Boiler fault codes that stop your hot water

A control board has no idea why your hot water has stopped. All it knows is that a reading it watches fell outside the window it expects, and that it has a number to display for that window. Everything between the number and the actual broken part is still guesswork until somebody tests it.

Take a flame sensing fault. The code says the board did not see a flame current. That is true of an electrode coated in white deposit, of a gas valve that never opened, of a supply that was not there to open onto, and of a board with a failing sensing circuit reading a flame that was burning perfectly well in front of it. One code, four different jobs, and they are not the same price or the same visit.

So the order of elimination is the whole skill. Outside the case before inside it, and the measurement before the part that takes the measurement. A sensor swapped on the strength of a code alone is the most commonly wasted part in this trade.

A lockout is the appliance refusing to try again until a person tells it to. That refusal is the safety function working, not the thing that has broken, and clearing it once is entirely reasonable. Boilers do trip on a genuine one off: a momentary dip in supply, a condensate that froze overnight and thawed by lunchtime, air pushed through the system after work somewhere else on the pipework. Reset it, and if it runs for days afterwards, that was the whole story.

The tell is how quickly the code comes back. Same hour, same day, or every time you ask for hot water and the reset is not fixing anything. It is wiping the record of the trip while whatever caused the trip is sitting there unchanged, waiting for the next attempt.

On a gas appliance that matters more than it sounds. Each reset is another full ignition sequence, which means another set of attempts to introduce gas into a chamber that has already proved it cannot light it. It also wears the igniter and the gas valve, and it overwrites the fault history an engineer would otherwise read off the board to see how long this has been building. Twice is diagnosis. The third time is a phone call, not another reset.

Two of them are the boiler never lighting at all. Vaillant F28 fault code is the appliance reporting that it asked for a flame and no flame appeared, which puts gas supply, the ignition electrodes and the gas valve in front of anything else. Baxi E133 fault code is the same event on a different manufacturer, and the causes rank in much the same order, which is worth knowing if the two of them are how you found this page. Ideal boiler L2 fault code is the ignition lockout on that range and belongs in the same family.

The fourth is a different failure and people confuse it with the other three constantly. Vaillant F29 is the burner lighting and then the flame going out, which is not a failure to ignite and does not have the same short list behind it. A flame that establishes and then disappears points at something interrupting supply once the burner is running, or at a flue and condensate problem, or at a sensing circuit losing the flame it can see. Checking it as though it were an ignition failure is how people replace an electrode that was never the issue.

Four codes is the whole list here on purpose. This is not an encyclopaedia of every number a boiler can display, it is the handful that reach people as cold taps, and the encyclopaedia belongs elsewhere.

On a combi the boiler is the water heater, so anything that locks it out takes the taps and the radiators together. That is the easy case, because the display is telling you something. The harder case is hot water gone while the heating is untouched, and there the display is often blank: no code at all, because nothing has failed in a way the board watches. What has usually happened is that the diverter valve is no longer moving across to the hot water side, or the plate heat exchanger that transfers the heat to your tap water has silted up.

That case is not a fault code page at all. No hot water from a combi boiler is where the blank display version of this belongs, and no hot water but the heating works is the page for the stored system version, because the diagnosis there runs through valves and a cylinder rather than through anything the boiler will admit to.

On a stored system the chain is longer and the codes reach you second hand. The boiler heats a circuit, a motorised valve sends that circuit to the cylinder coil, and the cylinder holds the result. A boiler lockout stops the whole chain, but so does a seized valve that never opened, and only one of those produces a code. If the cylinder is still warm at the top hours after the boiler stopped, the fault happened recently. If it is cold through, it stopped a while ago.

Immersion heaters sit outside all of this. They have no display and no code, they fail through the thermostat or the element, and they are the reason a property with a cylinder can still have hot water while the boiler is locked out on a fault. Immersion heater not working is the page for that side of it. Worth knowing before you assume the code is the whole problem.

Almost the entire London supply is hard water, and the codes that follow from it are misread constantly. Scale builds on the hottest surface it touches, which on a combi is the domestic hot water side of the plate exchanger, and on a cylinder is the coil. The boiler then starts throwing overheat or flow temperature codes because the heat it is making has nowhere to go. Change the sensor that reported it and the code comes straight back, because the sensor was right. Hard water and hot water cylinders in London covers what the scale is actually doing on the stored side of that.

Victorian conversions bring their own pattern. Condensate pipes on rear extensions were often run outside in plastic too narrow for the job, so the first hard frost of the year produces a wave of lockouts with nothing wrong with the boiler at all. Buried pipework in the same properties means a code that reads as a pressure fault can turn out to be a leak under a floor.

Purpose built blocks are different again. Plenty run Pulsacoil thermal stores rather than boilers, and a thermal store has no fault code to give you. Cold taps, nothing on any display, and the fault is an immersion, a pump or the control board inside the unit. How a pulsacoil works is the page to read if that description matches your property. If you are looking for a code and there is no code to find, check what is actually making your hot water before you go further.

Gas work is the clear line, and an unvented cylinder is the other one, because it holds mains pressure and is not something to experiment on. Our engineers hold the qualifications across the boiler, the cylinder and the pipework between them rather than one of the three, so a code that turns out to be a seized valve does not become a second appointment. The rate and the call-out are published on the pricing page, and neither moves for a difficult property or an evening visit.

Where the code has not narrowed anything down, boiler diagnosis in London is the service page this hub ends at, and unvented cylinder repair is where a code that turns out to be a stored system fault belongs instead.