Johnson & Starley 25 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 25 fault code mean?
Fault code 25 on Johnson & Starley WarmCair condensing warm air heaters (C Series and DW Series) indicates that the control board has detected an electrical problem with one or both of the water temperature sensors — commonly called thermistors. On the DW variant, this typically involves either the flow thermistor or the return thermistor. When the board can no longer read a reliable temperature signal from these sensors, the heater shuts down and displays this blocking code until the underlying fault is resolved. Note that this code applies to the WarmCair range only — older J25-32 warm air heaters use a completely different diagnostic system, and the Reno/QuanTec wet boiler ranges do not use this code numbering.
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
- Condensate leak saturating the thermistor Common
On WarmCair heaters, condensate leaks inside the casing are a particularly frequent culprit. If condensate from the heat exchanger or drain system drips onto the thermistor, it corrodes the sensor and its connections, pushing the resistance reading outside the acceptable range. The condensate drainage restrictor — a small red plastic insert where the pipe meets the drain pump — should be inspected at every service, as blockages here can cause internal flooding that reaches the sensor.
- Failed or degraded thermistor Common
Temperature sensors degrade over time. If the thermistor's resistance value no longer matches the manufacturer's expected temperature/resistance curve when tested with a multimeter, the sensor has failed and needs to be replaced. Johnson & Starley list a 'Kit: Temperature Sensor' (part 1000-0526515) and a 'Temperature Sensor Clip On 15mm' (part 1000-0526500) for the DW Series II.
- Loose or damaged wiring connections Sometimes
The thermistor connects to the PCB via a low-voltage 9-way connector. Over time, vibration, heat cycling, or condensate ingress can cause pins to corrode or become intermittent. A loose or broken wiring harness can produce exactly the same fault code as a fully failed sensor, even when the sensor itself is fine.
- System not fully filled with water Sometimes
On the DW (combined air and water) variant, the heater must have its wet system completely filled, vented, and all isolation and radiator valves open before the thermistors will read correctly. Low water volume or an air-locked circuit can cause abnormal sensor readings that trigger a thermistor fault code.
- PCB (control board) failure Rare
If the thermistor resistance tests within specification and the wiring is intact, the PCB itself may be misinterpreting the sensor signal. The WarmCair Series II PCB (part 1000-0527900) is a relatively expensive component, and its replacement cost can make it worth comparing against the cost of a new appliance, particularly on older units.
How to fix it
- Check that the gas supply is on and system pressure is normal DIY safe
Make sure your gas meter isolation valve is open and other gas appliances in the home are working. On the DW (wet system) variant, confirm the system pressure gauge is reading within the normal operating range — typically 1 to 1.5 bar when cold. If pressure is low, top up via the filling loop following the guidance in your user manual.
- Ensure all system valves are open and the circuit is fully filled DIY safe
On the WarmCair DW variant, check that all isolation valves, radiator valves, and zone valves are fully open. If the wet circuit has not been bled recently, trapped air can cause erratic sensor readings. Bleed any radiators that feel cold at the top.
- Attempt a boiler reset — no more than two or three times DIY safe
Locate the reset function on the WarmCair control panel (refer to your user manual for the exact button or sequence for your model). Hold or press as instructed and allow the heater to go through its start-up sequence. If it locks out again quickly, do not keep resetting — repeated resets without fixing the root cause can mask developing faults and cause additional wear.
- Engineer to inspect the condensate drainage system and internal components for leaks Gas Safe engineer
A Gas Safe engineer should open the casing and check whether condensate is pooling inside the unit. The small red restrictor in the condensate drain pipe must be inspected for blockage. Any internal condensate leak must be repaired at this stage, because simply replacing the thermistor without fixing the leak will lead to repeat failures.
- Engineer to test the thermistor resistance values Gas Safe engineer
Using a multimeter, the engineer checks the thermistor resistance against the manufacturer's temperature/resistance specification. Both the sensor connector and the low-voltage 9-way connector at the PCB should be inspected for corrosion, damaged pins, and secure seating. Wiring continuity along the harness should also be confirmed.
- Engineer to replace the thermistor if found to be out of specification Gas Safe engineer
If the sensor reading is outside the acceptable range, the appropriate replacement temperature sensor kit is fitted. Given that condensate damage is common on this range, the engineer should also clean and dry the surrounding area and address any ongoing moisture ingress before closing up the unit.
- Engineer to assess and replace the PCB if thermistor and wiring are both serviceable Gas Safe engineer
If the sensor and wiring test correctly but the fault code persists, the control board is likely misreading the signal. The engineer will advise on whether a PCB replacement is cost-effective given the age and condition of the appliance — on older WarmCair units, the labour and parts cost may prompt a conversation about replacement instead.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
Fault code 25 cannot be resolved by a homeowner beyond the basic checks above. All internal inspection, wiring testing, sensor replacement, and PCB work must be carried out by a Gas Safe registered engineer. Under the Gas Safety (Installation and Use) Regulations, it is a legal requirement that gas appliance repairs are performed by a competent, registered person.
Parts you may need
- Temperature Sensor Kit (part 1000-0526515) · from £35
- Temperature Sensor Clip-On 15mm (part 1000-0526500) · from £20
- PCB: WarmCair Series II (part 1000-0527900) · from £280
- Condensate drain restrictor (red plastic insert) · from £8
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–£400, depending on the underlying cause.
Frequently asked questions
Can I fix Johnson & Starley fault code 25 myself?
No. Beyond checking your gas supply is on, topping up system pressure if it is low, and attempting a reset once or twice, everything else requires a Gas Safe registered engineer. Thermistor testing, wiring inspection, and any internal work on the heater are not legally or safely something a homeowner should attempt on a gas appliance.
Why does condensate keep damaging the thermistor on my WarmCair?
WarmCair heaters produce condensate as part of normal operation, and this acidic liquid is meant to drain away through a dedicated pipe and pump. A common weak point is the small red plastic restrictor inside the condensate drain pipe where it enters the pump — if this becomes blocked, condensate can back up and pool inside the casing, dripping onto nearby components including the thermistor. Ask your engineer to inspect and clear this restrictor at every annual service, as it is often overlooked.
How much does it cost to fix fault code 25 on a Johnson & Starley WarmCair?
If the fault is a straightforward thermistor replacement — which is the most common outcome — the total bill including call-out, labour, and parts is typically in the region of £150 to £250. If wiring repairs are also needed, expect the upper end of that range. If the PCB turns out to be at fault, the board alone can cost around £280 in parts, pushing the total repair bill to £400 or more. At that point, it is worth asking your engineer to assess whether the appliance is worth repairing given its age.
Are spare parts easy to get for Johnson & Starley WarmCair heaters?
Parts availability can be variable. Johnson & Starley do maintain a spare parts programme and sensors are generally listed components, but the WarmCair is a specialist warm air appliance and not all heating engineers stock parts for it. It is worth confirming with your engineer that the required part is available before they attend, to avoid a delay if the component needs to be ordered from a specialist supplier.