1. General Information

Anarchy reigns in the principles of coding errors in heating boilers—every manufacturer sets their own rules for encrypting fault codes. It is generally accepted to use a combination of Latin alphabet letters and numbers from 0 to 9 for coding errors. Most often, to interpret a particular code, it is enough to consult the product's technical documentation. As a rule, it contains tables with the error name, cause of the malfunction, and a list of measures to eliminate it.

Problems with boilers usually arise due to untimely servicing of units and assemblies, so it is necessary to routinely conduct thorough checks of heating equipment with the help of relevant specialists. These specialists should also be consulted for any boiler problems, except for some elementary issues that can be resolved independently.

In gas heating boilers, the gas burner demands close attention, as it gets clogged most frequently. Periodic cleaning of the internal surfaces of working chambers from contaminants and chimney maintenance is also important. Notably, the risk of failure or malfunctions in dual-circuit models is significantly higher than in single-circuit models. This is due to the more complex construction of the first type of devices; additionally, the water heater in such a boiler has to operate year-round without a break.

Important! Do not entrust the installation and piping of the gas boiler to amateurs. Errors made during this process may not be immediately apparent, but they will gradually lead to the breakdown of elements in the heating system.

Some heating units lack a display, yet they can also be diagnosed for faults. If an issue arises, colored lights on the panel of such boilers will light up or flash. Their meaning is explained in the accompanying documentation for the device. Don't be lazy; flipping through the model's technical passport will exclusively be beneficial.

Below is a list of the most common error codes with their descriptions for six major players in the heating systems market.

2. BAXI Boilers

  • E 01 — failed to ignite the gas burner in normal mode. The error is considered generic, caused by a wide range of factors: input gas pressure not corresponding to set standards, incorrect polarity of the boiler connection, malfunction of the gas valve or electronic board, etc.
  • E 02 — activation of the overheating emergency thermostat. This is most often triggered by a clogged heat exchanger circuit. Less frequently, the problem should be sought in the thermostat or the heating supply temperature sensor.
  • E 03 — issues with the supply or removal of air from the combustion chamber. In boilers with an open chamber, the error almost certainly indicates a lack of proper draft, whereas, in turbocharged models, it's not so straightforward: reasons for the E 03 error may include the fan not starting or mere accumulation of condensate in the tubes.
  • E 05 — malfunction of the temperature sensor on the heating circuit supply. Most often, when this error occurs, the aforementioned sensor needs replacement.
  • E 06 — malfunction of the domestic hot water (DHW) system temperature sensor. Similar to the previous item.
  • E 10 — unacceptable pressure in the heating system. The error "pops up" due to low water pressure in the circuit, poor contact between the sensor and the control board, or due to a malfunction of the minimum pressure relay. In the vast majority of cases, the problem goes away when the system is filled with coolant.
  • E 25 — exceeding the maximum boiler temperature. The reason should be sought, mainly, in poor circulation of the coolant or in a malfunction of the heating system temperature sensor, which is installed at the outlet of the primary heat exchanger.
  • E 35 — parasitic flame (intermittent). The code may appear on the boiler display due to a malfunction of the electronic board, as well as incorrect positioning of the flame detection electrode.
  • E 40, E 41 — chimney clogging or overly low input gas pressure. The emergence of this pair of errors is influenced by several factors: from genuinely low supply pressure of "blue fuel" to gas valve malfunctions.
  • E 96 — reduced voltage in the power circuit (less than 170 V). If the error appears with enviable regularity, it wouldn't be amiss to acquire a voltage stabilizer to even out the voltage and protect the device from potential energy fluctuations in the power supply network.
  • E 00, E 99 — internal error of the electronic board.
Popular BAXI Boilers

3. Bosch Boilers

For Bosch Gaz 6000 Series:

  • A7 — malfunction of the DHW circuit temperature sensor. The sensor needs to be checked and replaced if necessary.
  • Ad — boiler temperature sensor not defined. The sensor and connection wire need checking.
  • C1 — low fan rotation speed. This may be caused by low power supply voltage or issues related to the flue gas removal system.
  • C4 — differential pressure switch not turning off when the fan is off. This pressure switch should be inspected.
  • C7 — fan malfunction. Resolving the problem involves checking the functionality of the fan and its connectors, and replacing the fan if necessary.
  • E2 — malfunction of the flow line temperature sensor (break). The error typically occurs due to damage or short-circuiting of the aforementioned sensor.
  • E9 — activation of the temperature limiter in the heat exchanger. Code E9 appears on the boiler's display due to various factors—from damage to the temperature limiter to problems in the heating circuit.
  • EA — flame not detected. This is one of the most common errors in Bosch heating boilers. There are numerous reasons for the error's occurrence and many ways to solve the issue. But it's best to first check the gas valve opening, gas supply pressure, and the flue gas removal system. Still not helping? Start cleaning the heat exchanger and, if necessary, replace the gas fittings.
  • F7 — boiler detects a flame in the off state. The issue should be sought in electrode contamination, in parallel with checking the control board for moisture.
  • FA — boiler detects a flame with the gas supply cutoff. The gas fittings and the flue gas removal system should be checked.

