Remeha E.02.35 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha E.02.35 fault code mean?
E.02.35 is a permanent lockout code (the 'E' prefix always indicates a lockout, as opposed to 'H' which signals a temporary blocking fault) displayed on Remeha Tzerra Ace and related boilers. It means the main control board has lost communication with a component it considers safety-critical — such as the CSU (Control Safety Unit) or an internal sensor board — and has shut the boiler down rather than risk operating in an unverified state. The boiler will not restart on its own; a manual reset is required, and if the fault returns, an engineer must investigate. A closely related code worth knowing: A.02.37 is the non-lockout equivalent, where communication is lost with a non-critical component such as the SCB (System Circuit Board). If you see A.02.37 rather than E.02.35, the boiler may still be running but flagging a wiring or SCB issue. E.02.15 (found on the Tzerra Ace-Matic) points specifically to the CSU key recognition timeout — a breakdown in communication between the main board and the CSU — and is diagnosed similarly.
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
- Loose or damaged internal wiring connector Common
A poor or intermittent connection between the main board and the safety-critical component is the most frequent starting point. Vibration over time, or movement during a previous service, can work a connector partially free, breaking the communication bus the boiler relies on.
- Defective SCB (System Circuit Board) Common
The SCB handles communication between the main PCB and various boiler sub-systems. If the SCB itself has failed or its firmware has become corrupted, the main board will report that a safety-critical component cannot be reached, triggering the E.02.35 lockout.
- Failed CSU (Control Safety Unit) Sometimes
The CSU is the component directly responsible for verifying safe operation. If it stops responding on the internal bus — due to component failure, moisture ingress, or a manufacturing defect — the boiler has no choice but to lock out permanently until the issue is resolved.
- Power surge or supply disturbance Sometimes
Sudden spikes or drops in mains voltage — from lightning, a nearby appliance fault, or grid disturbances — can corrupt or physically damage the onboard electronics. A board that has survived a surge may appear functional but lose its ability to communicate reliably over the internal bus.
- Main PCB failure Rare
If the primary control board itself is degraded, it may lose the ability to poll and detect other components correctly, generating a false or genuine E.02.35 even when the downstream boards are intact. This tends to be the cause when wiring checks and board replacements have not resolved the fault.
How to fix it
- Attempt a single boiler reset DIY safe
On the Remeha Tzerra Ace, press and hold the reset (R) button and the arrow button together for approximately two seconds. The boiler will attempt to restart. If it fires up and runs normally, monitor it closely over the next few hours. If E.02.35 returns promptly, do not keep resetting — repeated resets will not clear a genuine communication fault and may mask useful diagnostic information for the engineer.
- Check your gas supply is active DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier before calling a boiler engineer. If the gas supply is fine, the fault is internal to the boiler.
- Call a Gas Safe registered engineer to run the autodetect procedure Gas Safe engineer
Remeha's own guidance for E.02.35 is to perform an autodetect via the boiler's e-Smart control platform. This procedure re-surveys all modules connected to the internal communication bus and attempts to re-establish contact with the missing safety-critical component. Only an engineer with access to Remeha diagnostic tools and familiarity with the e-Smart platform should carry this out.
- Engineer to inspect internal wiring and connectors Gas Safe engineer
If the autodetect does not resolve the fault, the engineer should systematically check all wiring looms and connectors between the main PCB, the SCB, and the CSU. They will look for corrosion, pin damage, partial ejection of connectors, and any signs of heat damage or moisture ingress near the board housings.
- Engineer to replace the faulty board or boards Gas Safe engineer
Once wiring is confirmed to be intact, the engineer can use voltage and continuity testing to identify whether the SCB, the CSU, or the main PCB has failed. In some cases Remeha service guidance recommends replacing the main board and CSU together if no single point of failure can be isolated. Only a suitably qualified engineer should handle this work.
Parts you may need
- Remeha SCB (System Circuit Board) · from £180
- Remeha CSU (Control Safety Unit) · from £150
- Remeha Main PCB / Control Board · from £280
- Internal wiring loom / connector harness · from £45
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–£450, depending on the underlying cause.
Frequently asked questions
Can I fix Remeha E.02.35 myself by resetting the boiler?
A single reset is worth trying — hold the reset and arrow buttons for two seconds to attempt a restart. However, E.02.35 is a permanent lockout caused by a loss of communication with a safety-critical internal component, so if the code returns the underlying fault must be properly diagnosed. Repeatedly resetting will not repair a failed board or a broken wiring connection, and doing so could delay finding the real problem. Beyond the reset, this fault requires a Gas Safe registered engineer.
How much does it typically cost to repair a Remeha E.02.35 fault in the UK?
For most households the repair falls between £150 and £450 including parts and labour. A wiring or connector fix at the lower end, or an SCB or CSU board replacement in the middle, covers the majority of cases. If the main PCB has also failed — particularly on older boilers or after a power surge — the bill can reach £500–£700. London and the South East tend to sit at the top of these ranges; engineers in the North and Wales are often 10–20% cheaper. It is always worth asking for a written quote before authorising parts.
What is the difference between E.02.35 and A.02.37 on a Remeha boiler?
The letter prefix tells you the severity. An 'E' code is a permanent lockout — the boiler shuts down completely and will not restart without a manual reset and a fix. An 'H' or 'A' prefix indicates a temporary blocking fault or warning, where the boiler may still be attempting to operate. A.02.37 means communication has been lost with a non-critical component (usually the SCB) and is diagnosed similarly — check wiring and connectors, then replace the SCB if needed — but it does not necessarily shut the boiler down immediately. E.02.35 is the more serious, safety-critical version of the same class of fault.
My Remeha boiler is 10 or more years old — is it worth repairing E.02.35 or should I replace it?
Remeha boilers typically last 10 to 15 years with regular servicing. If yours is approaching or past the 10-year mark and the repair involves a main PCB, CSU, or SCB replacement (£300–£700), it is worth getting a new-boiler quote at the same time. A brand-new A-rated condensing boiler will be more energy efficient and comes with a manufacturer warranty of 5–10 years, which may make replacement more cost-effective than an expensive repair on an ageing appliance. Ask your engineer to give you an honest view of the boiler's overall condition before committing.