FAQs

Vaillant Fault Codes

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Code Meaning Cause
F.49 eBUS fault Short circuit on the eBUS, eBUS overload or two power supplies with different polarities on the eBUS
F.52 Mass flow sensor connection fault Mass flow sensor not connected/disconnected, plug not connected or incorrectly connected
F.53 Mass flow sensor fault Mass flow sensor faulty, filter below venturi filter cap wet or blocked, gas flow pressure too low
F.54 Gas pressure fault (in combination with F.28/F.29) No or too little gas supply pressure, gas valve closed
F.56 Fault: Mass flow sensor regulation Gas valve defective, cable harness to gas valve defective
F.57 Fault during comfort safety mode Ignition electrode highly corroded
F.61 Fault: Gas valve regulation
  • Short circuit/short to earth in cable harness for the gas valve
  • Gas valve defective (coils shorted to earth)
  • Electronic defective
F.62 Fault: Gas valve switch-off delay
  • Delayed shutdown of gas valve
  • Delayed extinguishing of flame signal
  • Gas valve leaking Electronics defective
F.63 EEPROM error Electronics defective
F.64 Electronics/NTC fault Supply or return NTC short circuited, electronics defective
F.65 Electronic temperature fault Electronics overheating due to external influences, electronics defective
F.67 Electronics/flame fault Implausible flame signal, electronics defective
F.68 Unstable flame signal fault Air in gas, gas flow pressure too low, wrong air ratio, condensate route blocked, wrong gas restrictor, ionisation flow interruption (cables, electrodes), flue gas recirculation, condensate route
F.70 Invalid device specific number (DSN) Spare part case: Display and PCB replaced at same time and DSN not set, wrong or missing output range coding resistor
F.71 Flow NTC fault Flow temperature sensor signalling constant value:

  • Flow temperature sensor incorrectly positioned at supply pipe
  • Flow temperature sensor defective
F.72 Flow/return NTC fault Flow/return NTC temperature difference too great → flow and/or return temperature sensor defective
F.73 Water pressure sensor signal in the wrong range (too low) Interruption/short circuit of water pressure sensor, interruption/short circuit to GND in supply line to water pressure sensor or water pressure sensor defective
F.74 Water pressure sensor signal outside correct range (too low) Cable to water pressure sensor has short-circuitedd at 5V/24 V or internal fault in water pressure sensor
F.75 Fault, no pressure change detection when starting pump Water pressure sensor and/or pump defective, air in heating installation, too low water pressure in boiler; check adjustable bypass, connect external expansion vessel to return
F.76 Overheating protection on primary heat exchanger has responded Cable or cable connections for safety fuse in primary heat exchanger or primary heat exchanger defective
F.77 Flue non-return flap/condensate pump fault No response from flue non-return flap or condensate pump defective
F.78 Interruption to DHW outlet sensor at external controller UK link box is connected, but hot water NTC not bridged
F.83 Flow and/or return temperature sensor temperature change fault
  • When the burner starts, the temperature change registered at flow and/or return temperature sensor is non-existant or too small.
  • Not enough water in the boiler
  • Flow and/or return temperature sensor not in correct position at pipe
F.84 Fault: Flow/return temperature sensor difference implausible Flow and return temperature sensors return implausible values

  • Flow and return temperature sensors have been inverted
  • Flow and return temperature sensors have not been correctly fitted
F.85 Fault: Flow and return temperature sensors incorrectly fitted Flow and/or return temperature sensors have been fitted to the same pipe/wrong pipe
F.92 Coding resistor fault The coding resistor on the PCB does not match the entered gas family. Check the resistor, repeat the gas family check and enter the correct gas family.
F.93 Gas group fault Combustion quality outside the permitted range: Wrong gas restrictor, recirculation, wrong gas family, internal pressure measuring point in venturi blocked (do not use lubricant on O-ring in venturi.). Gas family check prematurely terminated.
Communication fault No communication with PCB

Ideal Fault & Status Codes

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Ideal Status Codes
Display Code System Behaviour
0 The boiler is in standby mode awaiting either a central heating call or hot water demand
C The boiler has a call for central heating but the appliance has reached the desired temperature set on the boiler
D The boiler has a call for hot water but the appliance has reached the desired temperature set on the boiler
C The boiler is operating in central heating mode
d The boiler is operating in hot water mode
F The boiler is operating in frost mode
Ideal Fault Codes
Fault Code System Behaviour
C0 BCC Activation Fault
C2 BCC Fault
F1 Low Water Pressure
F2 Flame Loss
F3 Fan Fault
F4 Flow Thermistor
F5 Return Thermistor
F6 Outside Sensor Failure
F7 Low Mains Voltage
F9 Unconfigured PCB
L1 Flow Temperature Overheat or No Water Flow
L2 Flame Loss
L5 5 Boiler Resets in 15 minutes
L6 False Flame Lockout