For ZSC / ZWC Series:

  • A7 — DHW circuit temperature sensor malfunction. The sensor is commonly to blame, i.e., it needs replacement.
  • A8 — connection to the BUS bus is interrupted. This malfunction is to be resolved by checking the connecting cable and regulators.
  • A9 — incorrect installation of the DHW temperature sensor. When this error occurs, check the sensor's installation area and reconnect it using thermally conductive paste.
  • Ad — boiler sensor not detected. Perform a sensor diagnosis or replacement.
  • C1 — during boiler operation, the differential pressure switch disconnects. Inspect the differential switch, exhaust device, and connecting tubes.
  • C4 — differential pressure switch not disconnecting when the fan is off. Inspect this switch.
  • C6 — differential pressure switch not disconnecting. The first item of the action plan when this error occurs is to check the switch and exhaust gas pipes.
  • d4 — excessively high temperature difference. When the code appears, attention should be given to the circulation pump, bypass hose, and check the system pressure.
  • EA — flame not detected. This is one of the most common errors in Bosch heating boilers. There are numerous reasons for the error's occurrence and many ways to solve the issue. But it's best to first check the gas valve opening, gas supply pressure, and the flue gas removal system. Still not helping? Start cleaning the heat exchanger and, if necessary, replace the gas fittings.
  • F0 — internal malfunction. In this case, diagnose the electronics board.
  • F7 — boiler detects a flame in the off state. The issue should be sought in electrode contamination, in parallel with checking the control board for moisture.
  • FA — boiler detects a flame with the gas supply cutoff. The gas fittings and the flue gas removal system should be checked.
Popular Bosch Boilers

4. Buderus Boilers

Errors encountered in Buderus gas boilers are encrypted in a unique way. First, the main code (indicating the issue) is specified, followed by an additional code combination (this number uniquely identifies the fault signal), and finally, the class of the fault is designated. There are several classes:

  • O — operational messages during normal boiler operation;
  • R — non-blocking faults, during which the system continues to operate;
  • B — blocking faults, leading to a time-limited shutdown of the heating system;
  • V — locking faults, which block the operation of the heating system.

The list of possible errors in Buderus boilers is quite extensive. However, most often, users see these fault codes on the service display:

  • 2E 357 O — the air removal function is active, or the supply flow temperature is rapidly increasing.
  • 3C 217 V — the fan is not working. Here, it is important to determine at what stage the malfunction occurs: if the fan starts, but the relay does not click, the problem lies in the feeding tubes or chimney clogging. If the fan does not start at all, check if power is being supplied to it. If the fan is working, and there are no issues with the chimney, check the air pressure switch. Continuously closed contacts or their disconnection during the burner operation would also be considered a fault by the safety system.
  • 4C 224 B V — activation of the heat exchanger or flue gas temperature limiter. Both sensors and their power loops should be inspected, and if system pressure is inadequate, the boiler should be replenished. The issue might also be caused by circulation problems with the coolant or the formation of an airlock.
  • 4Y 351 B V — temperature sensor disconnection. The fault is solved by checking contacts and installing a new part.
  • 6A 227 B V — flame not recognized. In most cases, resolving ignition issues is better left to professionals. The simplest action for an ordinary user is a visual examination of the combustion chamber and electrodes for contamination. Cleaning is performed if necessary. If the boiler ignites but then goes out after some time, the fault is often related to the electronic board failure.
Popular Buderus Boilers

5. Hotpoint-Ariston Boilers

  • 101 — activation of the overheat protection system. The main reasons for the error code appearance include increased pressure at the gas valve outlet, dirt in the coarse filter, poor coolant circulation in the heating system, poor heat exchanger passage due to scale formation, and malfunction of the circulation pump.
  • 108 — drop in pressure in the heating circuit below the minimum. It is likely that a leak has occurred somewhere in the system.
  • 109 — pressure in the system has risen above 3 bar. This error sometimes occurs in the second heat exchanger of dual-circuit boilers. Due to a malfunction in it, hot and cold water mix, leading to increased pressure.
  • 201 — no contact with the DHW circuit sensor. Contacts must be tightened, or the sensor replaced.
  • 302 — loss of connection with the display and electronic module. The problem often lies in the wiring.
  • 501 — no flame during ignition. This error is one of the most common. First and foremost, check if the gas supply valve is not closed. A malfunction sometimes manifests due to a flame sensor malfunction.
  • SP3 (5P3) — flame separation. This issue regularly occurs in the coaxial chimney area. To eliminate it, adjust its position and clean it from debris.
  • 601 — no draft in the system. If the sensor works correctly, chimney cleaning will be required.
  • 604 — low fan rotation speed.
  • 6P1, 6P2 — delay in closing the air pressure switch contacts. It arises due to damage to the Venturi tube. The code difference is that 6P2 displays during boiler operation, while 6P1 appears when attempting to ignite with an already damaged tube.
Popular Hotpoint-Ariston Boilers

