Johnson & Starley E15 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley E15 fault code mean?
E15 on a Johnson & Starley unit is a sensor drift lockout: one of the temperature sensors the controller cross-checks has drifted outside the tolerance it expects — the reading is still changing, but no longer agrees with the physics of the appliance or with its companion sensors. Modern controls run plausibility checks precisely to catch a slowly failing thermistor before it causes a temperature-control problem, and E15 is that check firing. The remedy is diagnosis with a multimeter rather than guesswork: each candidate sensor's resistance is compared against the temperature table and the drifting one is replaced. Sensors are cheap; running a heater on a lying sensor is not.
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
- Ageing thermistor drifting out of tolerance Common
Thermistors drift with age and heat cycling; when a reading strays beyond the cross-check tolerance, E15 locks the unit out before control quality suffers.
- Poor connection skewing the reading Sometimes
A corroded or loose connector adds resistance in series with the sensor, offsetting the measured temperature — indistinguishable from drift until the loom is checked.
- Sensor in a disturbed position Sometimes
A probe dislodged from its pocket or clip reads the wrong location's temperature — plausible values, wrong physics. Position is checked alongside resistance.
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 compare each temperature sensor's resistance against the specification table at known temperatures, check connectors and sensor seating, and replace whichever probe has drifted. A verification heat cycle confirms readings track together afterwards.
Parts you may need
- Temperature sensor (NTC thermistor) · 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 £90–£250, depending on the underlying cause.
Frequently asked questions
What does E15 mean on my Johnson & Starley?
One of the unit's temperature sensors has drifted outside the tolerance the controller cross-checks — it still gives readings, but they no longer add up. The controller locks out rather than run on unreliable measurement. An engineer identifies and replaces the drifting sensor with a resistance check.
Why lock out over a slightly wrong reading?
Temperature sensing drives output control and backs up the safety limits. A sensor allowed to drift indefinitely eventually misleads both — catching it early, while it is merely inaccurate, is deliberately conservative design.
Is it expensive?
Usually not — a thermistor is one of the cheapest parts on the unit. Expect roughly £90–£250 including diagnosis, varying with access and region.