F13 error on a Fagor boiler: meaning and what to do

The F13 code on a Fagor boiler does not have a universal meaning. In the technical documentation available for the FE-24E, FE-27E, FE-30E and CE-30E models, it is associated with a fault affecting the thermistor at the main heat exchanger outlet. For the FE-24NOX and FEE-24NOX, however, the user instructions reviewed do not explain the cause and refer the user to a service technician. Identifying the model is essential before blaming a sensor, adding water or buying a replacement part.

If you have a problem with your boiler, you can use our free error code finder. It helps you identify errors and find straightforward, effective ways to address them.

The model determines what the warning means

The difference between model numbers is more than a catalogue detail. Fagor used different product ranges and control systems, and a code table for one family cannot automatically diagnose another. On the FE-24E, FE-27E, FE-30E and CE-30E, F13 is associated with temperature measurement at the outlet of the main heat exchanger, the component that transfers heat from combustion to the water.

A thermistor is a sensor whose electrical resistance changes with temperature. That change allows the boiler control system to monitor how the water is heating. If the signal is not valid, the appliance may lock out. However, identifying the affected measurement circuit does not establish which component has failed: the relevant system needs checking before parts are replaced.

For the FE-24NOX and FEE-24NOX, the available information from the user instructions includes F13 among the lockouts requiring technical assistance, without explaining its meaning. Applying the same diagnosis as for versions ending in E would therefore be an assumption. The Comfort or Isocomfort product name is no substitute for the full model reference, because a product family may contain different versions.

The identification details can usually be found in the appliance paperwork or on a label accessible from outside. Reading or photographing it is enough; there is no need to remove the casing. A photograph of the code exactly as it appears on the display is also useful. The combination of the exact model and displayed warning contributes more to diagnosis than a general description of a locked-out Fagor boiler.

F13 and F03 are not the same problem

Some confusion comes from search results discussing F3 or F03 when looking for information about F13. These are different codes. On the FE-24NOX and FEE-24NOX, F03 indicates heating circuit pressure below 0.6 bar or above 2.8 bar, according to the information in their instructions. It therefore does not mean only a lack of water: excessive pressure can also trigger it.

F13 should not be treated as a variation of that warning. Adding water because a page describes a solution for F3 may achieve nothing and raise the pressure unnecessarily. Reading the pressure indicator provides context, but does not justify topping up the system when a different code is displayed and the cause has not been identified.

The available references do not give one filling pressure for every Fagor boiler either. General information about FEE models cites between 1 and 1.2 bar, while model-specific information for the FE-24NOX and FEE-24NOX gives 1 bar in its procedure. Those figures should not become universal instructions. In this situation, the safe check is to read and record the pressure, without operating valves or working on the circuit.

The filling system also differs. The FE-24NOX documentation describes manual filling, while the FEE-24NOX has automatic filling. This explains why apparently simple advice may be unsuitable for another appliance. With F13, operating the filling system or bleeding the installation is not a supported remedy based on the known meaning of the code.

Related codes, separated by model range

The table distinguishes F13 from other lockouts appearing in the available documentation. Its purpose is to prevent confusion, not to turn every code into a diagnosis of a failed part. Where the user instructions do not publish a cause, that limitation remains: an undocumented meaning must not be filled in with assumptions or equivalents borrowed from another brand.

The distinction between F11 and F13 on models ending in E is particularly useful: both concern temperature, but they do not identify the same measurement location. Buying a sensor advertised simply as suitable for a Fagor boiler, without checking its specific application, may result in choosing a component that does not match either the fault or the appliance.

What you can check without opening the boiler

With the appliance locked out, external observations provide enough information to prepare for a service visit without taking risks. It helps to know whether F13 appears immediately after switching on, when a hot tap is opened or when the heating starts. Note whether it remains on the display or alternates with another indication. The aim is to describe when it happens, not repeatedly trigger the fault to see how long the boiler runs.

The pressure shown on the display or an accessible gauge can be recorded alongside the state of the appliance: cold, recently used or locked out after running. A single reading cannot establish whether a sensor has failed, but it helps avoid mixing up separate problems. Observe from outside, without removing covers or touching connections, even if they appear easy to reach.

You can also look for visible water leaks beneath the boiler or in accessible areas of the installation, without touching hot surfaces or approaching electrical components. A fresh stain, dripping water or a noticeable change in the indicator can be useful information. They do not justify dismantling a fitting or valve: leaks and pressurised circuits must be handled by qualified personnel.

Recent work on the appliance provides another relevant detail. If the lockout began after servicing, repair or cleaning, explain that sequence to the professional assessing it. The timing does not prove that someone caused the fault, but it may guide their checks. Trying to correct it by moving wires or parts yourself would undermine the benefit of safe observation.

