Alpha 0A37 Fault Code: Causes, Fixes & Repair Costs
What does the Alpha 0A37 fault code mean?
The 0A37 fault code on Alpha boilers signals that the central heating system pressure has fallen too low for the boiler to operate safely — typically below 0.6 bar. The boiler locks out as a protective measure until adequate pressure is restored. You may also see this fault referred to alongside the Alpha E10 code; both point to the same underlying condition (insufficient water pressure), with 0A37 being the numeric display version shown on certain digital-display Alpha models. Once pressure drops below the safe threshold, the boiler will not fire until the issue is resolved.
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
- Pressure naturally dropped over time or after bleeding radiators Common
System pressure gradually falls through normal use, and bleeding radiators deliberately releases water and air, which lowers the pressure reading. This is the most common trigger for 0A37 and is usually straightforward to resolve by repressurising via the filling loop.
- Water leak somewhere in the heating system Common
A slow leak from a radiator valve, pipe joint, or boiler component allows water to escape, causing pressure to keep dropping. The leak may be visible (damp patches, corrosion staining) or hidden behind a wall or under floorboards. If you find yourself topping up the pressure repeatedly, a leak is the most likely culprit.
- Faulty or weeping pressure relief valve (PRV) Sometimes
The PRV is designed to release water if pressure gets dangerously high. Over time the valve can fail or become stuck partially open, allowing water to slowly discharge through the overflow pipe outside. You may notice a dripping pipe on an external wall near the boiler.
- Faulty or waterlogged expansion vessel Sometimes
The expansion vessel absorbs the increase in water volume as the system heats up. If the vessel loses its air charge or the internal diaphragm fails, the system can shed pressure repeatedly even without an obvious external leak.
- Filling loop left partially open or damaged Rare
If the filling loop valve is not fully closed after repressurising, water can slowly back-siphon out of the system. A damaged or corroded filling loop connector can also cause slow pressure loss.
How to fix it
- Check the pressure gauge on your boiler DIY safe
Locate the pressure gauge — usually a dial or digital display on the boiler front panel. A cold system should read between 1 and 1.5 bar. A reading below 1 bar (and especially below 0.6 bar) confirms pressure is the cause of the 0A37 fault.
- Repressurise the system using the filling loop DIY safe
Find the filling loop — typically a silver or grey flexible braided hose with one or two valves, located beneath the boiler. Slowly open the valve(s) and watch the pressure gauge rise. Stop when the needle reaches around 1 to 1.2 bar on a cold system. Close both valves fully before doing anything else. Do not overfill past 1.5 bar.
- Reset the boiler DIY safe
Once pressure is restored, press and hold the reset button (refer to your Alpha boiler manual for the exact location — it is usually a dedicated button or a function accessed via the display). Allow the boiler up to 5 minutes to complete its start-up sequence. Repeat the reset no more than 2–3 times; persistent failure to restart suggests an additional fault.
- Check for obvious water leaks around the system DIY safe
Inspect radiator valves, visible pipework under the boiler, and any external overflow pipe on an outside wall. Damp patches, rust staining, or a dripping pipe are all signs of a leak that will need professional attention. Do not attempt to repair gas pipework or internal boiler components yourself.
- Monitor the pressure over the next 24–48 hours DIY safe
If the pressure holds steady after repressurising, the fault was likely a one-off drop from bleeding or normal use. If the pressure falls again within a day or two, there is an underlying problem — a leak, faulty PRV, or failing expansion vessel — that requires a Gas Safe engineer to diagnose.
- Call a Gas Safe registered engineer if pressure keeps dropping or you cannot identify the cause Gas Safe engineer
Repeated pressure loss, a dripping external overflow pipe, or visible leaks all require a qualified engineer. They can pressure-test the system, inspect the expansion vessel, replace a faulty PRV, and trace hidden leaks. Do not remove the boiler casing or attempt to work on internal components yourself — this is illegal without Gas Safe registration and is dangerous.
Parts you may need
- Pressure relief valve (PRV) · from £25
- Expansion vessel (replacement) · from £55
- Filling loop assembly · from £20
- Radiator valve (replacement pair) · from £15
- Expansion vessel Schrader valve / air charge kit · 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 £100–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Alpha fault code 0A37 myself?
In many cases, yes. If the pressure has simply dropped due to bleeding radiators or gradual loss over time, repressurising the system via the filling loop (as described in the steps above) is a safe DIY task and costs nothing. However, if the pressure drops again within a day or two, the underlying cause — such as a leak or a failed expansion vessel — needs a Gas Safe registered engineer. You must not remove the boiler casing or touch internal components.
What pressure should my Alpha boiler be at?
When the system is cold, the pressure gauge should read between 1 and 1.5 bar. During normal operation (when the heating is running and the water is hot) it is normal for the pressure to rise slightly, up to around 2 bar. If you consistently see readings below 1 bar when cold, or above 2.5 bar when hot, the system needs attention.
Why does my Alpha boiler keep losing pressure after I top it up?
If you find yourself repressurising every few days or weeks, something is causing the water to escape. The most common culprits are a small leak on a radiator valve or pipe joint, a weeping pressure relief valve (look for a dripping pipe on an external wall), or a waterlogged expansion vessel. A Gas Safe engineer can carry out a system pressure test to locate the source. In rare cases the repair can extend to replacing a heat exchanger, which can cost £400–£600 or more, but most pressure-loss faults are resolved for £150–£350.
What is the difference between Alpha fault codes 0A37 and E10?
Both codes mean the same thing — the system pressure has dropped too low for the boiler to operate safely. The E10 code tends to appear on older Alpha models with simpler LED or basic digital displays, while 0A37 is the full numeric code displayed on certain newer digital-display Alpha models. The diagnosis and fix are identical for both.