Worcester Bosch A8 362 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch A8 362 fault code mean?
The A8 362 fault code has two layers. The primary code — A8 — tells you the boiler's Energy Management System (EMS) has lost communication with its FX sensor controls. The sub-code 362 narrows this down further: it points specifically to a low or unstable mains voltage supply reaching the boiler, or to a Heat Control Module (HCM) that is invalid, incompatible, or internally faulty. When the boiler detects either of these problems it shuts down as a safety precaution and will not restart until the root cause is resolved and the fault is cleared. While other A8 sub-codes can indicate a broken FX sensor wiring connection or a defective FX sensor unit itself, the 362 variant is primarily an electrical supply and HCM issue.
General guidance only — not a substitute for professional advice. 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 or unstable mains voltage supply Common
The 362 sub-code specifically flags an issue with the incoming electrical voltage to the boiler. This could be caused by a faulty or undersized fused spur feeding the boiler, voltage fluctuations on the property's supply, or a wider issue with the incoming mains. Even brief dips in voltage can trigger the fault and cause the boiler to lock out.
- Faulty, incompatible, or poorly seated Heat Control Module (HCM) Common
The HCM is the module responsible for managing the boiler's heating functions. If the wrong HCM has been fitted during a previous service or installation, if the module has worked itself loose internally, or if the HCM has developed an internal fault, the boiler will flag the 362 code. An incorrectly specified module — even if it physically fits — can appear 'invalid' to the boiler's control system.
- Broken or loose FX sensor wiring (broader A8 family) Sometimes
Although 362 focuses on voltage and HCM issues, damaged, corroded, or disconnected wiring on the FX sensor circuit can cause the broader A8 communication fault. A break anywhere along the connection between the boiler and its FX sensor controls disrupts the data flow and may trigger an A8 code.
- Defective FX sensor unit Rare
If the wiring and HCM both check out, the FX sensor unit itself may have failed. A faulty sensor will prevent the EMS from receiving valid signals, keeping the A8 fault active. In some cases the code plug associated with the sensor may also be incorrect.
How to fix it
- Attempt a single boiler reset DIY safe
Press and hold the reset button on the boiler's control panel for approximately three seconds, then release. Wait two to three minutes to see whether the boiler fires up normally. A one-off reset is a perfectly reasonable first step — if the A8 362 code clears and the boiler runs without issue, monitor it over the next 24 hours. If the fault returns, do not keep resetting; repeated resets without fixing the underlying problem can mask a developing issue and cause further damage.
- Check that the boiler's electrical supply is switched on DIY safe
Make sure the fused spur or switched connection unit supplying the boiler is turned on and that the fuse has not blown. Check your consumer unit (fuse board) to confirm no relevant circuit breaker has tripped. This is a simple visual check only — do not attempt to open or work inside any electrical enclosures unless you are a qualified electrician.
- Call a Gas Safe registered engineer to inspect and test the mains voltage supply Gas Safe engineer
An engineer will use appropriate test equipment to verify that the voltage reaching the boiler is stable and within the manufacturer's specified range. If the supply is low or intermittent they will identify whether the problem lies with the boiler's own wiring, the fused spur, or a wider supply issue that may require an electrician.
- Have the engineer inspect, reseat, or replace the Heat Control Module (HCM) Gas Safe engineer
The engineer will check that the correct HCM specification is fitted for the boiler model and that it is properly seated and making good electrical contact. A loose module can often be reseated at minimal cost; an incompatible or failed HCM will need to be replaced with the correct part.
- Have the engineer test the FX sensor wiring and sensor unit Gas Safe engineer
If the voltage and HCM both prove to be in order, the engineer will trace and test the wiring between the boiler and the FX sensor controls, looking for breaks, corrosion, or damaged connectors. Any faulty wiring will be repaired or replaced. If the wiring is sound but the fault persists, the FX sensor unit or associated code plug will be tested and replaced as necessary.
Parts you may need
- Heat Control Module (HCM) · from £120
- FX sensor unit · from £55
- Fused spur / connection unit (13A) · from £18
- EMS BUS wiring harness · from £40
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 £150–£400, depending on the underlying cause.
Frequently asked questions
Is the Worcester Bosch A8 362 fault dangerous?
The boiler has locked itself out as a precaution, so it will not operate in an unsafe state. There is no immediate danger in the home, but you should not repeatedly reset the boiler hoping the fault will go away on its own. Arrange for a Gas Safe registered engineer to diagnose and resolve it promptly, particularly during cold weather when the boiler is essential.
Can I fix the A8 362 fault myself?
A single reset attempt and checking that the boiler's electrical supply is switched on are the only steps a homeowner should take. Everything beyond that — testing mains voltage, inspecting or replacing the HCM, and checking internal wiring — must be carried out by a qualified professional. Working inside the boiler casing without the appropriate qualifications is not only unsafe but also risks invalidating your warranty and potentially your home insurance.
How much does it cost to repair the A8 362 fault?
For the most common causes — a wiring repair, sensor replacement, or HCM reseating or replacement — most UK homeowners pay somewhere between £150 and £400 including parts and labour. A straightforward HCM reseat with a standard call-out will sit toward the lower end; a full HCM replacement with an emergency call-out in London or the South East can push toward the higher end. Repairs in London and the South East typically cost 20–35% more than in other regions due to higher labour rates. If diagnostics reveal a deeper PCB or electrical board fault, costs can rise to £500 or beyond, though this is less common with this specific code.
What is the difference between the A8 code and the A8 362 code on Worcester Bosch boilers?
A8 on its own (or with other sub-codes) is a broad EMS communication fault — it tells you the boiler has lost contact with its FX sensor controls, which can be caused by a faulty sensor, damaged wiring, or a loose connection. The three-digit sub-code 362 is much more specific: it points the engineer toward a low or interrupted mains voltage supply or an invalid Heat Control Module as the root cause. If your display is showing the full code A8 362, make sure to quote both parts to any engineer you contact, as it significantly narrows down the diagnosis.
Will an annual boiler service help prevent the A8 362 fault?
Yes, in many cases it can. During a service an engineer will check electrical connections, inspect the control modules, and pick up on any early signs of wiring deterioration or a module working loose. A service typically costs £80–£120 — often less than the call-out fee alone for an emergency breakdown visit — making it one of the most cost-effective ways to avoid unexpected faults like this one.