Johnson & Starley 45 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 45 fault code mean?
Fault Code 45 appears on Johnson & Starley WarmCair warm air boilers when the control system detects a short circuit in the flue gas temperature sensor circuit. In practice, the boiler displays 'E45' and shuts down to prevent unsafe operation. The boiler cannot reliably monitor flue gas temperatures, so it locks out as a precaution. This is closely related to Code 46, which indicates an open circuit on the same sensor — engineers often find the two codes alternate depending on the exact nature of the wiring fault, and the diagnostic approach is similar for both. All work to clear this fault must be carried out by a Gas Safe registered engineer with warm air appliance experience.
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
- Poor connection on the ESYS module multiway connector Common
The most frequently reported real-world cause is a loose or corroded connection at the ESYS module on the gas valve — specifically at Conductor No. 9 within the multiway connector. Johnson & Starley themselves have acknowledged this as a known weak point on WarmCair units, and specialist warm air engineers regularly find it responsible for E45 lockouts.
- Damaged or short-circuited wiring loom Common
The cable run between the flue gas temperature sensor and the ESYS control board can suffer from chafing, heat damage, or insulation breakdown over time. A short circuit anywhere along this loom will trigger the E45 code, even if the sensor itself is perfectly healthy.
- Failed flue gas temperature sensor Sometimes
The sensor itself can develop an internal short circuit as it ages or if it has been subjected to repeated overheating events. A Gas Safe engineer can confirm this by measuring the sensor's electrical resistance against the manufacturer's resistance-temperature curve.
How to fix it
- Do not attempt to reset the boiler repeatedly DIY safe
Unlike some fault codes, E45 is caused by an electrical fault in the sensor circuit that will not clear on its own. Resetting once or twice is acceptable to confirm the fault is persistent, but repeated resets will not resolve the underlying problem and could mask intermittent faults.
- Check that the boiler's gas supply is on and other gas appliances in the property are working DIY safe
Although E45 is not a gas supply fault, it is always worth ruling out an interrupted gas supply before calling an engineer. Check that a gas hob or fire lights normally.
- Contact a Gas Safe registered engineer experienced with warm air appliances DIY safe
Johnson & Starley WarmCair boilers are warm air units rather than conventional wet-system boilers. Make sure the engineer you book has relevant experience — not all Gas Safe engineers regularly work on warm air appliances. Mention the E45 code when booking so they can bring appropriate test equipment and potentially the correct sensor.
- Engineer to inspect the ESYS module multiway connector — especially Conductor No. 9 Gas Safe engineer
The most productive first check is a close inspection of the multiway connector on the ESYS module for signs of corrosion, pushed-back pins, or a loose fit at position nine. Re-seating or re-pinning this connection resolves the fault in many cases without any parts being needed.
- Engineer to check the full wiring loom between the sensor and the ESYS board Gas Safe engineer
Every section of cabling should be checked for chafing, heat damage, or insulation breakdown. A short circuit anywhere along the loom will produce the E45 code. Damaged sections of the loom should be repaired or replaced rather than temporarily taped.
- Engineer to measure the flue gas temperature sensor resistance Gas Safe engineer
Using a multimeter, the engineer should measure the sensor's resistance across its operating temperature range and compare the readings against the manufacturer's resistance-temperature graph. A reading that sits outside specification or shows a short to earth confirms sensor failure.
- Replace the flue gas temperature sensor if confirmed faulty Gas Safe engineer
If the wiring and connections are sound but the sensor itself is defective, it should be replaced with the correct Johnson & Starley part. After fitting, the engineer should clear the fault code, run the boiler through a full heat cycle, and verify that no further faults are logged.
- Replace or repair the ESYS module if the fault lies within the board itself Gas Safe engineer
In a minority of cases the short circuit originates inside the ESYS module rather than in the external wiring. If all wiring and the sensor check out correctly, the engineer may need to source a replacement ESYS module. This is the most expensive outcome and should only be concluded after all other causes have been ruled out.
Parts you may need
- Flue gas temperature sensor (WarmCair) · from £45
- ESYS module multiway connector / wiring loom · from £35
- ESYS control module · 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–£350, depending on the underlying cause.
Frequently asked questions
Can I reset my Johnson & Starley boiler to clear fault code 45 myself?
You can try a single reset to confirm the fault is genuine and not a one-off glitch, but E45 will almost certainly return immediately because it is caused by a wiring or sensor fault that does not clear itself. Resetting more than two or three times is not recommended and will not fix the underlying problem. You need a Gas Safe engineer to diagnose and repair the fault.
What is the difference between Johnson & Starley fault codes 45 and 46?
Code 45 indicates a short circuit in the flue gas temperature sensor circuit — where wires are making unwanted contact with each other or with earth. Code 46 points to an open circuit on the same sensor — meaning the circuit is broken rather than shorted. In practice the two codes can appear interchangeably when there is a wiring fault, because a damaged section of loom can produce either condition depending on how the insulation has failed. The diagnostic approach is the same for both: the engineer will inspect the ESYS module connector, check the loom, and measure the sensor resistance.
Why do I need a warm air specialist rather than any Gas Safe engineer?
Johnson & Starley WarmCair units are warm air boilers — quite different from the wet central heating boilers most domestic engineers encounter every day. The ESYS module, the multiway connector arrangement, and the sensor resistance charts are specific to this product range. An engineer without warm air experience may not carry the right test equipment or know the known weak points such as Conductor No. 9 on the ESYS connector. It is worth asking specifically when booking.
How much does it cost to fix fault code 45 on a WarmCair boiler?
For most households the repair falls between £150 and £350 all-in, covering the engineer's diagnostic time and any parts needed. A straightforward re-seat of the ESYS connector costs little beyond the call-out charge, while a sensor replacement adds £40–£60 for the part on top of labour. In the relatively rare situation where the ESYS module itself needs replacing, costs can rise to around £400 or more. Out-of-hours or emergency call-outs will add to these figures.