Remeha H.02.05 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha H.02.05 fault code mean?
The H prefix in Remeha's fault-code system identifies a blocking fault — the boiler shuts down and will not operate until the underlying problem is resolved. H.02.05 specifically signals that the boiler's main control board (PCB) is unable to recognise or communicate correctly with the configuration storage unit (CSU), the small memory component that holds the boiler's identity settings and operating parameters. Because the PCB and CSU are effectively out of sync, the boiler locks out as a safety measure. Resolving this fault requires a Gas Safe registered engineer to reconfigure the CN1/CN2 boiler-identity settings, replace the CSU, or replace the PCB itself — there are no homeowner-accessible fixes for this code.
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
- Incorrect CN1/CN2 configuration settings Common
The CN1 and CN2 settings are internal boiler-identity parameters that tell the PCB which type of unit it is controlling. If these are set incorrectly — most commonly following a PCB swap or service intervention — the board will fail to match the stored configuration and trigger H.02.05. Resetting these settings is listed as the first-line fix in Remeha's own technical guidance.
- Incompatible or mismatched CSU fitted after a component replacement Common
The configuration storage unit must be compatible with the PCB it is paired with. If a replacement PCB or CSU was sourced without verifying compatibility, the mismatch will cause this blocking fault. This is a frequent pitfall when non-OEM or incorrect-revision parts are used.
- Faulty or failed CSU (configuration storage unit) Sometimes
The CSU can fail on its own — losing stored data or developing a communication fault — even if the PCB is healthy. A failed CSU will appear to the PCB as an incompatible or missing memory component, producing the same H.02.05 code.
- Damaged or failing PCB Sometimes
Wear, age, or physical damage to the main control board can prevent it from reading the CSU correctly. A common trigger for PCB damage is moisture ingress from a slow internal leak — even minor drips from a pump seal or heat exchanger can cause corrosion on the board over time.
- Moisture or water damage to internal electronics Rare
A small leak elsewhere inside the boiler — from the pump, heat exchanger, or pipework — can allow condensation or drips to reach the PCB or CSU connectors. Water contamination disrupts the communication between components and can cause this configuration mismatch fault.
How to fix it
- Do not attempt to open the boiler casing or touch any internal components DIY safe
H.02.05 is an electronic configuration fault that has no homeowner-accessible fix. Opening the boiler casing or handling the PCB without Gas Safe registration is unsafe and may void any remaining warranty. Do not attempt a boiler reset repeatedly — it will not clear this fault.
- Check that your gas supply is working DIY safe
Before calling an engineer, confirm that other gas appliances in your home (hob, fire) are working normally. If you have no gas supply, contact your gas supplier rather than a boiler engineer.
- Note whether any recent boiler work has been carried out DIY safe
If a PCB, CSU, or any other component was recently replaced, this information is valuable for the engineer. An incompatible part fitted during a recent repair is a common trigger for H.02.05, and knowing the service history will speed up diagnosis.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will access the boiler's internal settings and attempt to reset the CN1/CN2 configuration parameters first, as this is the least invasive fix. If that does not resolve the fault, they will test and if necessary replace the CSU. If the PCB itself is damaged or incompatible, a full control board replacement will be required. The engineer should also inspect for any signs of internal leaks that may have caused moisture damage to the electronics.
Parts you may need
- Configuration Storage Unit (CSU) · from £55
- Main PCB / Control Board (Remeha Tzerra Ace series) · from £280
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 £200–£500, depending on the underlying cause.
Frequently asked questions
Can I reset the boiler to clear the H.02.05 fault?
No — a standard boiler reset will not clear H.02.05. This is a blocking fault caused by an electronic configuration mismatch between the PCB and CSU, and it requires an engineer to reconfigure internal settings (CN1/CN2) or replace faulty components. Repeatedly pressing the reset button will not help and may mask other issues.
Is the H.02.05 fault dangerous — can I keep using the boiler?
The boiler will have shut itself down, so it will not fire or produce heat while the fault is active. This is a safety feature rather than an immediate gas-safety hazard, but you should not attempt to bypass the lockout. Leave the boiler off and arrange for a Gas Safe engineer to attend promptly, especially in cold weather.
What is the difference between replacing the CSU and replacing the PCB — and which is more likely?
The CSU (configuration storage unit) is a smaller, cheaper memory component that stores the boiler's identity and settings. Replacing it alone costs significantly less than a full PCB swap. An engineer will typically try reconfiguring the CN1/CN2 settings first, then replace the CSU if needed, before moving on to a full PCB replacement. In many cases the CSU or settings adjustment resolves the fault without needing a new board. Full PCB replacement is the most expensive outcome — parts alone can run to £250–£350 or more for Ace-series models, and in exceptional circumstances (specialist or discontinued boards) costs can exceed £600 including labour. Mention this possibility when getting quotes.
Could this fault have been caused by recent boiler repairs?
Yes — this is one of the more common triggers. If a PCB or CSU was recently replaced and the replacement part was not the correct revision or was not properly configured afterwards, H.02.05 can appear immediately after the work. Inform your engineer of any recent service history; if the fault follows a repair, the previous contractor may be responsible for returning to correct their work.