Boiler Fault Codes: What to check

Boilers are a sophisticated piece of machinery and have may working parts which all need to be in synch. This is why we are always recommending an annual boiler service so you can head of any issues before they catch you at the worst possible time – on a cold weekend!

Luckily if something is not working the boiler flashes a fault code so no guess work is needed. Of course each model of boiler has a different fault codes so we look at the three most common here – Ideal, Worcester and Valliant. Refer to your boiler manual for more information.

Some error codes are more common than others such as:

D2 (Worcester) or F27 (Vaillant) – blocked condensate pipe.
U9, A1 or No code (Worcester), F75 (Vaillant), or F1 (Ideal) – low water pressure. Follow your manufacturer’s manual to top up your boiler.
F75 (Vaillant) – sensor change. This is common and usually means the boiler’s sensor is unaware that the pump has started running.

Ideal Boiler Fault 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

Worcester Bosch Boilers

Locking error – A flashing fault code & warning triangle. Try the reset button if this occurs.

Block error – No data is shown and your boiler will stop working. Find the 3 digit cause code by pressing the spanner / return button.

Fault error – An alpha numeric code will indicate the error.

Maintenance code – H codes show there is a maintenance issue.

Common Locking Errors

9A – Incorrect HCM fitted
9U – Faulty or disconnected code plug
B7 – Replace control board
C6 – Running too fast / too slow
C7 – Fan not running or airflow blocked
D1 – Wet or damaged sensor
D1 – Overheating
E2 – Flow sensor shorted, disconnected or damaged
E5 – Primary flow sensor overheated
E9 – Main heat exchanger overheated. Max temperature 105°C
E9 – Maximum temperature sensor on the main heat exchanger has failed or 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, or 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.

Common Blocking Errors

A1 – Little or no water in the system – check pressure gauge and repressurize to 1 bar.
C1 – Fan has stopped
EA – No flame 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 repressurize.
260 – Sensor disconnected or no water content – check pressure gauge and repressurize.

Vaillant Boiler Fault Error Codes

F.49 – eBUS, short circuit, overload or two power supplies with different polarities
F.52 – Mass flow sensor disconnected
F.53 – Mass flow sensor filter below venturi filter cap wet or blocked, gas flow pressure too low
F.54 – Gas pressure fault – too little gas supply pressure, gas valve closed
F.56 – 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 – Gas valve regulation – Short circuit in cable harness for the gas valve
F.62 – Gas valve switch-off delay
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 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-circuited at 5V/24 V or internal fault in water pressure sensor
F.75 – 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
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-existent or too small. Not enough water in the boiler. Flow and/or return temperature sensor not in correct position at pipe
F.84 – Flow/return temperature sensor difference implausible – sensors have been inverted or incorrectly fitted
F.85 – Flow and/or return temperature sensors have been fitted to the same pipe/wrong pipe
F.92 – 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 – 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.

M P Plumbing

A qualified plumber at mpplumbingservices can demystify the boiler fault codes for you and get a quick fix in place. Remember to first check whether your boiler is in warranty as you may be able to get the manufacturer to fix it for free – though they often have a two day lead time and you may need your heating and hot water sooner than that!

Call M P Plumbing today on 07779 303880 or complete our contact form.

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