A reset does not replace diagnosis

Not all Fagor boilers reset in the same way. The documentation for the FE-24E, FE-27E, FE-30E and CE-30E describes moving the main selector to position O and then returning it to its previous position. The FE-24NOX and FEE-24NOX have instructions involving RESET. These are different procedures, so follow the manual for the model, rather than a generic sequence found for another boiler.

If the manual allows a user reset and there are no abnormal warning signs, it can be carried out using the external controls as instructed. If F13 returns, stop testing. Repeated resets will not correct an invalid temperature signal or explain a lockout whose cause has not been published. They simply make the boiler attempt to run again without resolving the reason for the warning.

The code disappearing after a reset does not prove that the fault has been repaired either. An intermittent fault may not occur on every start. If it returns or the appliance behaves irregularly, the temporary disappearance should be reported as part of its history. It is not enough to assume reliable operation or rule out an inspection.

F03 on the FE-24NOX and FEE-24NOX illustrates why procedures must not be mixed: their instructions state that the lockout clears automatically when the pressure returns to the permitted range. That behaviour is specific to that warning and does not establish that F13 will correct itself. Each protection responds to a different control condition.

The sensor is a clue, not an instruction to buy one

On models where F13 is associated with the outlet thermistor, professional diagnosis must establish why the temperature signal is invalid. A failed sensor is one possibility, but its connections and the circuit interpreting its signal also need consideration. These are possibilities to investigate, not confirmed causes. The code identifies an affected function; it does not provide a list of parts that must be replaced.

Resistance, continuity, wiring and electronics checks involve accessing the interior and must be carried out by an appropriately qualified technician. Switching the appliance off does not make every task safe: boilers combine electricity, hot water and gas. It is therefore inappropriate to give users instructions for disconnecting sensors, bridging contacts or testing components.

It would also be inaccurate to blame F13 directly on the electronic control board. The documentation for versions ending in E has a separate code, F5, associated with an internal control fault. This does not rule out every possible electronic issue behind an abnormal reading, but it shows that F13 does not automatically mean a damaged board. Replacement should be supported by checks that substantiate the diagnosis.

Before buying a spare part, both the fault and compatibility with the exact appliance reference need confirmation. A photograph of the casing or a shared product name does not establish that match. On the NOX models, whose user instructions do not explain F13, buying a thermistor based on FE-E information would be particularly premature: the equivalence has not been established.

When to stop using it and arrange an inspection

If F13 remains or returns after a permitted reset, the prudent course is to leave the boiler out of service using its controls and arrange diagnosis. On models linking the warning to temperature, forcing further starts cannot guarantee correct control. On those without a published meaning, there is no reliable basis for claiming that normal use can safely continue either.

Testing hot water only or heating only does not establish that the other mode is safe. The main circuit and its controls may affect more than one demand, depending on the appliance design. Hot water flowing for a few minutes does not confirm that the fault has disappeared. With a recurring lockout, do not look for a way around the protection.

If there is also a smell of gas, smoke, a burning smell or a significant water leak, the situation is no longer simply a question about the display. Do not carry out tests or operate electrical switches if a gas leak is suspected; move away and contact the emergency services or gas network operator from a safe place. With these warning signs, safety takes priority over any reset.

The cost depends on the confirmed fault, not the code

The available information contains no verifiable repair tariff, and F13 alone cannot establish a cost. Replacing a sensor, correcting a faulty connection and diagnosing a different problem are different jobs. On ranges whose manuals do not publish the meaning, the initial uncertainty is greater. Any amount presented as a fixed F13 repair price would lack a sufficient basis.

Before authorising a repair, the quotation should identify the diagnosis and separate callout, testing, labour and materials where applicable. It should also clarify whether the visit is chargeable if no repair is carried out and whether that charge is deducted from subsequent work. There is no universal condition that can be assumed: the financial terms need confirming with whoever will carry out the work.

The decision to repair or replace involves more than the age of the appliance. Its overall condition, fault history, availability of the correct component and scope of the proposed repair all matter. A localised fault does not automatically justify replacing the entire boiler; an expensive repair on an appliance with other problems deserves a broader comparison. The useful approach is to assess the complete repair, rather than decide solely on the code name.

Accurate identification prevents guesswork repairs

The decisive distinction is between knowing what a warning means for a specific model and applying it to every boiler from the brand. On the FE-24E, FE-27E, FE-30E and CE-30E, F13 points towards temperature measurement at the main heat exchanger outlet. On the FE-24NOX and FEE-24NOX, the available user documentation requires leaving the cause open to professional diagnosis.

The user’s role is to identify the model, record what the display shows and observe the exterior without working on the installation. That information makes it possible to describe the lockout accurately and avoids topping up the system through confusion with F03 or buying a sensor without confirmation. With a persistent F13, the next step is diagnosis, not replacing parts by trial and error.