Johnson & Starley E17 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley E17 fault code mean?
E17 on a Johnson & Starley unit is a sensor stuck lockout on the second of the monitored sensor channels: a temperature reading the controller supervises has stopped responding while the appliance heats or cools. A flat-lining thermistor is a failed thermistor — real temperatures in a firing heater always move — so the controller locks out to avoid controlling gas output against a dead reading. It sits alongside E16 and E18, which apply the identical supervision to the other sensor channels; the code number simply directs the engineer to the right circuit. Diagnosis and repair follow the standard routine: live-reading observation, resistance check against the table, then sensor or wiring replacement.
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
- Sensor element failed frozen Common
The affected thermistor's output has stopped tracking real temperature — the element has failed in place. Confirmed by comparing its resistance against the temperature table while the unit heats and cools.
- Wiring holding the reading constant Sometimes
A partial short or a damaged lead can pin the measured value so it no longer moves; the loom is checked before the sensor is condemned.
- Board measuring-input fault Rare
If a known-good sensor still reads flat at the board, the measuring input itself has failed and the PCB is replaced.
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 watch the affected sensor's live reading through a heat-up cycle — a healthy thermistor moves continuously; a stuck one sits flat. They will then measure its resistance against specification and replace the sensor, repair the wiring, or (rarely) replace the board.
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 E17 mean on my Johnson & Starley?
One of the unit's temperature sensors has stuck — its reading no longer follows the appliance's real temperature, so the controller locks out. The E16/E17/E18 codes distinguish which sensor channel; the fix in every case is a confirmed-and-replaced probe, one of the cheaper heating repairs.
Could it be wiring rather than the sensor?
Yes — a partial short can pin a reading just as a dead element can. That is why the engineer checks the loom and measures resistance before fitting parts, rather than swapping the sensor on spec.
Is the unit safe while showing E17?
It is safe precisely because it has locked out — it will not fire without trustworthy temperature readings. Leave it off and have the sensor replaced; there is no workaround worth attempting.