Alpha 0A31 Fault Code: Causes, Fixes & Repair Costs
What does the Alpha 0A31 fault code mean?
The 0A31 fault code appears on Alpha Pro Tec Plus boilers when the flow temperature sensor (thermistor) is reading a value outside the acceptable operating range. This tells the boiler's control system that the outgoing water temperature is either unusually high, implausibly low, or simply inconsistent — so the boiler locks out as a safety precaution. The fault is closely related to overheating conditions: if you also see a flashing red light on the 'A' indicator, or codes 2, 25, or 0A3 alongside it, overheating is the likely trigger. The root cause could be the sensor itself, a circulation problem starving the heat exchanger of water flow, or — less commonly — the PCB misreading the sensor signal.
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
- Poor water circulation (air locks, sludge, or seized pump) Common
If water isn't circulating properly around the system, heat builds up rapidly in the heat exchanger and the flow sensor registers an out-of-range temperature. Trapped air, a pump that has seized over summer, or sludge deposits narrowing the pipework are all frequent culprits — especially in older systems without a magnetic filter.
- Faulty or drifting flow thermistor Common
The NTC thermistor that monitors the flow temperature can develop an open circuit, a short, or simply drift out of calibration over time. When the resistance value it returns no longer matches the expected curve, the boiler logs 0A31 even if the actual water temperature is perfectly normal.
- Overheat thermostat fault or tripped safety cut-out Sometimes
The Alpha Pro Tec Plus has an overheat thermostat that acts as a secondary safety device. If it has tripped — or its connections have become loose or corroded — the boiler may log 0A31 alongside overheating-related codes. A tripped thermostat often means an underlying circulation issue has already caused a heat event.
- Flue or combustion restriction Sometimes
A partially blocked flue, incorrectly pitched flue run, or restricted air intake can cause incomplete combustion and excessive heat build-up. The flow temperature then spikes, pushing the sensor out of its normal range.
- PCB fault Rare
The printed circuit board processes the signal from the flow sensor. If the PCB develops a fault on the relevant input channel, it may flag 0A31 even when the sensor and pipework are both healthy. PCB failure is the least common cause but should be considered once all other possibilities have been ruled out.
- Loose or damaged sensor wiring Sometimes
Vibration, heat cycling, or rodent damage can cause the sensor wiring loom to develop an intermittent connection. This produces erratic resistance readings that the boiler interprets as an out-of-range flow temperature.
How to fix it
- Check that the gas supply is turned on DIY safe
Confirm the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Check that other gas appliances in the property are working normally to rule out a supply interruption.
- Check the system pressure DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top it up via the filling loop — turn the valves slowly until the gauge reaches 1.2 bar, then close them fully. Low pressure can contribute to poor circulation.
- Bleed any radiators that feel cold at the top DIY safe
Trapped air reduces water flow through the system. Use a radiator key to open each bleed valve until water dribbles out, then close it. Re-check the boiler pressure afterwards and top up again if it has dropped below 1 bar.
- Reset the boiler — once or twice only DIY safe
Turn the boiler off and wait five minutes to allow it to cool. Then press and hold the reset button for approximately five seconds until the display changes. If the fault clears and the boiler runs normally, monitor it over the next few hours. If 0A31 returns, do not keep resetting — repeated resets on an overheating fault can cause further damage.
- Engineer: test the flow thermistor resistance Gas Safe engineer
A Gas Safe engineer will disconnect the flow sensor connector and measure its resistance with a multimeter at a known temperature, comparing the reading against the manufacturer's NTC resistance table. A reading that is open circuit, shorted, or significantly off the expected value confirms a faulty thermistor requiring replacement.
- Engineer: inspect the pump and check for circulation restrictions Gas Safe engineer
The engineer will verify that the pump is running at the correct speed and generating adequate flow. They will also check for sludge build-up in the heat exchanger and pipework, and inspect system valves to ensure none are partially closed. A powerflush may be recommended if heavy sludge is found.
- Engineer: inspect the overheat thermostat and its wiring Gas Safe engineer
The overheat thermostat and its associated wiring harness will be checked for continuity, correct seating, and signs of heat damage or corrosion. A faulty or tripped thermostat will be reset or replaced as appropriate.
- Engineer: inspect the flue run for blockages or incorrect installation Gas Safe engineer
The flue terminal, condensate trap, and the full flue run will be checked for obstructions (bird nests, debris, ice in cold weather) and for correct fall and joint integrity. Any restrictions will be cleared or the flue re-routed if necessary.
- Engineer: replace the flow sensor or PCB if all other checks pass Gas Safe engineer
If the thermistor tests faulty, it will be replaced with a genuine Alpha part. If the sensor, pump, thermostat, and flue all check out correctly, the PCB input circuitry will be investigated and the PCB replaced if a fault is confirmed.
- Call a Gas Safe registered engineer if the fault persists after a reset DIY safe
If the 0A31 code returns after one or two resets, or if you are unsure about any of the homeowner checks above, contact a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Do not attempt to open the boiler casing or work on internal components yourself.
Parts you may need
- Alpha Pro Tec Plus flow thermistor (NTC temperature sensor) · from £25
- Circulating pump (compatible replacement) · from £90
- Overheat thermostat / high-limit sensor · from £20
- Alpha Pro Tec Plus PCB · from £180
- Sensor wiring loom / harness · from £35
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 Alpha fault code 0A31 myself?
The only safe homeowner actions are checking the gas supply is on, topping up system pressure to around 1.2 bar via the filling loop, bleeding radiators to release trapped air, and attempting one or two resets. All internal work — testing or replacing the thermistor, inspecting the pump, or touching any wiring — must be carried out by a Gas Safe registered engineer. Opening the boiler casing yourself is both dangerous and potentially illegal unless you hold the relevant Gas Safe qualification.
Is 0A31 the same as the Alpha E1 error code?
They are closely related but not identical. The E1 code on Alpha boilers is a general thermistor or temperature sensor fault, whereas 0A31 specifically indicates the flow sensor reading is outside the accepted range. Both point toward the flow thermistor or a circulation/overheating problem as the likely cause, and both are resolved by the same diagnostic process. If your display shows 0A31 alongside a flashing 'A' indicator or codes 2 or 25, overheating due to poor circulation is a strong possibility.
How much does it typically cost to fix fault code 0A31 on an Alpha Pro Tec Plus?
Most people with this fault pay somewhere between £120 and £380 including parts, labour, and VAT. A straightforward thermistor replacement at the lower end costs roughly £120–£200. Pump replacement sits around £250–£350. If the PCB turns out to be the cause, expect £280–£380 for the part and fitting. Heat exchanger work is a less common outcome and can cost considerably more — in that situation it is worth getting a quote for a new boiler alongside the repair estimate, particularly if the boiler is over ten years old. Rates in London and the South East tend to be higher than the rest of the UK.
Why does 0A31 keep coming back after I reset it?
A fault that clears on reset but returns within hours or days almost always means the underlying problem — poor circulation, a drifting thermistor, or intermittent wiring — has not been addressed. Repeatedly resetting an overheating-related fault risks damaging the heat exchanger or other components. Book a Gas Safe engineer to carry out a proper diagnosis rather than continuing to reset the boiler.