Johnson & Starley 43 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 43 fault code mean?
Code 43 on a Johnson & Starley warm-air unit points to a short circuit on the return air temperature sensor — the probe measuring the temperature of air being drawn back into the heater from the house. The controller uses this reading to manage heat output and fan behaviour; a sensor shorted (typically to earth, or across its own conductors) returns an impossible value, so the unit flags code 43. The documented action is to inspect the wiring connections and check the sensor's resistance. Its neighbouring code 44 covers the return sensor's other failure modes (open or short on the central-heating return on combined models), making 43/44 a diagnostic pair around return-side temperature sensing.
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 shorted internally or to earth Common
The thermistor element or its insulation fails, pinning the reading. Confirmed by a resistance check against the temperature table; replacement resolves it.
- Crushed or chafed sensor wiring Common
A pinched loom shorts the conductors and mimics a failed sensor — the documented wiring inspection comes first for this reason.
- Connector corrosion or moisture Sometimes
Moisture bridging a connector produces intermittent shorts, often clearing when dry and returning in damp conditions.
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 — Johnson & Starley's warm-air units lock out for a reason, and repetition only masks the fault.
- Keep return air grilles clear DIY safe
While not the cause of a short, blocked return grilles stress the airflow sensors' operating conditions — keep them unobstructed as general good practice.
- Have a Gas Safe registered engineer complete the documented checks Gas Safe engineer
The engineer will inspect the sensor wiring and connectors, measure the return air sensor's resistance against specification, and replace the sensor or repair the loom as found — the manufacturer's documented remedy for this code.
Parts you may need
- Return air temperature sensor · 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 code 43 mean on a Johnson & Starley warm-air heater?
The return air temperature sensor is reading as a short circuit — usually the sensor shorted to earth or a crushed lead. The fix is the documented one: inspect the wiring, check the sensor's resistance, and replace whichever has failed. A quick job for an engineer.
Can the heater run without this sensor?
No — return air temperature feeds output and fan control, so the unit restricts operation rather than run on an impossible reading. That is protective behaviour.
Is this the same as code 44?
They are companions: 43 is the return air sensor short, 44 covers return sensor error (open or short) on the heating-water side of combined models. Both resolve through the same inspect-measure-replace routine.