Worcester Bosch H07 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch H07 fault code mean?
H07 is a maintenance/blocking code — identified by its 'H' prefix — that appears on Worcester Bosch Greenstar boilers to flag a condition that needs attention but may not immediately shut the boiler down. The most common interpretation across the range is low system water pressure, though on certain models H07 can instead point to a circulation pump problem, a software or range-rating configuration fault, or (less frequently) low gas pressure. Because the code carries different meanings depending on the exact model, it is worth confirming which applies to yours before taking action. In all cases the boiler may continue to run intermittently or at reduced efficiency until the underlying 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
- Low system water pressure Common
The most frequent trigger for H07 across the Greenstar range. Pressure drops gradually through normal operation, small air bleeds, or minor seeping leaks at joints and radiator valves. If the gauge reads below 1 bar — or is in the red zone — this is almost certainly your cause.
- Faulty or waterlogged expansion vessel Common
An expansion vessel that has lost its pre-charge pressure or developed a ruptured membrane causes the system to shed pressure repeatedly. You may top the boiler up successfully only to find it loses pressure again within days or weeks.
- Circulation pump fault or blockage Sometimes
On some Greenstar models H07 is linked to pump performance rather than pressure. A pump that is seized, blocked with debris, or running too slowly will not circulate water effectively. You may notice radiators failing to heat evenly or hear no gentle hum from the pump when the heating is running.
- Small leak in the heating system Sometimes
A weeping joint, corroded radiator, or deteriorating flexible hose can bleed pressure slowly. The leak may be too small to create an obvious puddle but enough to bring the system below the acceptable threshold over time.
- Software or range-rating configuration fault (code plug / KIM) Sometimes
Particularly on Greenstar i and CDi Compact models, H07 can signal that the boiler identification module (KIM) or code plug does not match the boiler's rating — for example after a PCB replacement where the wrong code plug was fitted. The control board cannot verify the correct settings and raises the fault.
- Low gas supply pressure Rare
Less commonly, H07 is associated with insufficient gas pressure reaching the boiler — most likely in very cold weather when the meter regulator can freeze, or if there is a wider supply issue. If other gas appliances in the home (such as a hob) are also misbehaving, this is the likely culprit.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's fascia or pipework. A healthy reading is between 1 and 1.5 bar when the system is cold. If the needle is below 1 bar or sitting in a red zone, low pressure is almost certainly your H07 trigger.
- Top up the system pressure using the filling loop DIY safe
Turn the boiler off and allow it to cool. Locate the filling loop — usually a braided or plastic hose with one or two small valves, found beneath or beside the boiler. For an internal (keyway) loop, turn the key slowly anticlockwise. For an external loop, open both quarter-turn valves. Watch the gauge rise and stop at 1.0–1.5 bar (the green zone), then close the valves fully. Do not overfill beyond 1.5 bar.
- Reset the boiler and monitor DIY safe
Once pressure is restored, press and hold the reset button as described in your user manual. The boiler should fire up normally. Watch the pressure gauge over the following few days — if it holds steady, a one-off air loss or minor top-up was all that was needed. Limit yourself to two or three resets; if the fault returns quickly, stop resetting and move to the next steps.
- Check whether other gas appliances are working DIY safe
If you suspect a gas supply issue rather than water pressure, try a gas hob or other gas appliance. If nothing is getting gas, the problem lies upstream of the boiler — contact your gas supplier or National Gas Emergency (0800 111 999) immediately. Do not attempt to work on the gas meter or regulator yourself.
- Bleed the radiators and check all radiator valves are open DIY safe
Trapped air can contribute to poor circulation and pressure loss. Use a radiator bleed key to release any air from each radiator, starting with the ground floor. After bleeding, re-check the boiler pressure and top up again if needed. Confirm all thermostatic and lockshield valves are open so the pump can circulate water freely.
- Listen for the circulation pump DIY safe
With the heating running, place your hand or ear near the pump housing (usually a cylindrical unit on the pipework close to the boiler). You should detect a gentle hum and slight warmth. Silence or grinding noises suggest the pump may be seized or failing — note this and report it to an engineer.
- Check whether the pressure keeps dropping after topping up DIY safe
If the system loses pressure again within a few days, inspect visible pipework, radiator connections, and the area beneath the boiler for signs of moisture or drips. Even a small damp patch is significant. Do not keep topping up indefinitely — recurring pressure loss means there is a leak or a failed expansion vessel that needs professional attention.
- Call a Gas Safe registered engineer for diagnosis and repair Gas Safe engineer
If the fault persists after repressurising, if pressure keeps dropping, if the pump sounds abnormal, or if you suspect a range-rating or PCB configuration issue, a Gas Safe registered engineer must inspect the boiler. Internal components — including the gas valve, pump replacement, expansion vessel, PCB, and code plug — must only be worked on by a qualified professional. You can verify any engineer on the Gas Safe Register at gassaferegister.co.uk.
Parts you may need
- Expansion vessel (Greenstar compatible) · from £60
- Circulation pump (Worcester Bosch Greenstar) · from £95
- Filling loop assembly (internal keyway type) · from £18
- Pressure relief valve · from £22
- Boiler code plug / KIM module · 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 Worcester Bosch H07 myself by topping up the pressure?
If your H07 is caused by low water pressure — which is the most common cause — then yes, topping up via the filling loop is a straightforward homeowner task and costs nothing. Bring the gauge up to 1.0–1.5 bar, reset the boiler, and see if the fault clears. However, if the pressure drops again within a few days, there is an underlying issue (a leak, a failing expansion vessel, or a pump fault) that a Gas Safe engineer needs to diagnose. Never keep topping up repeatedly without finding out why the pressure is falling.
Why does my Worcester Bosch keep showing H07 even after I've repressurised it?
Recurring H07 after topping up usually points to a slow leak somewhere in the system or a waterlogged expansion vessel that can no longer absorb pressure fluctuations properly. Less commonly, it can mean the circulation pump is underperforming and creating irregular pressure readings. A Gas Safe engineer can pressure-test the system to find leaks and check the expansion vessel's pre-charge. Expansion vessel replacement typically costs £130–£300 including labour; leak repairs vary from around £100 for a minor seal to more for hidden pipework issues.
My boiler is a Greenstar i or CDi — could H07 mean a software fault rather than pressure?
Yes. On Greenstar i and CDi Compact models in particular, H07 can be triggered by a range-rating or configuration fault linked to the boiler identification module (KIM) or code plug. This sometimes happens after a PCB has been replaced with an incorrectly matched code plug, or if the module itself has developed a fault. This is not a DIY repair — an engineer will need to check the code plug specification against the boiler model and replace it if necessary. The part itself is relatively inexpensive (around £30–£50), but correct diagnosis is essential.
Is H07 dangerous? Should I stop using the boiler?
H07 is classed as a maintenance or blocking code rather than a full safety lockout, so the boiler will often continue to run. However, you should not ignore it. Low pressure reduces heating efficiency and can eventually cause the boiler to shut down completely. A suspected gas supply issue is more serious — if other gas appliances in your home are also not working, treat it as an emergency and call National Gas Emergency on 0800 111 999. For all other H07 causes, arrange for an engineer to inspect the boiler promptly rather than leaving it running indefinitely without a fix.