Alpha 0A15 Fault Code: Causes, Fixes & Repair Costs
What does the Alpha 0A15 fault code mean?
Fault code 0A15 appears on Alpha boilers — most commonly the Pro Tec Plus range — when the control board detects that one or both of the flow and return temperature sensors are reporting values it considers outside a trustworthy range. Rather than continue operating on potentially false data, the boiler locks out as a built-in safety measure. It is worth knowing where 0A15 sits within Alpha's 0A fault family: 0A15 flags an incorrect sensor value, 0A16 means the flow temperature is rising too slowly, and 0A17 means the return temperature is rising too slowly. All three point towards the sensing circuit and circulation, but 0A15 is specifically about the sensor reading being wrong rather than a temperature rise that is simply too sluggish.
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 flow or return thermistor Common
The thermistors are small temperature-sensing components clipped directly onto the flow and return pipework. Over time they can degrade, drift out of calibration, or fail outright, causing them to report temperatures the PCB judges as implausible. This is the single most frequent cause of 0A15.
- Loose or corroded sensor wiring Common
The wiring harness connecting the thermistors to the PCB can suffer from corrosion at the connector pins, loose terminals, or chafed cable insulation — particularly in older or damp installations. Any of these can introduce resistance that distorts the signal reaching the board.
- Poor circulation or air in the system Sometimes
If water is not flowing freely through the heat exchanger — whether due to a sluggish pump, sludge deposits, or an airlock — the temperature at the sensors can behave erratically. The PCB may then flag the reading as incorrect even though the sensor itself is fine.
- Limescale or sludge restricting the heat exchanger Sometimes
In hard-water areas or systems without adequate inhibitor, scale and magnetic sludge can accumulate inside the heat exchanger. This restricts flow and causes localised hot spots that produce temperature readings inconsistent with normal operation.
- PCB fault Rare
If all sensors, wiring, and circulation check out as healthy, the fault may lie with the PCB itself misinterpreting the sensor signals. PCB failures are less common but should be considered after all other causes have been ruled out by an engineer.
How to fix it
- Check your gas supply is working DIY safe
Before assuming a boiler fault, confirm other gas appliances in the property — such as a hob or gas fire — are working normally. If you have no gas supply at all, contact your gas network operator rather than a boiler engineer.
- Reset the boiler once or twice DIY safe
Alpha boilers typically have a reset button on the control panel. Turn the boiler off, wait around five minutes, then press and hold the reset button for approximately five seconds. If the fault clears and the boiler runs normally, monitor it over the next hour. If 0A15 returns, do not keep resetting — repeated lockouts indicate a genuine fault that needs a professional diagnosis.
- Check and correct the system pressure DIY safe
Low pressure can sometimes contribute to poor circulation and knock-on sensor anomalies. Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when cold. If it is below 1 bar, top it up using the filling loop following your boiler's user guide, then attempt a reset. Do not overfill beyond 2 bar.
- Have a Gas Safe engineer inspect the thermistor connections and wiring Gas Safe engineer
An engineer will open the boiler casing and check the wiring harness to both the flow and return sensors. They will look for corrosion, loose spade connectors, and any signs of heat damage to the cable. Securing a loose connector can sometimes resolve the fault immediately.
- Have the thermistors tested and replaced if necessary Gas Safe engineer
Using a multimeter, the engineer can measure the resistance of each thermistor at a known water temperature and compare it against the manufacturer's specification. A sensor reading outside tolerance should be replaced. On most Alpha models this is a straightforward component swap.
- Check circulation — pump, system pressure, and bleed the primary circuit Gas Safe engineer
The engineer will verify that the pump is running at the correct speed and delivering adequate flow. They will also check for airlocks in the primary heat exchanger and inspect the system for sludge or scale restriction. A powerflush may be recommended if contamination is significant.
- Have the PCB inspected or replaced Gas Safe engineer
If sensors and circulation are confirmed to be functioning correctly, the PCB is the next suspect. The engineer will check for signs of burning or component failure on the board. PCB replacement is a moderate repair and is typically only recommended once all cheaper possibilities have been eliminated.
- Call a Gas Safe registered engineer Gas Safe engineer
If you have not already done so, book a Gas Safe registered engineer to carry out a full fault-finding inspection. You can verify an engineer's registration at gassaferegister.co.uk. Do not attempt to remove the boiler casing or interfere with any internal components yourself.
Parts you may need
- Flow thermistor (NTC sensor) · from £18
- Return thermistor (NTC sensor) · from £18
- Wiring harness / sensor loom · from £35
- Central heating pump · from £75
- PCB (main control board) · from £180
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–£350, depending on the underlying cause.
Frequently asked questions
Can I reset an Alpha 0A15 fault myself?
Yes — a single reset attempt is fine and entirely safe for a homeowner to try. Turn the boiler off, wait five minutes, then hold the reset button for around five seconds. If the boiler runs without returning the code, keep an eye on it. However, if 0A15 comes back after one or two resets, you should stop resetting and call a Gas Safe engineer. Repeatedly resetting a boiler with a genuine sensor or circulation fault will not fix the underlying problem and could mask a worsening condition.
What is the difference between Alpha fault codes 0A15, 0A16, and 0A17?
All three belong to the same family of Alpha sensor anomaly codes. 0A15 means one or both temperature sensors are reporting a value the PCB considers incorrect — the reading itself is out of range. 0A16 means the flow temperature is rising too slowly, suggesting a circulation or combustion problem rather than a bad reading. 0A17 means the return temperature is rising too slowly, which similarly points to poor heat transfer or restricted flow. If your display shows 0A16 or 0A17, share that specific code with your engineer as it narrows down the diagnosis differently from 0A15.
How much does it cost to fix an Alpha 0A15 fault in the UK?
For the most common repair — replacing one or both thermistors — you can typically expect to pay between £120 and £250 including parts, labour, and VAT. If the fault turns out to be wiring or a loose connection, the bill may be at the lower end once the engineer's callout is included. A pump replacement, if needed, usually falls in the £200–£350 range. PCB replacement is a bigger job and can cost £300–£500 or more depending on parts availability for your specific Alpha model. If your boiler is over ten years old and requires a PCB, it is worth getting a new-boiler quote alongside the repair estimate.
Could a powerflush fix my Alpha 0A15 fault?
Possibly, if the root cause is sludge or scale restricting flow through the heat exchanger and causing erratic temperature readings. A powerflush clears magnetic iron oxide and debris from the entire central heating circuit and can restore normal circulation. However, if the thermistors or PCB are genuinely faulty, a powerflush alone will not resolve 0A15 — your engineer should diagnose the actual cause before recommending this service. A powerflush on a typical UK home typically costs £400–£600 and is most worthwhile when the system has not been cleaned or had inhibitor added for many years.