Johnson & Starley 21 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 21 fault code mean?
Fault code 21 on Johnson & Starley warm air units points to a problem with the Air Circulation Fan (ACF) — the fan responsible for pushing heated air through the duct system and into each room. The boiler's control system has detected that this fan is either not running at all, or is failing to reach the required maximum speed. On WarmCair condensing warm air models (such as the C10D, C16D, C26D, and C36U), this is a diagnosed ACF defect as defined in the manufacturer's own service documentation. On older Economaire warm air units, a code 21 is treated as a serious unresolved fault, and even trained engineers are directed to contact Johnson & Starley directly before proceeding — it is not a fault with a simple off-the-shelf fix. In both cases, this is not something a homeowner can safely address and specialist warm air engineer attendance is essential.
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
- Air Circulation Fan motor failure Common
The ACF motor itself has worn out or seized, meaning it either does not start at all or cannot sustain the speed required by the control system. This is the most common cause on older WarmCair units and can sometimes be accompanied by screeching or grinding noises when the fan attempts to start or stop.
- Fan running but not reaching maximum speed Common
The fan motor may still spin but is degraded to the point where it cannot hit the required RPM. The control board detects this shortfall and triggers the fault. Low voltage at the fan terminals — where the engineer expects to see approximately 10 VDC between the blue and yellow terminals — can confirm this scenario.
- ESYS module fault or software incompatibility Sometimes
The ESYS module acts as the communication bridge between the control board and the rest of the system. If the ESYS module firmware is incompatible with the installed control board software version, a CRC communication error can occur and surface as a code 21. Replacing the ESYS module with a compatible version resolves this.
- Flue or air pathway blockage Sometimes
A blocked flue or restricted air pathway can cause the fan to labour and ultimately fail to meet the speed demand placed on it. Birds' nests, debris, or damaged flue terminals are worth checking externally before the engineer strips down internal components.
- Flame detection lead or limit switch fault Rare
A damaged or disconnected flame detection lead between the sensor and the ignition module, or a limit switch that has lost continuity, can cause the control board to misinterpret system conditions and log a fan-related fault even when the fan itself is serviceable.
- Serious undiagnosed fault (Economaire models only) Rare
On older Economaire warm air units, code 21 does not map to a single identified component. The manufacturer's own guidance directs the attending engineer to phone Johnson & Starley for support whilst on site. If you have an Economaire unit displaying 21, assume the fault is complex and requires specialist diagnosis.
How to fix it
- Check that the gas supply to the property is active DIY safe
Confirm other gas appliances such as a gas hob or fire are working normally. If there is no gas supply to the property, contact your gas supplier. Do not proceed further until a gas supply is confirmed.
- Check for any obvious external flue blockage DIY safe
Look at the external flue terminal from outside the property. Check for visible debris, bird activity, or physical damage that could be restricting airflow. Do not touch or modify the flue — simply observe and report findings to the engineer.
- Reset the boiler once or twice as instructed in your user manual DIY safe
A single reset attempt is reasonable. Press and hold the reset button as described in your specific model's user guide. If the fault returns immediately or after a short period, do not keep resetting — repeated resets without a fix can mask the underlying problem and may cause further damage.
- Engineer to check ACF voltage at the fan terminals Gas Safe engineer
A Gas Safe registered engineer with warm air experience should measure the DC voltage between the blue (negative) and yellow (positive) terminals on the Air Circulation Fan. Approximately 10 VDC should be present when the fan is commanded to run. If voltage is present but the fan is not running at full speed, the ACF assembly requires replacement.
- Engineer to inspect flame detection lead, ignition module connection, and limit switch Gas Safe engineer
The attending engineer should check the condition and continuity of the flame detection lead running between the sensor and ignition module. The limit switch should also be tested for continuity. Faulty connections here can contribute to or mimic an ACF fault.
- Engineer to assess ESYS module compatibility Gas Safe engineer
If the ACF checks out as electrically sound, the engineer should verify whether the installed ESYS module software version is compatible with the control board. An incompatible ESYS module will need to be swapped for a compatible replacement. This is particularly relevant if the boiler has recently had any control components changed.
- On Economaire models, engineer to contact Johnson & Starley technical support Gas Safe engineer
If the boiler is an older Economaire warm air unit, the manufacturer's own guidance is that the attending engineer should telephone Johnson & Starley for technical assistance whilst on site. This is an unusual step that reflects the complexity of the fault on that platform.
- Call a Gas Safe registered engineer with warm air heating experience Gas Safe engineer
Johnson & Starley warm air systems are specialist appliances that not all Gas Safe engineers routinely work on. When booking, specifically ask for an engineer with warm air or ducted heating experience, or contact Johnson & Starley directly for a list of approved service agents. Attempting any internal repair without appropriate qualifications is unsafe and may invalidate any remaining warranty.
Parts you may need
- Air Circulation Fan (ACF) assembly · from £120
- ESYS module (compatible replacement) · from £95
- Limit switch · from £25
- Flame detection electrode and lead assembly · from £35
- Control board (PCB) · from £220
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 £200–£450, depending on the underlying cause.
Frequently asked questions
Can I fix Johnson & Starley fault code 21 myself?
No. Fault code 21 relates to the internal Air Circulation Fan system and, on older Economaire units, potentially a complex control fault requiring manufacturer guidance. Neither scenario is safe or practical for a homeowner to tackle. You will need a Gas Safe registered engineer — ideally one with specific warm air heating experience, as these systems are more specialised than conventional combi boilers.
Why does my Johnson & Starley warm air unit need a specialist engineer?
Johnson & Starley warm air systems distribute heat through a ducted air circuit rather than radiators and wet pipework. The components, fault-finding procedures, and service documentation are quite different from a standard gas boiler. Many general Gas Safe engineers rarely work on warm air units, so it is worth specifically requesting someone with warm air or ducted heating experience, or contacting Johnson & Starley's service line directly to find an approved engineer in your area.
How much does it cost to repair a Johnson & Starley fault code 21?
Most people with this fault pay somewhere between £200 and £450 once parts and labour are included. An Air Circulation Fan assembly and the labour to fit it typically sits in the lower part of that range, while an ESYS module replacement or more involved diagnostic work can push towards the upper end. If the fault turns out to involve the main PCB or heat exchanger — which is less common — costs can rise considerably higher, so it is worth asking the engineer for a written quote before authorising work.
What is the ESYS module on a Johnson & Starley warm air boiler?
The ESYS module is a communication and control component that links the main control board to other parts of the warm air system. If the firmware version on the ESYS module does not match the software on the control board — for example, after a component has been replaced — a CRC communication error can occur. This mismatch can trigger fault code 21 even if the Air Circulation Fan itself is physically undamaged. The fix is to replace the ESYS module with a version that is compatible with the installed control board.