Vaillant F.87 fault codewhat it means and how to fix it
The ignition transformer, or on the ecoFIT pure the electrodes, are not connected as the board expects. A wiring fault on the ignition side; the boiler will not attempt to light.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.87
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.87 means
F.87 is one of three connection-check codes — F.87, F.88 and F.89 — that Vaillant added to the ecoFIT and ecoTEC sustain boards. Before a boiler tries to light, the board confirms that the parts it is about to drive are actually wired to it, and reports a missing one rather than sparking into nothing. The wording shifts a little between ranges. The ecoFIT pure prints "fault: electrodes — electrodes not connected or connected incorrectly, short circuit in the cable harness".
The ecoTEC sustain says "fault: ignition transformer cable — ignition transformer cable harness not connected to the main PCB or connected incorrectly, short circuit in the cable harness or damaged ignition transformer". The open-vent ecoTEC plus reads "ignition transformer not connected — plug not plugged in or has come loose, multiple plug on the PCB not plugged in correctly, interruption in cable harness, electronics defective", and the 2022 CS/1-5 shows F.087, "the ignition transformer is not connected to the PCB", with the harness and the transformer as its two causes.
Read together they describe the same circuit from board to spark, and the same practical fact: the code follows work on the boiler far more often than it appears spontaneously, because plugs do not fall out of a boiler that nobody has opened. It is not a row on the ecoTEC plus ..6/5-5, pro or exclusive.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Ignition transformer plug not connected or loose | Vaillant's first cause on the open-vent and CS/1-5 tables. Displaced during a service or an electrode change and not pushed home. | Common |
| Multiple plug on the PCB not seated | The board end of the same circuit. Named on the open-vent list; the sustain's "not connected to the main PCB". | Common |
| Short circuit or break in the harness | On every version of the row. A chafed ignition lead is the classic finding. | Occasional |
| Ignition transformer failed | The sustain and CS/1-5 name a damaged or defective transformer. Checked after the wiring, and replaced as a unit. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£200
Time on site
45 minutes
Reseating a plug or repairing a lead is a call-out of £80 to £120. An ignition transformer is £120 to £200 fitted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant ignition transformer and lead
£35–£80Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.28Stop — call an engineerThe boiler tried to light and failed every attempt it is allowed — three on an ecoTEC, five on the 2022 CS/1-5 — so it has shut the gas off and locked out.Related: ignition and flame
- F.29Stop — call an engineerThe boiler lit correctly and then lost the flame while it was running. It has shut the gas off and locked out, and it needs an engineer.Related: ignition and flame
- F.27Stop — call an engineerThe boiler is detecting a flame when the burner should be off. Switch it off and get a Gas Safe engineer out rather than resetting it.Related: ignition and flame
- F.68Get it checkedThe flame is lighting but its signal is unstable. Vaillant's causes run through the gas supply, the air ratio, the condensate route and the flue, ending with the electrode and the board.Related: ignition and flame
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: same brand
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: same brand
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.