Common Boiler Fault Codes
Although most new boilers are very reliable, all machines have the potential to encounter a fault at some point in their life. To make things easy to diagnose, boilers often display an Error Code when they are not working. This can be used to identify the issue, cutting down on investigation time.
Ableforce mainly install Worcester Bosch, Vaillant, and Ideal boilers. These units all tend to be reliable, but just in case, we have detailed the descriptions of common fault codes for each manufacturer below.
Despite this useful information, we would still strongly advise that any further diagnostics or repair is performed by a fully qualified Gas Safe registered company such as Ableforce Services.
Worcester Bosch Boilers
As the leading manufacturer of gas boilers in the UK, there are a significant number of these units installed in thousands of homes. Whilst these tend to be some of the most reliable boilers available, it’s still good to know what the issue is if something went wrong.
Locking Errors (Reset button needs to be pressed)
Incorrect HCM fitted
Problem with the code plug (faulty or not connected)
Replace control board
Running too fast / Running too slow
Fan not running / Fan not running or airflow blocked
Sensor wet or damaged (the boiler will block for 300 seconds before this condition)
Sensor disconnected or damaged (the boiler will block for 300 seconds before this condition)
Flow sensor shorted or damaged
Flow sensor disconnected or damaged
Primary flow sensor overheated
The main heat exchanger has overheated. Maximum temperature 105°C
Maximum temperature sensor on the main heat exchanger has failed
Maximum temperature sensor on the main heat exchanger has not been recognised
Flue gas thermostat overheat
Failure of four ignition attempts. The boiler waits 30 seconds before another attempt is made
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
Flame not established during the first 4 seconds ignition period
Gas valve coil disconnected
There is an error where the pump is not detecting water – check pressure gauge and repressurise to 1 bar.
Blocking Errors (will clear automatically)
Little or no water in the system – check pressure gauge and repressurise to 1 bar.
Fan has stopped
No flame is detected. The boiler will block during 4 attempts before going to Locking fault
The boiler is operating at minimum burner load and there is a greater than 18°C Flow & Return temperature difference
Air lock or reduced water content – check pressure gauge and repressurise to 1 bar
Sensor disconnected or no water content – check pressure gauge and repressurise to 1 bar
As a pioneer of the Combi Boiler and continuing to deliver excellent quality products, many homes have chosen a Vaillant Boiler for their heat and hot water requirements.
Short circuit on the eBUS, eBUS overload or two power supplies with different polarities on the eBUS
Mass flow sensor connection fault
Mass flow sensor not connected/disconnected, plug not connected or incorrectly connected
Mass flow sensor fault
Mass flow sensor faulty, filter below venturi filter cap wet or blocked, gas flow pressure too low
Gas pressure fault (in combination with F.28/F.29)
No or too little gas supply pressure, gas valve closed
Fault: Mass flow sensor regulation
Gas valve defective, cable harness to gas valve defective
Fault during comfort safety mode
Ignition electrode highly corroded
Fault: Gas valve regulation
Short circuit/short to earth in cable harness for the gas valve
Gas valve defective (coils shorted to earth)
Fault: Gas valve switch-off delay
Delayed shutdown of gas valve
Delayed extinguishing of flame signal
Gas valve leaking Electronics defective
Supply or return NTC short circuited, electronics defective
Electronic temperature fault
Electronics overheating due to external influences, electronics defective
Implausible flame signal, electronics defective
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
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
Flow NTC fault
Flow temperature sensor signalling constant value:
Flow temperature sensor incorrectly positioned at supply pipe
Flow temperature sensor defective
Flow/return NTC fault
Flow/return NTC temperature difference too great → flow and/or return temperature sensor defective
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
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
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
Overheating protection on primary heat exchanger has responded
Cable or cable connections for safety fuse in primary heat exchanger or primary heat exchanger defective
Flue non-return flap/condensate pump fault
No response from flue non-return flap or condensate pump defective
Interruption to DHW outlet sensor at external controller
UK link box is connected, but hot water NTC not bridged
Flow and/or return temperature sesnsor 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
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
Fault: Flow and return temperature sensors incorrectly fitted
Flow and/or return temperature sensors have been fitted to the same pipe/wrong pipe
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.
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.
No communication with PCB
UK Manufacturer Ideal has been producing boilers for over 100 years and offer a great value and dependable reliability.
All Error Codes
BCC Activation Fault.
Low Water Pressure.
Outside Sensor Failure.
Low Mains Voltage.
Flow Temperature Overheat or No Water Flow.
5 Boiler Resets in 15 Minutes.
False Flame Lockout.