Baxi 200 Combi/Baxi Combi 400 Fault Codes

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Fault Code Possible Fault
H.01 – .00 Temporary Fault – Temporary loss of communication between gas valve and boiler board. Fault in air/gas unit.
Loose connection to gas valve.
H.01 – .05 Temporary Fault – Maximum difference between the flow and return temperature exceeded. Non-existent or insufficient circulation. Sensor error.
H.01 – .08 Temporary Fault – Flow temperature increasing too fast in heating mode. Non-existent or insufficient circulation.
Sensor error.
H.01 – .14 Temporary Fault – Maximum flow temperature value exceeded. Non-existent or insufficient circulation. Non-existent or insufficient circulation.
H.01 – .18 Temporary Fault – No circulation of water. Non-existent or insufficient circulation. Sensor error.
H.01 – .21  Temporary Fault – Flow temperature increasing too fast in DHW mode. Non-existent or insufficient circulation. Sensor error.
H.02 – .02 Temporary Fault – No input of parameters C1/C2. PCB not configured.
H.02 – .03 Temporary Fault- Incorrect configuration settings C1/C2.
H.02 – .04 Temporary Fault – Parameter error. Incorrect parameters.
H.02 – .06 Temporary Fault – System water pressure. Water leak on boiler. Water leak on system.
H.03 -.00 Temporary Fault – Communication error with NTC flue sensor. Sensor not, or badly connected. Bad connection. Sensor fault.
H.03 – .01 Temporary Fault – Communication error with the HMI PCB. HMI not connected.
H.03 – .02 Temporary Fault – No flame during operation. No ionization current.
E.00 – .04 Permanent Fault –  Return temperature sensor open-circuit. Sensor not or badly connected. Bad connection. Sensor fault.
E.00 – .05 Permanent Fault – Return temperature sensor short-circuit. Sensor not or badly connected. Bad connection. Sensor fault.
E.01 – .02 Permanent Fault – Temperature measured by return sensor greater than flow sensor. Bad connection. Sensor fault. Sensor not or badly connected.
E.01 – .04 Permanent Fault – Loss of flame detected 5 times in 24 hours (With burner on). No ionization current.
E.01 – .11 Permanent Fault Requiring Reset – Incorrect fan speed. External draught over the boiler. Defective gas/air unit.
E.01 – .12 Permanent Fault Requiring Reset – Temperature measured by return sensor greater than flow temperature.
E.01 – .17 Permanent Fault Requiring Reset – No circulation. Sensor not or badly connected. Sensor fault.
E.01 – .20 Permanent Fault  – Maximum flue gas temperature value reached (>140 Deg C). No circulation. Sensor fault.
E.02 – .00 Permanent Fault  – Boiler reset in progress. Reset button has been pressed.
E.02 – .07 No or low system water pressure. Low system pressure (less than 0.5bar) boiler will not function.
E.02 – .16 Permanent Fault –  On board CSU time-out. Check PCB for damage.
E.02 – .17 Permanent Fault – Permanent loss of communication between gas valve and boiler PCB. Fault in air/gas unit. Loose connection to gas valve.
E.02 – .19 Permanent Fault – Jumper 1 changed. Check position of jumper 1.
E.02 – .20 Permanent Fault – Jumper 2 changed. Check position of jumper 2.
E.02 – .21 Permanent Fault – Jumper 3 changed. Check position of jumper 3.
E.02 – .47 Permanent Fault – Connection to external device unsuccessful. Check wiring to external device.
E.02 – .48 Permanent Fault – Configuration to external device unsuccessful. Check pairing to external.
E.04 – .00 Permanent Fault  – Gas valve fault. Gas valve not detected.
E.04 – .01 Permanent Fault  – Flow temperature sensor short-circuited. Sensor not or badly connected. Bad connection. Sensor fault.
E.04 – .02 Permanent Fault  – Flow temperature sensor open-circuited. Sensor not or badly connected. Bad connection. Sensor fault.
E.04 – .03 Permanent Fault – Critical flow temperature reached. Non-existent or insufficient circulation.
E.04 – .04 Permanent Fault  – Flue temperature sensor short-circuited. Sensor not or badly connected. Bad connection. Sensor fault.
E.04 – .05 Permanent Fault – Flue temperature sensor open-circuited. Sensor not or badly connected. Bad connection. Sensor fault.
E.04 – .06 Permanent Fault – Critical flue gas temperature value reached. No circulation. Sensor fault.
E.04 – .08 Permanent Fault – Maximum safe temperature value reached. (Safety thermostat open-circuited). No circulation. Sensor not or badly connected. Bad connection. Sensor fault.
E.04 – .10 Failure to light (could be gas supply issue).
E.04 – .12 Permanent Fault – False flame signal. Short-term fluctuation of the mains. Ionization current present even though there is no flame. The burner remains very hot (CO2 too high). Defective gas/air unit.
E.04 – .13 Permanent Fault – Fan rotor blocked. Fan seized. Wiring to fan damaged.
E.04 – .17 Permanent Fault – Problem with gas valve. Defective gas/air unit.

