Baxi H.01-.18 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi H.01-.18 fault code mean?
The H.01-.18 fault code on a Baxi boiler indicates that the boiler has detected absent or inadequate water circulation through the system, or a possible temperature sensor error. As a temporary (non-locking) fault, the boiler shuts down as a safety precaution and will not resume normal operation until the root cause is addressed and the fault is cleared. This code sits within the broader H.01 fault family, which covers circulation and temperature-related issues — H.01-.18 specifically flags a system-wide circulation problem rather than one confined to domestic hot water (DHW) mode alone.
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
- Seized or failing circulation pump Common
The circulation pump is responsible for pushing heated water around the central heating system. If it seizes, loses power, or the internal impeller breaks, water movement stops entirely and the boiler triggers the H.01-.18 fault. This is the single most frequent cause engineers find on site.
- Airlock in the system Common
Air trapped in the pipework or pump housing can prevent water from circulating effectively. This often follows radiator bleeding, a recent drain-down of the system, or a period of low system pressure. The boiler may fire briefly before the fault appears.
- Low system pressure Common
Baxi boilers operate correctly at between 1.0 and 2.0 bar. If pressure has dropped below 1.0 bar — through a small leak, a recently bled radiator, or a failing pressure relief valve — there may not be enough water in the circuit to circulate properly, triggering this fault.
- Faulty NTC flow or return temperature sensor Sometimes
NTC thermistors monitor water temperature at the flow and return points. A drifting or failed sensor can send incorrect readings to the PCB, causing the boiler to believe there is no circulation even when the pump is working normally.
- Blockage or sludge restricting flow Sometimes
Over time, magnetite sludge and scale can build up in the heat exchanger, pipework, or pump body, restricting water flow to the point where the boiler's internal checks flag it as insufficient circulation.
- PCB fault Rare
In rarer cases the printed circuit board itself may be misinterpreting sensor signals or failing to drive the pump correctly, producing a false H.01-.18 reading even when mechanical components are in good order.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button on the boiler's front panel for 1–3 seconds until the boiler restarts. A momentary glitch can occasionally trigger this code. If the fault clears and does not return within a normal heating cycle, monitor the boiler over the next day or two. Do not reset more than once or twice — repeated resetting without resolving the root cause can mask a developing problem.
- Check system pressure DIY safe
Look at the pressure gauge on the front of the boiler. The reading should sit between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, use the filling loop (usually located beneath the boiler) to top up slowly until the gauge reaches around 1.2–1.3 bar, then close the filling loop valves fully. If pressure drops again within a few days, there is likely a leak somewhere that needs an engineer to locate and repair.
- Bleed radiators to release trapped air DIY safe
Turn off the boiler and allow the system to cool. Starting with the radiators furthest from the boiler, use a radiator bleed key to open the bleed valve slightly until a steady stream of water (not air) emerges, then close it again. Work your way back towards the boiler. Once finished, recheck the system pressure and top up again if needed, as bleeding releases a small amount of water from the system.
- Check that all isolation valves on the pump and pipework are fully open DIY safe
Locate the pump — usually inside the boiler casing or on the flow pipework nearby — and confirm that any isolation valves on either side are turned to the fully open position (inline with the pipe). A partially closed valve is a surprisingly common cause of restricted flow after any recent maintenance work.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
If the steps above do not clear the fault, the most likely culprit is the circulation pump itself. A Gas Safe registered engineer can test pump operation, check its electrical supply, and confirm whether it needs freeing off (sometimes possible on older pumps) or replacing entirely. Pump replacement on a Baxi boiler typically takes 2–3 hours and includes pressure-testing the system afterwards.
- Have a Gas Safe engineer test the NTC temperature sensors Gas Safe engineer
An engineer will use a multimeter to check the resistance values of the flow and return NTC sensors against the manufacturer's specification. A sensor reading outside tolerance will need replacing — a straightforward job but one that involves accessing internal boiler components and should not be attempted by a homeowner.
- Have a Gas Safe engineer inspect for sludge or blockage and consider a power flush Gas Safe engineer
If the pump and sensors are found to be in good order, the engineer may recommend checking the heat exchanger and pipework for magnetite sludge. A power flush or chemical flush of the system can restore circulation and protect components from further damage. Fitting a magnetic filter (such as a Magnaclean) afterwards helps prevent recurrence.
- Have a Gas Safe engineer evaluate the PCB if no other cause is found Gas Safe engineer
As a last resort, the PCB may need testing or replacement. This is a specialist task requiring the correct replacement board for your specific Baxi model. An engineer will confirm whether a repair or full boiler replacement is the more cost-effective route given the boiler's age.
Parts you may need
- Circulation pump (Grundfos or Wilo — Baxi compatible) · from £95
- NTC flow temperature sensor · from £22
- NTC return temperature sensor · from £22
- PCB (model-specific) · from £220
- Magnetic system filter (e.g. Magnaclean Pro2) · from £65
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 the Baxi H.01-.18 fault myself?
There are a few safe checks a homeowner can carry out: resetting the boiler (once or twice at most), topping up the system pressure via the filling loop, and bleeding radiators to clear an airlock. These steps resolve the fault in a minority of cases. However, if the fault persists, the cause is almost certainly a failed pump, faulty sensor, or internal blockage — all of which require a Gas Safe registered engineer to diagnose and repair safely.
How much does it cost to fix a Baxi H.01-.18 fault?
Most people with this fault end up paying between £120 and £380 all-in, depending on what needs replacing. Bleeding radiators and topping up pressure costs nothing if you do it yourself. An NTC sensor swap will typically come to £100–£160 including labour. Circulation pump replacement is the most common paid repair, usually falling between £150 and £380 for parts and labour combined. In rarer cases where the PCB is at fault, costs can rise to £400 or more — if your boiler is older and facing a big-ticket repair like that, it may be worth comparing the repair cost against a new boiler installation.
Why does my Baxi boiler keep showing H.01-.18 after I reset it?
If the fault returns every time you reset, the boiler is consistently detecting the same underlying problem rather than a one-off glitch. The most likely culprits are a circulation pump that is on its way out, a persistent airlock that bleeding has not fully cleared, or a temperature sensor giving erratic readings. Continuing to reset without resolving the root cause will not fix the issue and could delay getting heat and hot water restored. At this point you should call a Gas Safe registered engineer.
Will my Baxi warranty cover the H.01-.18 fault?
Baxi offers a minimum two-year warranty on new boilers, extendable to up to ten years when installed by a Baxi-accredited engineer and serviced annually. If your boiler is within the warranty period and you have kept up with annual services, it is worth contacting Baxi directly before booking an independent engineer, as the repair may be covered at no cost to you. Skipping annual services can invalidate the extended warranty, so always keep service records.