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)

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

Flame not established during the first 4 seconds ignition period

EA

Gas valve coil disconnected

EA

There is an error where the pump is not detecting water – check pressure gauge and repressurise to 1 bar.

Blocking Errors (will clear automatically)

A1

Little or no water in the system – check pressure gauge and repressurise to 1 bar.

C1

Fan has stopped

EA

No flame is detected. The boiler will block during 4 attempts before going to Locking fault

EF

The boiler is operating at minimum burner load and there is a greater than 18°C Flow & Return temperature difference

No Code

Air lock or reduced water content – check pressure gauge and repressurise to 1 bar

260

Sensor disconnected or no water content – check pressure gauge and repressurise to 1 bar

Vaillant Boilers

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.

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 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

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 Boilers

UK Manufacturer Ideal has been producing boilers for over 100 years and offer a great value and dependable reliability.

All Error Codes

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.