Johnson & Starley 46 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 46 fault code mean?
Fault code 46 on Johnson & Starley WarmCair warm air heaters indicates that the control board has detected an open circuit on the flue gas temperature sensor. In plain terms, the boiler's electronics cannot read a valid signal from that sensor — either because the sensor itself has failed, a wiring connection has come loose, or, very commonly, because there is a poor electrical connection on the ESYS PCB module that sits piggy-backed on the gas valve. As a safety precaution the unit shuts itself down and holds this code until the underlying problem is resolved.
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 at the ESYS PCB on the gas valve Common
The red-boxed ESYS module that piggy-backs onto the gas valve has a multiway connector whose terminals can loosen or corrode over time. Terminal 13 (which carries the flue sensor signal) is a known trouble spot. When contact is poor here the control board sees an apparent open circuit even though the sensor itself tests fine with a meter. Johnson & Starley have acknowledged this as a common pattern — a closely related code (fault 45, open circuit flue overheat sensor) shows the same behaviour at conductor 9 in the same connector.
- Disconnected or damaged sensor wiring Sometimes
The wire loom running from the flue temperature sensor to the PCB can develop a break, chafing damage, or a connector that has simply worked loose — particularly where the wiring passes through vibrating panels or tight bends. When continuity is lost anywhere along this run the boiler interprets it as an open circuit sensor.
- Failed flue temperature sensor Sometimes
The sensor element itself can fail, either due to age, heat cycling, or moisture ingress. A genuine open-circuit sensor will show no continuity when tested with a multimeter across its terminals and must be replaced before the boiler will fire.
How to fix it
- Do not attempt to reset the boiler more than two or three times DIY safe
Repeatedly resetting a boiler that has locked out on a sensor fault will not fix the underlying problem and may mask useful diagnostic information for the engineer. Try one reset; if it locks out again, leave it and arrange a service visit.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in the property (hob, fire) are working normally. If there is no gas supply, contact your gas supplier rather than an engineer.
- Arrange for a Gas Safe registered engineer to inspect the ESYS module connector Gas Safe engineer
Because the most common cause is a poor connection at Terminal 13 of the multiway connector on the red ESYS PCB (on the gas valve), the engineer's first action should be to inspect, clean, and reseat that connector before condemning any parts. This alone resolves many code-46 lockouts without the need for replacements.
- Have the engineer test the flue temperature sensor for continuity Gas Safe engineer
If the connector checks out, the engineer will test the sensor directly with a multimeter. A sensor showing no continuity across its terminals has failed and needs to be replaced. The flue temperature sensor is a listed serviceable part on the WarmCair range.
- Have the engineer check and reseat all wiring connectors in the sensor circuit Gas Safe engineer
The full wiring run from the sensor to the PCB should be inspected for chafing, corrosion, or loose plugs. Reseating or replacing an intermediate connector is a straightforward fix if a wiring fault is found.
- Replace the flue temperature sensor if it has failed Gas Safe engineer
If the sensor tests open circuit, the engineer will fit a replacement. This is a relatively straightforward parts-and-labour job on the WarmCair range and should restore normal operation.
- If the ESYS PCB module itself is faulty, have it replaced by a Gas Safe engineer Gas Safe engineer
In a small number of cases the ESYS PCB itself has an internal fault that causes it to misread the sensor circuit. PCB replacement is an engineer-only task and pushes the repair cost higher — see the FAQ below for typical costs.
Parts you may need
- Flue gas temperature sensor (WarmCair compatible) · from £25
- ESYS PCB module (gas valve piggy-back board) · from £220
- Wiring harness / sensor connector repair kit · from £15
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–£350, depending on the underlying cause.
Frequently asked questions
Why does fault code 46 appear even when the flue sensor tests as fine?
This is the most confusing aspect of code 46. Because the fault is frequently caused by a poor connection at Terminal 13 on the ESYS PCB module (not by the sensor itself), the sensor can measure perfectly on a multimeter while the boiler still reports an open circuit. The control board only sees what the connector delivers, so a corroded or loose contact at that terminal is enough to trigger the lockout. An engineer who knows this pattern will check the ESYS connector first, before ordering any parts.
Is fault code 46 a Gas Safe job, or can I fix it myself?
It is a Gas Safe registered engineer job. The most likely fix — reseating the connector on the ESYS PCB module that sits on the gas valve — involves working directly on gas valve components, which is not permitted for unregistered persons. Even if the fix turns out to be as simple as pushing a connector home, it must be done by a qualified engineer who can verify the repair is safe and that the boiler is functioning correctly before leaving.
How much does it cost to fix fault code 46 on a Johnson & Starley WarmCair?
If the fault is a loose or corroded connector on the ESYS module — the most common scenario — you are typically looking at a call-out plus an hour's labour: roughly £120–£200 all-in. If the flue sensor itself needs replacing (the part costs around £20–£30), add the parts cost to a similar labour charge, bringing the total to around £150–£280. In the less common situation where the ESYS PCB module needs to be replaced, the part alone can cost £180–£250, pushing the total to £300–£450. Emergency or out-of-hours call-outs will cost more.
What is the ESYS module and why does it cause so many sensor faults on WarmCair boilers?
The ESYS module is a small red-boxed PCB that piggy-backs directly onto the gas valve and handles certain safety interlock and sensor signals. It connects to the main wiring harness via a multiway plug, and the individual terminals in that plug are prone to developing high resistance through vibration and corrosion over time. Johnson & Starley themselves have noted that poor connections on this module are behind a number of apparent sensor faults — including both fault 45 (flue overheat sensor open circuit) and fault 46 (flue temperature sensor open circuit). Reseating or cleaning the relevant terminal is often all that is needed.