What a boiler PCB does
The PCB (printed circuit board) is the main control board inside the boiler. It reads the sensors, runs the fan and pump, opens the gas valve, starts the ignition sequence, checks for a flame and shows codes on the display. When it spots something wrong it locks the boiler out for safety.
Because the board is involved in almost everything, a PCB fault can look like many other faults. That makes it one of the most commonly suspected parts and, sometimes, one of the most wrongly replaced. A careful engineer tests the parts around the board before condemning it.
That testing usually includes checking the incoming mains voltage and earth, the fuses on the board, the connections to the fan, pump, gas valve and sensors, and the resistance of each sensor. Only when those are all fine does the board itself become the likely cause. This takes time, but it can save you from paying for a new board that does not fix the problem.
Symptoms of a failing PCB
- A blank display, or a display that flickers, freezes or shows random characters.
- The boiler turns itself on and off, or does not respond to the controls.
- Faults that come and go with no obvious pattern.
- Codes that point to the board or its memory, listed below.
- A burning smell or visible scorch marks, which an engineer may find inside the casing.
- The boiler stopped working after a power cut, a thunderstorm or a water leak inside the casing.
Fault codes that point to the PCB
| Brand and code | What it usually means |
|---|---|
| Ideal F9 | Unconfigured or faulty PCB, and on some newer models a gas valve short circuit |
| Vaillant F63 | EEPROM fault, a memory problem on the main board |
| Baxi E168 | General fault, often linked to the PCB or supply voltage |
| Viessmann FD | Burner control unit fault, after electrodes and interference are ruled out |
| Vaillant F61 | Gas valve control fault, which can be the valve, its wiring or the PCB |
Other codes, such as fan faults like Worcester C6 215, can occasionally trace back to the board, but the fan and wiring are more common causes. Meanings can vary between models, so check the code page for your boiler.
Typical PCB replacement cost in the UK
| Item | Typical cost |
|---|---|
| Genuine replacement PCB, part only | Typically £150 to £400 |
| Labour to fit and set up the board | Typically £80 to £180 |
| Total PCB replacement, fitted | Typically £250 to £550 |
| Diagnosis and repair of wiring or connectors instead | Typically £80 to £150 |
What pushes the cost up or down
- Brand and model: boards for premium boilers from Worcester Bosch, Vaillant and Viessmann often cost more than those for budget models.
- Set-up: many new boards need configuring to your exact model, using a data chip, a code card or settings entered by the engineer. Ideal F9, for example, can show on a new board that has not been configured.
- Part supply: boards for older or discontinued models can be expensive or hard to find, which may mean a wait or a refurbished unit.
- Labour rates: London and the South East usually cost more than other regions.
- Warranty: a boiler still under manufacturer warranty may get a new board free if it has been serviced every year.
Genuine, refurbished or repaired boards
Some engineers offer refurbished boards or a board repair service, where specialists replace the failed components on your existing board. These can be cheaper, and for discontinued models they may be the only option. The trade-off is usually a shorter part warranty and, with a repair service, a wait while the board is sent away. For a boiler that is still under manufacturer warranty, stick with a genuine part fitted by an approved engineer so the warranty is not affected.
How long it takes
Once the part is available, fitting and setting up a new board usually takes one to two hours. The engineer then runs the boiler on heating and hot water and checks the combustion to make sure everything is working as it should.
Why PCBs fail
- Water: a leak from a pump, valve or seal that drips onto the board is a common cause. Our guide to a boiler leaking water covers the usual sources.
- Power problems: surges, lightning and unstable supply can damage components.
- Heat and age: components wear over years of heating and cooling.
- Another failed part: a shorted fan, pump or gas valve can damage the board it is connected to, so the engineer should check these too, or a new board may fail again.
To protect a new board, have any leaks fixed straight away, keep the annual service up to date so worn seals are spotted early, and ask an electrician about surge protection if your area has frequent power problems.
What you can check first
A PCB diagnosis needs an engineer, but you can rule out a few simple things:
- Check the fused spur or plug for the boiler and your fuse box. A tripped circuit can look like a dead board.
- Check the thermostat and timer are calling for heat and have power or batteries.
- Check the pressure gauge reads 1 to 1.5 bar with the system cold.
- Switch the boiler off at the spur, wait a minute, switch it back on and reset once. Stop after two or three attempts.
Never remove the casing to look at or touch the PCB. It carries mains voltage and sits next to gas parts. Only a Gas Safe registered engineer should test or replace it.
Is a new PCB worth it on an older boiler?
On a boiler under about ten years old, a new board is usually a sensible repair. On an older boiler, especially one with other faults or where the board is discontinued, it is worth comparing the repair price with a replacement. Our guides to repairing or replacing a boiler and boiler repair costs can help you decide.
If you do go ahead with a new board, ask the engineer to explain what caused the old one to fail. If the cause was a leak or another faulty part, that needs fixing at the same visit, otherwise the new board is at risk from the same problem.