Worcester Bosch Error Codes

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Fault Code Possible Fault
9A Incorrect HCM fitted
9U Problem with the code plug (faulty or not connected)
B7 Replace control board
C6 Running too fast / Running too slow
C7 Fan not running / Fan not running or airflow blocked
D1 Sensor wet or damaged (the boiler will block for 300 seconds before this condition)
D1 Sensor disconnected or damaged (the boiler will block for 300 seconds before this condition)
D1 Boiler overheat
E2 Flow sensor shorted or damaged
E2 Flow sensor disconnected or damaged
E5 Primary flow sensor overheated
E9 The main heat exchanger has overheated.
Maximum temperature 105°C
E9 Maximum temperature sensor on the main heat exchanger has failed
E9 Maximum temperature sensor on the main heat exchanger has not been recognised
E9 Flue gas thermostat overheat
EA Failure of four ignition attempts. The boiler waits 30 seconds before another attempt is made
EA An established flame has been extinguished.
This could be caused by:
moisture in the sump, due to a blocked sump/condensate trap.
a strong wind into the flue
EA Gas valve coil disconnected
EA Flame not established during the first 4 seconds ignition period
EA There is an error where the pump is not detecting water – check pressure gauge and repressurise to 1 bar.

How do I know if I need a new boiler?

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  • Is your boiler more than 15 years old?
  • Is the energy efficiency rating of your boiler below ‘A’?
  • Does it take a while for your boiler to heat up?
  • Does your boiler need frequent repairs?
  • Is the cost of maintaining your boiler on the up?
  • Are you heating a large hot water tank regardless if it gets used or not?
  • Does your boiler make strange noises?
  • Are your fuel bills on the up?

If you find yourself answering yes to any of these questions, it may be time to replace your old boiler.

Efficiency benefits of replacing your boiler

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Replacing a boiler that has been worn-out will definitely impact your account balance. Still, there are positive impacts of replacing it and upgrading your heating system. This is especially obvious if you don’t have the room thermostat system or you don’t use TRVs.

The estimates from the relevant sources say that the heating system participates with no less than 60% of the total energy consumption of your household! It is easy to fathom that the more energy efficient boilers can contribute to significant savings in this department. The majority of the new boilers that has been “A” rated have up to 90% energy efficiency or even more! Your old boiler can fall in many energy efficiency groups, but the majority of them are labelled as the “G” devices, which means that their energy efficiency stands somewhere between 55% and 70%. This comes as a consequence of using a non-condensing technology. Your annual saves on gas bills can be as high as £200-300, Depending on your gas consumption.

If you are turning your boiler on just two times a day, if you do not use the hot water in them, it is a pure waste of energy. The water in the tank will cool down if left there for too long, even if the insulation is good, and the energy used will be used irreversibly. In these cases, a combi boiler is a perfect solution. It will provide all the hot water you need, ONLY when you need it, so the losses generated by the redundant heating will become non-existing!

Combi boilers pros & cons

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Pros:

  • Compact design is ideal for smaller places where the space is limited
  • Combi boilers are an ideal option when there is no loft
  • Decreased home pipework
  • Perfect for freshly built premises
  • Energy efficiency increase

Cons:

  • The main water pressure needs to be sufficient
  • Can’t run multiple hot taps at the same time
  • Water pressure in the shower is directly dictated by the main water pressure
  • In cases of breakdowns, there is no alternate source of hot water

Pros & cons of conventional boilers

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Pros:

  • Ideal for older buildings
  • Perfect in scenarios where a lot of hot water is being spent (large buildings and buildings with multiple occupants)
  • Ideal for homes that have more than one bathroom
  • Ideal for areas where water pressure is low.
  • Can be combined with various solar water heating system

Cons:

  • They require huge spaces
  • They are not energy efficient, especially in the cases where the tanks have poor insulation.
  • The amount of hot water at your disposal is defined by the size of the tank.
  • The main components (heating system and the water pump) are separated, thus require more space.

System boilers pros & cons

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Pros:

  • Lower space requirements as all the most important components are built in.
  • Extremely economical, easy to install, and fit in limited spaces.
  • Faster hot water response on demand.

Cons:

  • Loses heat if the cylinder is not insulated properly.
  • Insufficient cylinder insulation results in waiting for the boiler to heat up again if you wish to take a hot bath.
  • The size of the tank dictates the quantity of hot water at your disposal.

What is the difference between conventional boilers and system boilers?

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Using a system boiler is an extremely economical and efficient option for heating your home. For many people it can be difficult to tell the difference between a system boiler and regular boiler. What is the most important difference? System Boilers take up far less space to be installed than the regular boilers. This is because they incorporate the cistern tank as well as all other major components, but also still provides a constant flow of hot water. A system boiler is ideal for larger premises with not enough space for storage purposes.

All the system boilers already have the majority of vital components of a functional heating system built in. This makes installation quicker and easier. The hot water is being pumped through the radiators as well as the hot water cylinder when you use a system boiler. This brings a faster response, and as a consequence your heating becomes more economical and has lower running costs. Since all of the major components are built into the single casing, the space requirements are much lower, and the installation is much easier.

Useful videos

  • How to top up your boiler pressure
  • How to repressurise a heating system with an Internal Filling Key
  • How to repressurise a Vaillant ecoTEC Pro
  • How to bleed your radiators
  • How to safely unfreeze a frozen boiler condensate pipe
  • Handy Greenstar boiler checks