause can mask the problem and risks further damage, so it is best to book an engineer rather than continuing to reset.