Remeha H.03.05 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha H.03.05 fault code mean?
H.03.05 is a temporary stop code on Remeha boilers. The 'H' prefix tells you the boiler has halted itself but has not permanently locked out — once the underlying problem is resolved, normal operation can resume. The '03' group points to the safety and control unit, and '.05' identifies a specific internal fault within that unit. In plain English, the boiler's main control PCB has detected an internal error within its safety circuit and has shut down as a precaution. This is distinct from a full lockout: the boiler is protecting itself while it waits for the fault to be cleared, rather than requiring a manual reset key or engineer override to unlock it. The most common root cause is a fault on the main PCB itself (the CU-GH control board) or a problem with the interconnection board and its CN1/CN2 connectors that link the safety circuit together.
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
- Faulty or failing main PCB (CU-GH control board) Common
The main control board continuously monitors every sensor, switch, and safety interlock in the boiler. If an internal component on the PCB degrades — a capacitor, microprocessor, or onboard relay — the safety unit detects the inconsistency and raises H.03.05. This is the most frequent underlying cause identified in Remeha's own service documentation for this code.
- Loose or damaged interconnection board / CN1 or CN2 connectors Common
The interconnection board bridges the main PCB to the rest of the safety circuit. Vibration over time can cause the CN1 or CN2 connectors to work loose, and corrosion or water ingress can degrade the contact pins. Even a partially seated connector can disrupt the internal communication the safety unit relies on, triggering this fault without the PCB itself being defective.
- Wiring harness damage between PCB and safety unit Sometimes
Chafed, pinched, or heat-damaged wiring within the boiler casing can introduce resistance or intermittent open circuits in the control signal paths. This can mimic an internal PCB fault, causing H.03.05 to appear — particularly on older installations or boilers that have had previous repairs.
- Transient electrical disturbance or voltage spike Rare
A momentary power surge, a brief drop in mains supply voltage, or static discharge during other electrical work in the property can occasionally cause the PCB's safety unit to latch into a fault state. In these cases a single reset clears the code and it does not return.
How to fix it
- Check the boiler display confirms H.03.05 DIY safe
Before doing anything else, note the exact code shown. On Remeha Tzerra Ace and Ace-Matic models the fault code is displayed on the front panel screen. Confirm it reads H.03.05 so you can rule out a similar code being misread.
- Attempt a boiler reset (once or twice only) DIY safe
On the Remeha Tzerra Ace-Matic, press and hold the reset button for approximately 2–3 seconds until the display acknowledges the reset. Allow the boiler a full minute to attempt restart. If the fault clears and the boiler fires successfully, monitor it over the next few hours. If H.03.05 returns, do not keep resetting — repeated resets will not fix an underlying PCB or wiring fault and could mask a developing problem.
- Check your mains power supply is stable DIY safe
Confirm no other appliances in the property suffered an outage or power surge around the time the fault appeared. If there was a recent power cut or storm, a one-off reset is reasonable. If power supply issues are ongoing, have a qualified electrician investigate before calling a boiler engineer.
- Do not attempt to open the boiler casing or touch internal components Gas Safe engineer
H.03.05 points directly to the PCB and interconnection board — both of which sit inside the sealed boiler casing and involve mains-voltage wiring. Removing the casing, touching the PCB, or attempting to reseat connectors yourself is unsafe and will invalidate your warranty. This work must only be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
A qualified engineer will remove the casing safely, inspect the CN1 and CN2 connectors and interconnection board for signs of corrosion, damage, or poor seating, and test the PCB. Remeha's own service guidance for H.03.05 directs engineers to check or replace the interconnection board first, then replace the main CU-GH PCB if the fault persists. Ask the engineer whether a specialist PCB repair service (which can be significantly cheaper than a new board and typically carries a 2-year guarantee) is appropriate for your boiler's age and condition.
Parts you may need
- Remeha CU-GH main control PCB · from £320
- Interconnection board (verbindingsprint) · from £85
- CN1 / CN2 connector harness · 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 £250–£600, depending on the underlying cause.
Frequently asked questions
Is H.03.05 a full lockout or will the boiler try to restart itself?
H.03.05 is a temporary stop, not a permanent lockout. Remeha uses the 'H' prefix specifically for faults where the boiler halts but remains capable of restarting once the cause is resolved. That said, if the PCB or interconnection board is genuinely faulty, the boiler will keep returning to this state — it will not self-heal. A single reset attempt is worthwhile, but if the code comes back you need an engineer rather than more resets.
How much does it cost to fix H.03.05 on a Remeha boiler in the UK?
Because this fault almost always involves the PCB or interconnection board, repair costs tend to be higher than average. Most homeowners pay somewhere between £250 and £600, which typically covers an engineer's diagnostic visit plus parts and labour. If the main CU-GH PCB needs replacing with a new unit, costs can push toward or beyond the top of that range. One way to reduce the bill is to ask about specialist PCB repair services — these refurbish the existing board rather than replacing it, often for significantly less, and reputable companies offer a 2-year guarantee on the repair. If your Remeha is more than 10–12 years old, it's worth getting a new boiler quote alongside the repair quote for comparison.
Can I fix H.03.05 myself, or do I need a Gas Safe engineer?
This fault requires a Gas Safe registered engineer. The repair involves working inside the boiler casing with mains-voltage components — specifically the main PCB and its connector boards. There is no homeowner action beyond a careful reset attempt (once or twice at most). Attempting to open the boiler yourself, remove the PCB, or reseat internal connectors is both dangerous and illegal without Gas Safe registration.
Why does my Remeha keep showing H.03.05 after I reset it?
If the code returns each time you reset, the transient-fault explanation (a one-off power blip) is unlikely. A persistently returning H.03.05 strongly suggests the PCB itself is developing a hardware fault, or the interconnection board or its CN1/CN2 connectors have a physical problem — corrosion, a cracked solder joint, or a loose connector that vibration keeps dislodging. Each reset is just clearing the symptom temporarily. You need an engineer to inspect and test the control board assembly properly.