Johnson & Starley F07 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley F07 fault code mean?
F07 on a Johnson & Starley unit reports an exhaust sensor fault: the temperature probe monitoring the flue gases has failed electrically or is returning implausible readings. The exhaust sensor is a quiet but important guardian — abnormally high flue-gas temperature is an early sign of poor combustion or a fouling heat exchanger, so the controller watches it continuously and restricts operation when it loses the reading. The fault itself is usually mundane: a failed sensor element, heat-damaged wiring near the flue connection, or a corroded connector. Because it sits in the combustion-monitoring chain of a gas appliance, diagnosis and replacement are strictly a Gas Safe engineer's work.
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 exhaust temperature sensor Common
The probe lives in the hottest, most corrosive environment on the appliance and fails accordingly — open, short, or drifted. A resistance check settles it.
- Heat-damaged wiring or connector Sometimes
Leads near the flue outlet harden and crack with age; connectors corrode in the humid flue-adjacent zone. The loom is inspected before the sensor is condemned.
- Genuine flue-gas temperature abnormality Rare
Occasionally the sensor is honest and the flue gases really are out of range — pointing at combustion or heat-exchanger problems the engineer must investigate with an analyser.
How to fix it
- Reset once and observe DIY safe
Use the reset control (or switch off at the fused spur for 30 seconds and back on) a single time. If the code returns, stop resetting and book a Gas Safe registered engineer.
- Have a Gas Safe registered engineer complete the documented checks Gas Safe engineer
The engineer will test the exhaust sensor's resistance and wiring, replace the failed element, and — importantly — run a combustion check with a flue-gas analyser to confirm the reading it now gives is genuinely healthy, not just electrically valid.
Parts you may need
- Exhaust/flue temperature sensor · from £35
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–£280, depending on the underlying cause.
Frequently asked questions
What does F07 mean on my Johnson & Starley?
The flue-gas temperature sensor has failed or is reading implausibly, so the unit restricts operation — that sensor is part of how it watches its own combustion health. An engineer replaces the probe and should verify combustion with an analyser at the same visit.
Is a flue sensor fault dangerous?
The fault is usually electrical and the lockout is precautionary. But because the sensor's job is spotting combustion problems early, it must be repaired promptly and properly — and if you ever smell fumes or gas, switch off, ventilate, and call 0800 111 999.
Why does the engineer need an analyser for a sensor swap?
Because occasionally F07 is the sensor telling the truth about hot flue gases — a combustion or heat-exchanger problem. Fitting a new sensor without confirming combustion would just silence the messenger.