Johnson & Starley 31 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 31 fault code mean?
Fault code 31, displayed as E31 on Johnson & Starley boilers (including the Reno HE range and WarmCair warm air units), indicates that the boiler's control system has detected a problem and shut the appliance down as a safety precaution. Based on technical documentation for the Reno HE series, E31 is primarily associated with a temperature sensor (NTC thermistor) fault — typically the boiler return temperature sensor — where the control board is receiving an abnormal or absent signal. On WarmCair warm air models, a code in this region can also point to a combustion fan fault or flue-related issue. In all cases the boiler locks out until the underlying fault is resolved. Note the exact fault shown on your display: some Johnson & Starley controls show the letter E followed by the number (e.g. E31), while others may display it differently — the number 31 is the key identifier to note before calling an engineer.
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
- Faulty or failed NTC return temperature sensor Common
The boiler return thermistor monitors the temperature of water coming back from the heating system. If the sensor fails, develops a short circuit, or drifts outside its expected resistance range, the control board interprets this as a fault and locks the boiler out. This is the most commonly identified cause of E31 on the Reno HE series.
- Wiring or connector fault to the temperature sensor Common
Even when the sensor itself is in good condition, a loose plug, corroded terminal, or damaged cable running between the sensor and the PCB can produce the same abnormal signal. Engineers check wiring continuity and inspect connectors carefully before condemning the sensor itself.
- Combustion fan fault (warm air models) Sometimes
On WarmCair and Economaire warm air appliances, a code 31 type lockout can be triggered by the combustion air fan (CAF) failing to run correctly. This may be caused by wiring that has lost continuity between the fan and the PCB, or by dirt and debris contaminating the venturi inside the fan assembly — a particularly common finding on older units.
- Flue blockage or restricted flue path Sometimes
A blocked or partially obstructed flue prevents combustion gases from clearing properly, which can cause the boiler to lock out on a safety code. Birds' nests, debris at the terminal, or a collapsed flue liner are typical culprits.
- PCB (printed circuit board) fault Rare
If the control board itself has failed or is misreading sensor inputs, it may generate a spurious E31 lockout even when sensors and wiring are intact. PCB faults are usually confirmed only after all sensors and wiring have been tested and found to be within specification.
- Heat exchanger deterioration Rare
A cracked or partially blocked heat exchanger can cause abnormal temperature readings and trigger thermal safety lockouts. Engineers inspect the heat exchanger visually by removing the burner and using a light source to check for cracks or sooting.
How to fix it
- Note the fault code and turn the boiler off DIY safe
Write down exactly what appears on the display — the letter and number combination — before doing anything else. Turn the boiler off at the programmer or room thermostat. This is useful information to pass on to your engineer and helps confirm you have fault code 31 rather than a different code.
- Attempt a single boiler reset DIY safe
Johnson & Starley documentation advises turning the temperature control to the RESET position and holding it there until the fault code clears. If the boiler fires up and runs normally, monitor it closely over the next hour. If E31 returns promptly, do not keep resetting — repeated resets on a genuine sensor or wiring fault will not fix the underlying problem and may mask important diagnostic information.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in the property (cooker, hob) are working normally. If there is no gas elsewhere, contact your gas supplier rather than an engineer — this is a supply issue, not a boiler fault.
- Engineer: inspect sensor wiring and measure thermistor resistance Gas Safe engineer
A Gas Safe registered engineer will disconnect the return temperature sensor plug and use a multimeter to check that the sensor's resistance matches the manufacturer's specification at the current water temperature. They will also check all wiring for continuity, inspect connectors for corrosion, and reseat any loose plugs. This is the primary diagnostic step for E31 on the Reno HE range.
- Engineer: inspect and test the combustion fan (warm air models) Gas Safe engineer
On WarmCair and Economaire units the engineer will verify the fan receives the correct voltage (230 Vac) from the PCB, check all wiring connections at both ends for continuity, and inspect the venturi for dirt or blockage. If voltage is present but the fan does not run, the fan motor is replaced. If the venturi is contaminated, the connecting tubes can sometimes be cleared with compressed air.
- Engineer: inspect flue terminal and flue run Gas Safe engineer
The engineer will check the flue terminal externally for obstruction, and where accessible, inspect the flue run for blockage, condensate pooling, or physical damage. Any obstruction must be cleared before the boiler is recommissioned.
- Engineer: replace faulty sensor or wiring harness if required Gas Safe engineer
If the thermistor is confirmed out of specification, the engineer will fit a like-for-like replacement sensor, ensuring the correct gaskets are used and all connections are remade securely. On warm air models a fan assembly replacement may be required if the motor has failed.
- Engineer: test PCB and replace if necessary Gas Safe engineer
Only after sensors, wiring, and mechanical components have been tested and found to be in order will the engineer consider the PCB as the fault source. PCB replacement on Johnson & Starley boilers is a specialist task requiring correct commissioning of the new board.
- Contact a Gas Safe registered engineer Gas Safe engineer
If your reset attempt has not resolved E31, or if the fault returns after a successful reset, you need a Gas Safe registered engineer. Johnson & Starley boilers — particularly the warm air range — require a technician familiar with the specific model. You can find registered engineers at the Gas Safe Register website (gassaferegister.co.uk). Always ask the engineer to confirm their Gas Safe registration number before work begins.
Parts you may need
- NTC return temperature thermistor (Reno HE compatible) · from £22
- Combustion air fan assembly (WarmCair/Economaire) · from £155
- PCB/control board (Reno HE series) · from £290
- Wiring harness/sensor loom · from £35
- Flue terminal cowl (replacement) · from £18
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Johnson & Starley fault code E31 myself?
No. While you can safely attempt a single reset and check that your gas supply is on, the diagnostic work required to resolve E31 — measuring sensor resistance, testing wiring continuity, inspecting the fan, and working near gas components — must be carried out by a Gas Safe registered engineer. Johnson & Starley's own documentation states that occupiers should note the fault code, switch off the appliance, and contact a competent person. Attempting DIY repairs on a gas boiler is both dangerous and a criminal offence under the Gas Safety (Installation and Use) Regulations 1998.
How much does it cost to fix E31 on a Johnson & Starley boiler?
For the most common repair — a faulty NTC temperature sensor and associated wiring — you are typically looking at £120 to £250 including parts and labour for a standard weekday call-out. If the combustion fan needs replacing on a warm air model, expect £250 to £380 all-in. A PCB replacement is more expensive, often £350 to £550 with labour, and is usually only needed when all other components have been ruled out. Major components such as a heat exchanger can cost significantly more and may make a full boiler replacement worth considering on older units.
My Johnson & Starley boiler is a WarmCair or Economaire — is E31 the same fault?
Not necessarily. On warm air models, codes in the 31 region are more commonly associated with combustion fan issues or flue problems rather than water-side temperature sensors. The most common finding on older Economaire units is contamination of the venturi inside the combustion air fan, which restricts airflow and causes a lockout. A warm air technician familiar with Johnson & Starley units is the best person to diagnose this, as the fault logic differs from conventional wet-system boilers.
Why does my Johnson & Starley keep showing E31 after I reset it?
A fault that returns immediately or within a short time after resetting indicates that the underlying cause has not been resolved. Common reasons include a thermistor that has failed outright, a persistent wiring fault, or — on warm air models — a fan that is running intermittently or a partially blocked flue. Repeatedly resetting the boiler without investigating the root cause can mask useful diagnostic information and, in a worst case, allow the boiler to operate in an unsafe condition. If E31 returns after one reset attempt, stop resetting and call a Gas Safe registered engineer.