Johnson & Starley 3 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 3 fault code mean?
Fault Code 3 on a Johnson & Starley warm air unit signals that the Air Pressure Switch (APS) has not detected the pressure differential it needs to allow the ignition sequence to begin. In the vast majority of cases, this points directly at the Combustion Air Fan (CAF) — either it has stopped working altogether, or it is running but not generating enough airflow to satisfy the APS threshold of around 0.48 mBar. When the boiler enters this fault state it shuts down safely and the LED display shows the number 3. On units fitted with the Intejan hot water circulator, both space heating and domestic hot water will be lost simultaneously, since both depend on the same combustion air fan.
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
- Combustion Air Fan (CAF) complete failure Common
The fan motor has seized, burned out, or suffered bearing failure. With no fan running, there is zero airflow, so the APS never sees any pressure differential and the boiler locks out immediately. This is the most common cause on the Economaire range. A completely failed fan is often identifiable by total silence from the unit, or by a grinding noise prior to final failure.
- Insufficient pressure output from the CAF Common
The fan is spinning but not developing enough pressure — the APS requires roughly 0.48 mBar differential to close and allow ignition to proceed. A worn motor, degraded impeller, or partially restricted airpath can all leave the fan running below this threshold without the fan appearing to have failed outright. A digital manometer is the most reliable way to confirm this.
- Dirty venturi or restricted air pressure tubing Sometimes
Dust and debris can accumulate inside the venturi within the CAF assembly, on the fan blades themselves, or in the small restrictor fitted in the positive air hose to the APS. Even partial blockages can drop the pressure differential enough to prevent the switch from closing. In some cases blowing through the connecting tubes clears the fault without any parts being replaced.
- Split or disconnected air pressure tubes Sometimes
The small-bore hoses connecting the CAF outlet to the APS can split, perish, or become dislodged over time. A leak in these tubes means pressure bleeds away before it reaches the switch, mimicking a fan failure. The official Johnson & Starley service guide specifically directs engineers to inspect these hoses as part of the fault-finding procedure.
- Incorrect flue length, blocked flue, or faulty APS Rare
An over-long flue run, a partially blocked flue terminal, or an incorrect orifice/venturi configuration can increase back-pressure and upset the air pressure balance. If all of the above checks pass, the APS itself may have drifted out of calibration or failed internally and may need replacement.
How to fix it
- Check that the gas supply and electricity supply to the unit are both on DIY safe
Make sure the gas meter isolation valve is open and that other gas appliances in the property are working normally. Also confirm the warm air unit is receiving mains power and that no fuse or circuit breaker has tripped. These are simple checks that rule out supply issues before an engineer visits.
- Attempt a single boiler reset DIY safe
Consult your user guide for the reset procedure for your specific model — on most Johnson & Starley units this involves pressing and holding the reset button until the fault code clears. If the boiler fires up and runs normally, monitor it closely. If Code 3 returns promptly, do not keep resetting; a persistent lockout needs professional diagnosis.
- Call a Gas Safe registered engineer with warm air experience Gas Safe engineer
Johnson & Starley warm air units are specialist appliances; not all Gas Safe engineers work on them regularly. Request an engineer familiar with warm air heating or Johnson & Starley products specifically. All further diagnostic and repair work must be carried out by a qualified professional.
- Engineer checks CAF operation and measures pressure differential Gas Safe engineer
Using a digital manometer, the engineer checks whether the fan is running and whether the pressure differential across the venturi meets the 0.48 mBar minimum required to close the APS. This reading immediately distinguishes between a failed fan, an underperforming fan, and a pressure path problem.
- Engineer inspects and cleans the venturi, fan blades, and air pressure tubes Gas Safe engineer
The engineer checks for dust on the fan blades, debris in the venturi, blockages in the restrictor on the positive air hose, and any splits or loose connections in the small-bore pressure tubes. In some cases cleaning these components restores sufficient airflow without any parts being replaced.
- Engineer inspects flue length, terminal, and internal orifice Gas Safe engineer
As directed in the official Johnson & Starley fault-finding guide (ZZ1317-2), the engineer verifies that the flue installation is within permitted length limits, the terminal is clear of obstruction, and the correct venturi orifice is fitted. On horizontal flue installations the protrusion distance is also checked.
- Replace the Combustion Air Fan assembly if required Gas Safe engineer
If the fan has failed or cannot meet the required pressure output after cleaning, it must be replaced. The standard Economaire fan assembly (part N310-0504005) should be used where available. For older or discontinued sizes where new units are no longer stocked, a refurbished unit with a new motor may be the only viable option.
- Replace or recalibrate the Air Pressure Switch if the fan proves sound Gas Safe engineer
If the fan is delivering adequate pressure but the boiler still locks out on Code 3, the APS itself is the likely culprit. The engineer replaces or recalibrates the switch and retests ignition before signing off the repair.
Parts you may need
- Combustion Air Fan (CAF) Assembly — Johnson & Starley Economaire (N310-0504005) · from £160
- Air Pressure Switch (APS) · from £45
- Air pressure hose / venturi tubing kit · from £15
- Fan motor (for refurbished fan rebuild on discontinued sizes) · from £80
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix a Johnson & Starley Fault Code 3 myself?
No. Unlike a gas boiler where a homeowner can safely top up the pressure or thaw a condensate pipe, warm air units have no equivalent DIY fix for Code 3. Every step beyond a single reset involves access to gas-carrying or electrically live components and must be performed by a Gas Safe registered engineer. On older Economaire models it is especially important to use an engineer familiar with warm air appliances.
Why has my Johnson & Starley lost both heating and hot water at the same time?
If your unit is fitted with the Intejan hot water system, the domestic hot water and the warm air heating share the same Combustion Air Fan. When that fan fails or underperforms — triggering Code 3 — neither function can operate. Restoring the fan restores both.
Can the Economaire fan be repaired rather than replaced?
Potentially yes. New fan assemblies for the most popular Economaire size (the 32) are no longer manufactured, which means a direct like-for-like replacement is not always available. An experienced engineer may be able to rebuild the existing fan housing with a new motor, which costs less than sourcing a scarce new-old-stock unit and avoids scrapping an otherwise sound boiler. Discuss this option with your engineer before committing to a full replacement.
How much does it cost to fix Code 3 on a Johnson & Starley warm air unit?
A diagnostic visit plus a clean of the venturi and pressure tubes — if that is all that is needed — typically costs £100–£180 including call-out. If the fan itself needs replacing, expect to pay roughly £225–£350 all-in for parts and labour, though on discontinued models where parts are scarce the cost can be higher. If only the Air Pressure Switch requires replacing, the total is usually in the £100–£160 range. Very large bills (above £400) are more likely on older or out-of-production models where refurbishment work is needed; in those cases it is worth asking your engineer whether continued repair still makes financial sense.