Johnson & Starley 34 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 34 fault code mean?
Fault code 34 on a Johnson & Starley warm air unit indicates that the boiler's control electronics have detected an insufficient or unstable incoming mains electrical supply. As a safety precaution, the appliance shuts down rather than continuing to run on degraded power. Once a stable electrical supply is restored, the boiler should be able to resume normal operation — often after a reset. The fault points to something external (your property's electricity supply or the circuit feeding the boiler) or internal (wiring connections within the appliance itself).
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
- Temporary mains voltage dip or power interruption Common
A brief drop in your property's mains voltage — caused by grid fluctuations, a momentary supply interruption, or a tripped circuit — can be enough to trigger code 34. If power is fully restored and the fault clears on reset, this is the most likely explanation.
- Loose or deteriorated wiring at the fused spur or boiler isolator Common
The dedicated fused spur or isolator that supplies power to the boiler can develop loose terminals or corroded connections over time. This creates a voltage drop specifically at the boiler's supply point even when the rest of the property's electrics appear normal.
- Faulty internal wiring connections or wiring harness Sometimes
Wiring inside the appliance — including connections to the control board and other internal components — can loosen, chafe, or degrade, particularly on older units. Poor contact at any terminal can cause the control board to read a low-voltage condition.
- Failing consumer unit or upstream wiring fault Sometimes
A deteriorating consumer unit, a faulty breaker, or degraded cable between the consumer unit and the boiler circuit can cause persistent undervoltage at the appliance. This typically requires an electrician to investigate rather than a Gas Safe engineer.
- Control PCB fault Rare
In rarer cases, the control board itself misreads the incoming supply voltage due to an internal component failure. Because the PCB is supplied as a complete kit on Johnson & Starley units, the whole board must be replaced if this is confirmed.
How to fix it
- Check that your property's electricity supply is functioning normally DIY safe
Confirm other appliances are working and look at your consumer unit for any tripped breakers. If a breaker has tripped, reset it once. If it trips again immediately, do not keep resetting — contact a qualified electrician.
- Check the boiler's fused spur or isolator switch DIY safe
Locate the dedicated fused spur or double-pole isolator switch that supplies power to the boiler (usually on a nearby wall). Make sure it is switched on and that the fuse has not blown. A blown fuse can be replaced like-for-like by a competent person, but if it blows again, stop and call an engineer.
- Reset the boiler DIY safe
If the mains supply appears normal, press the boiler's reset button as described in your user guide. Attempt this no more than two or three times. If fault code 34 returns each time, further DIY action is not appropriate and you should move to the next step.
- Arrange for a Gas Safe registered engineer to inspect the boiler's wiring and supply Gas Safe engineer
A qualified engineer will use a multimeter to measure the actual voltage at the boiler's supply terminals, inspect the wiring from the isolator through to the control board, check all internal connections and the wiring harness, and identify any deterioration or loose terminals that are causing the undervoltage condition.
- Engineer to test or replace the control PCB if wiring checks out Gas Safe engineer
If the mains supply and all wiring are confirmed healthy but code 34 persists, the control board may be misreading the voltage due to an internal fault. On Johnson & Starley warm air units the PCB is supplied as a complete replacement kit. Your engineer will advise whether replacement is cost-effective given the age of the appliance.
Parts you may need
- Control PCB (complete replacement kit) · from £280
- Fused spur / double-pole isolator · from £20
- Internal wiring harness · from £55
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 £100–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Johnson & Starley fault code 34 myself?
The initial checks — confirming your electricity supply is on, looking for tripped breakers, and pressing the reset button — are safe for a homeowner to carry out. Beyond that, diagnosing and repairing wiring faults or replacing a control board on a warm air unit requires a Gas Safe registered engineer. Attempting electrical work on the boiler yourself risks both safety and invalidating any warranty or service plan.
Will the boiler restart automatically once power is restored?
Johnson & Starley warm air units are designed to return to a safe off-state during a supply failure. Once a stable mains supply is resumed, you will typically need to press the reset button to clear fault code 34 and allow the boiler to restart. If it does not restart normally after one or two resets, the underlying cause has not resolved itself and an engineer's visit is needed.
Could the problem be with my house electrics rather than the boiler itself?
Yes — a failing consumer unit, a deteriorating circuit breaker, or degraded cabling between your consumer unit and the boiler can all cause a persistent undervoltage at the appliance even when the rest of your home's sockets and lights seem fine. In that scenario, a registered electrician would need to inspect the circuit rather than (or in addition to) a Gas Safe engineer checking the boiler's internal wiring.
How much does it cost to replace the PCB on a Johnson & Starley warm air unit if needed?
PCB replacement, including parts and labour, typically falls in the £250–£400 range, though older or less common Johnson & Starley models may have discontinued or hard-to-source boards that push costs higher. The headline repair estimate on this page (£100–£380) reflects the more typical outcome where only wiring repairs or connection work is needed — your engineer should confirm the diagnosis before ordering a PCB.