Alpha 0A30 Fault Code: Causes, Fixes & Repair Costs
What does the Alpha 0A30 fault code mean?
The 0A30 code appears when the boiler's PCB detects that the flow temperature sensor (an NTC thermistor fitted to the flow pipe) has developed a short circuit. Instead of returning a resistance value that corresponds to a real water temperature, the sensor is reading as a dead short, so the boiler locks out to protect itself. This is a purely electrical fault on the sensing circuit — not a gas or combustion issue — but the boiler will not fire until it is resolved. It is worth knowing the adjacent sensor codes on Alpha boilers so you can confirm you are reading the right one: 0A15 indicates a flow/return sensor anomaly (an incorrect temperature value rather than a short), 0A30 is the flow sensor short circuit covered here, 0A31 means the flow sensor is reading a temperature outside its normal operating range, and 0A43 is a return sensor short circuit. Also note that the visually similar code 0A3 (without the trailing zero) relates to the overheat thermostat — a different component and a different fault entirely.
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
- Failed NTC flow temperature sensor Common
The thermistor itself has developed an internal short circuit, collapsing its resistance to near zero and triggering the fault. This is by far the most frequent finding — the sensor is a wear item exposed to constant heating and cooling cycles.
- Damaged or chafed sensor wiring Sometimes
The cable running from the flow sensor to the PCB can become pinched, abraded, or melted against the heat exchanger casing over time. A break in the insulation anywhere along the loom can create a short to earth that mimics a failed sensor.
- Moisture or corrosion at the connector Sometimes
Water ingress around the sensor plug — from a minor system leak or condensation — can bridge the terminals and produce a short-circuit resistance reading at the PCB.
- PCB fault or damaged sensor input circuit Rare
If the sensor and its wiring test as healthy, the input channel on the PCB that reads the flow sensor may itself be damaged, either by a previous short circuit event or by a manufacturing fault.
How to fix it
- Check whether any other fault codes are showing DIY safe
Before doing anything else, note the exact code on the display. Confirm it reads 0A30 and not the similar-looking 0A3 (overheat thermostat) or 0A31 (sensor out of range), as each has a different cause and fix. Take a photo of the display if helpful.
- Attempt a single boiler reset DIY safe
Press and hold the reset button for the duration described in your boiler's user guide (usually around 3 seconds). If the boiler fires and runs normally, monitor it over the next hour. If 0A30 returns, do not keep resetting — repeated resets without fixing the underlying fault can mask the problem or cause further damage.
- Do not remove the boiler casing or touch internal components Gas Safe engineer
All further diagnosis — checking wiring, testing sensor resistance with a multimeter, and replacing components — must be carried out by a Gas Safe registered engineer. Removing the casing yourself is unsafe and may invalidate any remaining warranty.
- Engineer to inspect the flow sensor wiring and connector Gas Safe engineer
The engineer will check the wiring loom from the flow sensor back to the PCB for signs of chafing, burning, moisture, or corrosion at the connector plug. A visual inspection often reveals the cause before any electrical testing is needed.
- Engineer to test the NTC flow sensor electrically Gas Safe engineer
Using a multimeter, the engineer measures the resistance of the flow NTC sensor at a known temperature and compares it against the manufacturer's specification. A short circuit will show resistance close to zero ohms. This confirms whether the sensor itself is at fault or the problem lies in the wiring or PCB.
- Engineer to replace the flow NTC sensor if confirmed faulty Gas Safe engineer
The flow sensor on Alpha boilers sits on the right-hand side of the heat exchanger. The engineer will isolate the boiler, drain down as necessary, unscrew the sensor, and disconnect its wiring. A new sensor must be fitted with a fresh sealing washer to prevent leaks on restart.
- Engineer to assess the PCB for secondary damage Gas Safe engineer
If the short circuit was present for an extended period before the boiler locked out, the PCB's sensor input circuit may have been damaged. The engineer will test the PCB and advise whether it needs repair or replacement before the boiler is safe to return to service.
- Reset and commission after repair Gas Safe engineer
Once parts have been replaced, the engineer will reset the boiler, check system pressure, run the boiler through a heating cycle, and verify that 0A30 does not return. They should also check for any related fault history on the boiler's diagnostics.
Parts you may need
- NTC flow temperature sensor (Alpha compatible) · from £20
- Sensor wiring loom / harness · from £35
- Main PCB (model-specific) · from £220
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–£250, depending on the underlying cause.
Frequently asked questions
Can I fix Alpha fault code 0A30 myself?
No. While you can safely attempt a single reset, all further work — checking wiring, testing the sensor with a multimeter, and replacing components — requires a Gas Safe registered engineer. It is a legal requirement in the UK that only Gas Safe registered engineers work on the internal components of a gas boiler.
How much does it cost to fix an Alpha 0A30 fault?
In most cases the repair involves replacing the flow NTC sensor, which is an inexpensive part (typically under £25). With a standard callout and an hour's labour you would normally expect to pay between £100 and £250 all in. If the PCB has also been damaged by the short circuit, a replacement PCB for an Alpha boiler can add £180–£300 to the bill, so it is worth getting a written quote before authorising any work.
What is the difference between Alpha fault codes 0A30 and 0A3?
They are two entirely separate faults. Code 0A30 (with a trailing zero) indicates a short circuit on the flow temperature NTC sensor. Code 0A3 (without the trailing zero) relates to the overheat thermostat — a different safety device that monitors the boiler against overheating. Always read the full code from the display carefully, as the two require different repairs.
My Alpha boiler is still under warranty — does 0A30 affect my claim?
Alpha boilers typically carry a manufacturer's warranty of 7–10 years depending on the model and registration. If your boiler is within the warranty period and you have had it serviced annually by a Gas Safe engineer as required, contact Alpha directly before booking an independent engineer. They should arrange for the fault to be assessed and repaired under warranty terms, potentially at no cost to you.