Johnson & Starley E33 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley E33 fault code mean?
E33 on a Johnson & Starley unit reports a return thermistor fault: the sensor measuring return temperature — return air on warm-air models, heating return water on the combined and water-side variants — has failed electrically, reading open or short circuit rather than a plausible temperature. The controller uses the return reading to judge how much heat the house is absorbing and to modulate output accordingly, so it restricts operation when that probe fails. Diagnosis is the classic thermistor routine — inspect the wiring, measure resistance against the temperature table, replace the failed part — and it sits in the same family as codes 43 and 44, which cover specific return-sensor failure modes.
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 return thermistor Common
The element fails open or short with age and thermal cycling; a resistance check against the table settles it in minutes.
- Wiring or connector damage Sometimes
Chafed leads or corroded connectors reproduce either failure mode; the loom is inspected before condemning the sensor.
- Board input fault Rare
Rarely the measuring input fails; suspected only when a known-good sensor still misreads at the board.
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 locate the return thermistor for your model, check its wiring and connectors, measure its resistance hot and cold, and replace the failed element, verifying the reading tracks correctly through a heating cycle afterwards.
Parts you may need
- Return thermistor (NTC) · 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–£240, depending on the underlying cause.
Frequently asked questions
What does E33 mean on my Johnson & Starley?
The return temperature sensor has failed electrically — open or short circuit — so the controller has lost the reading it uses to modulate heat output. An engineer confirms sensor versus wiring with a resistance check and replaces the failed part; thermistors are inexpensive.
How does E33 relate to codes 43 and 44?
They all concern return-side temperature sensing: 43 is specifically the return air sensor shorted, 44 the CH return sensor open/short on water-carrying models, and E33 the return thermistor fault generally. Which appears depends on model and failure mode — the repair path is the same.
Will the heating work meanwhile?
Operation is restricted rather than fully locked out on some models, but running degraded is not a plan — the sensor exists to keep output matched to demand. Book the repair; it is quick and cheap as heating faults go.