Johnson & Starley E18 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley E18 fault code mean?
E18 on a Johnson & Starley unit is the third of the stuck-sensor lockouts: a supervised temperature channel has frozen — the reading refuses to move while the unit's actual temperature changes. The controller cross-checks its sensors against expected behaviour every cycle, and a static value during heat-up fails that test immediately. As with E16 and E17 (the same supervision on the other channels), the code's job is to point the engineer at the right circuit; the underlying fault is a failed thermistor, a lead pinning the signal, or — rarely — the board input. The repair is short: confirm with a resistance check, replace the failed element, verify readings track through a full cycle.
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 E18 mean on my Johnson & Starley?
A temperature sensor has failed 'stuck' — its output stopped tracking the unit's real temperature, so the controller locked out. It is the same fault class as E16 and E17 on a different sensor channel. An engineer confirms which probe with a resistance check and replaces it.
Why are there three codes for the same fault?
Each code maps to a specific sensor circuit, which saves diagnostic time — the engineer goes straight to the right probe instead of testing them all. For the homeowner the meaning is identical: one failed temperature sensor.
How long is the repair?
Typically well under an hour once the engineer is on site with the sensor — access, measurement, replacement and a verification heat cycle. Cost usually lands £90–£250 depending on region and call-out terms.