6. Protherm Boilers

  • F00/01 — malfunction of the heating temperature sensor. The sensor should be checked and replaced if necessary.
  • F13 — boiler sensor malfunction. Similar to the previous point.
  • F20 — overheating malfunction (temperature over 97 °C). To resolve the error, check the circulation pump, ensure the heating valves and the bypass valve are open, and diagnose the thermal filter.
  • F22 — no coolant in the boiler. Fill the system and ensure there are no leaks.
  • F24 — malfunction in the coolant circulation. The problem here lies in both the pump and closed heating valves.
  • F26 — lack of necessary current through the gas valve motor. The stepper motor requires diagnosis and replacement.
  • F28/29 — flame not detected during ignition and loss of flame during burner operation. As with peers: personally verify the gas valve is open, and seek the help of qualified specialists for more severe issues.
  • F84 — constant temperature difference between supply and return sensors. The sensor most likely requires replacement.
Popular Protherm Boilers

7. Vaillant Boilers

  • F.10/11 — short circuit in the supply/return sensor. The sensor requires diagnosis or replacement.
  • F.13/14 — short circuit in the hot start sensor. Similar to the previous point.
  • F.22 — insufficient water quantity in the system or complete absence. In the majority of cases, the problem is negated when the system is filled with coolant.
  • F.28 — electronic board malfunction.
  • F.29 — gas burner not igniting. One of the most commonly known issues with gas boilers. Its appearance is influenced by gas supply pressure not matching set standards, incorrect polarity of the boiler connection, gas valve malfunction, or electronic board issues.
  • F.36 — emission of gaseous combustion products into the room. The error arises with a closed air duct or gas outlet. It manifests due to poor draft and ventilation, as well as high room temperature where the boiler is installed.
  • F.61 — error in gas valve management. The problem lies in the valves themselves, their wiring, or the electronic control board.
  • F.75 — the gas boiler pump is started, yet the pressure does not change. To promptly correct the situation, a check of the adjustable bypass, pressure sensor, circulation pump, and correct connection to the expansion tank line is warranted.
Popular Vaillant Boilers

8. Ferroli Boilers

Errors in Ferroli gas boilers are divided into two groups:

  • critical (with a Latin alphabet letter A and two digits) — lead to equipment lockout, though sometimes they can be resolved by rebooting the boiler;
  • non-critical (with a Latin alphabet letter F and two digits) — result in only temporary stoppage of the boiler or one of its components.

Common errors from group A include the following:

  • A01 — lack of signal about flame presence. No gas is supplied to the burner, the ignition electrode is faulty, or ignition power is set too low.
  • A02 — false signal of flame presence when the burner is off. The issue arises from ignition electrode or control board malfunctions, less frequently from wiring faults.
  • A03 — activation of the overheat protection system. The error arises from temperature sensor or circulation pump problems, inadequate water circulation in the heating circuit, due to system airlocks.
  • A05 — water pressure sensor malfunction. It needs checking for functionality or replacement.
  • A06 — no flame after ignition. This results from insufficient pressure in the gas line or an overly low minimum pressure for gas flow.
  • A08 — overheating sensor malfunction. This may be caused by clogging or failure of the mentioned sensor, or an open electrical circuit from it.
  • A09, A16 — gas valve issues. Error arises from malfunction of gas fittings or a break in the valve power wire.

Common issues from group F include:

  • F04 — flue gas thermostat overheating. Typically, the fault manifests due to gas boiler flue problems. It may also be caused by flue gas temperature control sensor malfunctions.
  • F05 — flue gas removal fan failure. Thoroughly check its functionality. The error may also arise due to flue clogging.
  • F08 — heat exchanger overheating. This results from temperature sensor or circulation pump malfunction, due to system airlocks or inadequate water circulation. The F08 error often precedes A03 (see above).
  • F10, F11 — malfunctions of the heating water and DHW temperature sensors, respectively. Short circuit or broken electrical circuit frequently lead to these issues.
  • F20 — combustion quality issues. The gas valve, fan, flue gas removal system, and ignition electrode need checking.
  • F34 — low voltage. Occurs due to voltage sag in the power grid below 180 V.
  • F40 — excessive pressure in the heating system (in closed-loop circuits). Pressure may need to be reduced, and the safety valve's operation checked.
Popular Ferroli Boilers