What the F63 code means
EEPROM stands for electrically erasable programmable read-only memory. It is a small memory chip on the boiler's main circuit board that stores the settings the boiler needs to run: its model data, the gas type, the output limits and adjustments made by the installer. Unlike normal memory, it keeps its contents when the power is off.
Every time the boiler starts up, and at intervals while it runs, the board checks that this stored data is intact. On most ecoTEC models, if the check fails the boiler shows F63, which Vaillant's manual describes as an EEPROM fault with the cause listed as defective electronics. The boiler will not run on data it cannot trust, so it stays locked out.
F63 is therefore different from most other codes. It is not about water pressure, gas or flame. It is about the brain of the boiler.
Common causes of F63
A failing circuit board
Circuit boards age like any electronic device. Heat cycling over many years can cause the memory chip or the parts around it to fail. This is the most common reason for F63 on an older boiler.
Power surges and interruptions
A spike in the mains supply, such as during a storm, or a power cut at the moment the boiler was writing to its memory, can corrupt the stored data.
Water damage
Water reaching the board, from an internal leak or from pipework above the boiler, can damage the electronics.
A replacement board not set up correctly
Installer sources note that F63 can appear after a new PCB is fitted if the board has not been configured correctly for the boiler. This is something the engineer who fitted it should put right.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Ageing PCB | F63 on an older boiler with no obvious trigger | Gas Safe engineer |
| Power surge or cut | F63 appears after a storm or outage | Gas Safe engineer if it will not clear |
| Water on the board | F63 with drips or damp near the boiler | Gas Safe engineer |
| New PCB not configured | F63 straight after a board replacement | The engineer who fitted the board |
Safe checks you can do yourself
- Write down the code and whether anything happened just before it appeared, such as a power cut, a storm or electrical work in the house.
- Check the fused spur for the boiler is on and that other electrical items in the house are working normally.
- Switch the boiler off at the fused spur, wait about a minute, then switch it back on. This lets the board restart and recheck its memory. It may clear a one-off glitch.
- If the code is still there, press reset once as your manual describes.
- Look under and around the boiler for any sign of water, without removing any covers.
- If F63 returns after two or three attempts in total, leave the boiler off and call an engineer.
If F63 appeared straight after a new board was fitted, contact the engineer who did the work first. The fault may simply mean the board needs setting up, which should be part of the original job.
When to call a Gas Safe engineer
If F63 does not clear after a power cycle and a reset, the board almost certainly needs attention. Only a Gas Safe registered engineer should work inside the boiler. They will check the power supply and connections, look for signs of water or heat damage and confirm whether the board has failed.
In many cases the fix is a replacement PCB. After fitting it, the engineer needs to set it up correctly for your model, then run the boiler and check its combustion. Do not buy a board online and ask a non-registered person to fit it. The board controls the gas valve and ignition, so this is a gas safety job.
It helps to have the boiler model and serial number ready when you call. Boards are specific to the model, so the engineer may need to order the right one before the repair visit.
Never remove the casing or unplug anything inside the boiler. If you smell gas at any point, call the National Gas Emergency Service on 0800 111 999.
Typical repair costs in the UK
PCB faults are among the more expensive boiler repairs. These typical ranges include parts and labour and vary by region.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Repairing a loose or damaged connection | Typically £80 to £150 |
| Replacing the main PCB | Typically £250 to £500 |
If your boiler is still under warranty, speak to Vaillant or your installer first, as a board failure may be covered. If the boiler is well out of warranty and has had several other repairs, ask the engineer for an honest view on whether replacement makes more sense.
How to stop F63 coming back
You cannot completely prevent a circuit board from ageing, but you can reduce the risks. Keep the boiler dry by fixing leaks promptly, and do not store damp items against it. If your area has frequent power problems, ask an electrician about surge protection for the circuit that supplies the boiler. A yearly service lets an engineer spot scorch marks, corrosion or loose connections on the board early, and our boiler service cost guide explains what is included. Other electronic faults, such as F61, are covered on our Vaillant fault codes page.