Johnson & Starley E35 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley E35 fault code mean?
E35 on a Johnson & Starley unit reports a flow thermistor fault: the sensor measuring the flow-side temperature — the heated air leaving the unit on warm-air models, or the heating flow water on combined and water-side variants — has failed electrically, reading open or short. The flow reading is the controller's primary handle on output temperature: it modulates the burner against it and backs the overheat protection with it, so a failed flow probe restricts operation immediately. It is the counterpart of E33 (return thermistor); together the pair give the controller the temperature rise across the heat exchanger. Diagnosis and repair are the standard thermistor routine — wiring inspection, resistance check against the table, 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
- Failed flow thermistor Common
The flow probe lives at the hottest point of the measurement set and fails accordingly — open or short circuit confirmed by resistance check.
- Wiring or connector damage Sometimes
Heat-hardened insulation and vibration take out leads and connectors near the heat exchanger; the loom is checked before the sensor is condemned.
- Board input fault Rare
The measuring input rarely fails, and is only suspected once a known-good sensor still misreads.
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 flow thermistor, inspect its wiring and connectors, measure its resistance against specification, and replace the failed element — then confirm the flow reading tracks properly and the overheat chain behaves through a full heat cycle.
Parts you may need
- Flow 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 E35 mean on my Johnson & Starley?
The flow temperature sensor — the one the controller steers output by — has failed open or short circuit. Operation is restricted because the unit would otherwise be heating without knowing how hot it is running. An engineer replaces the probe after confirming it with a resistance check.
Is a flow sensor fault more serious than a return one?
Slightly — the flow reading also backs up overheat protection, so the controller is stricter about it. The repair is identical and similarly priced; the difference is only in how firmly the unit restricts itself meanwhile.
What will it cost?
Typically £90–£240 including the visit — the thermistor itself costs little. Regional rates vary; ask for a written quote.