Worcester Bosch Gas Boiler Error Codes – What They Mean and What to Do

If your Worcester Bosch gas boiler is showing an error code, don’t panic — it’s the boiler’s way of letting you know something isn’t right.
Most issues can be sorted quickly, especially with the help of a qualified professional.

At TP Gas, we specialize in Worcester Bosch gas boiler repairs in Limerick.
If you’re seeing an error code, we can diagnose the problem and get your heating system back up and running safely and efficiently.

Here’s a quick guide to some of the most common Worcester Bosch error codes — and what they usually mean.

Common Worcester Bosch Gas Boiler Error Codes

Error Code	Meaning	Typical Cause
EA	No flame detected	Gas supply issue or ignition fault
E9	Overheating	Blockage, pump failure, or sensor fault
C6	Fan not running or running slowly	Blocked air vents or faulty fan
A1	Low water pressure	Leak in the system or pressure drop
E5	Flow sensor error	Sensor failure or blockage
D1	Sensor fault	Faulty temperature sensor
E2	Faulty NTC thermistor	Temperature sensing issue
F0	Internal control error	PCB (circuit board) fault
E4	Flow temperature sensor fault	Faulty or disconnected sensor
E6	Flow temperature differential too high	Blockage or restricted circulation
H07	Low system pressure warning	System needs topping up
CE	Communication error	Fault between boiler control and display
C7	Fan speed fault	Fan not running at correct speed
A7	Outside sensor fault	External temperature sensor issue
A8	Return sensor fault	Sensor problem or wiring issue
C1 264	Air pressure switch fault	Faulty switch or flue obstruction
E8	Water pressure sensor fault	Sensor failure or low water pressure
H11	Gas valve control fault	Gas valve issue
9A	Boiler configuration error	Fault in boiler setup parameters
9U	Control unit fault	Issue with control board or communication
2970 B	Flame detected when not expected	Possible valve fault or control board error

These are just a few examples — Worcester Bosch boilers are very good at spotting issues early so you can take action before something bigger goes wrong.

Worcester Bosch Greenstar i System/Combi Boiler Error Codes

The Worcester Bosch Greenstar i was the brand’s best-selling combi boiler, with over 1 million units installed across UK homes. Available in both combi and system versions (sharing the same error codes), it came in two sizes: the 25i, ideal for smaller homes, and the 30i, suited for average three-bedroom properties. It has since been replaced by the upgraded Greenstar 4000.

Fault Code	Description
FD	Power interrupted
C7	Fan not running on start-up
C6	Fan speed too high or too low
C1	Fan stopping during operation
C4	Fan has failed to stop
E2	Flow temperature issue
E9	Primary flow temperature exceeding 95°
FA	Ionisation detected after gas valve closed
EA	No ionisation detected after ignition
F7	Ionisation detected before burner start
9U	Control box defective or loose
C4	Control box or heat control module defective
F0	Gas valve/control box error
A8	Heat control module invalid
CC	Outdoor sensor defect
A7	Domestic hot water sensor defect
A1	Pump stuck or running dry
E9	Flue or high limit thermostat activated
D4	Primary flow temps rising too fast

PS Typically a Worcester Bosch error code is accompanied by a blue flashing light.

Worcester Bosch Greenstar Si Compact Error Codes

The Worcester Bosch Greenstar Si is similar to the Greenstar i and is ideal for small to medium-sized homes. Its standout feature is its compact design, making it suitable for installation in tight spaces like kitchen cupboards.

	Fault code description
T1	Ignition test
T2	Fan test
T3	Pump test
T4	Three way valve test
T6	Ionisation oscillator test
9A 362 error	Incorrect HCM fitted
9U 233 Heat Control Module (HCM) error	Problem with code plug
B7 257 Internal error	Potential control board problem
C6 215 Fan problem	Fan running too fast
C7 214 Fan problem	Fan not running
D1 240 Return sensor error	Sensor may be wet or damaged
E2 222	Flow sensor short circuit error
E5 218	Flow temperature too high
E9 219 Safety sensor fault	Temperature too high, sensor short circuit or open circuit
EA 227	No flame detected or flame signal loss during operation
F0 237	Internal error
F7 228 Flame error	False flame or flame detected before burner started
FA 306 False flame fault	Flame detected after burner stop
FD 231 Mains power fault	Electrical power interruption
FA 364 Gas valve EV2 leak test failed	Gas valve leak
FB 365	Gas valve EV1 leak test failed
A1 281	Pump stuck or running with air in the system
C1 264	Airflow stopped during operation
C4 273	Airflow present during last 24 hour
D4 271	Temperature difference between flow and safety sensor exceeds limit
D1 240	Return sensor short circuit
E9 224 Max thermostat activated	Flue gas thermostat overheat
EA 227	No ionisation detected after ignition
EF 349 Central heating boil detected	Boiler operating at minimum burner load with temperature difference greater than 18°C between Flow & Return
NO CODE 212	Safety or flow temperature rising too fast

Worcester Bosch CDi Classic Regular Boiler

The Worcester Bosch CDi Classic range has been discontinued and replaced by the Greenstar 8000 Style. Available in multiple outputs (29kW to 42kW) and models including combi and heat-only, it suited everything from standard homes to larger properties with higher hot water demands.

	Fault code description
B1	Code plug not detected
C6	Fan speed to low or too high
E2	Central heating (CH) flow negative thermistor coefficient (NTC) sensor defective
E9	Safety temperature limiter in central heating (CH) flow has tripped
EA	Flame not detected
F0	Internal error
F7	Flame detected even though appliance switched off
FA	Flame detected after gas shut off
FD	Reset button pressed by mistake

Worcester Bosch CDi Compact Error Codes

The Worcester Bosch CDi Compact is a popular combi boiler available in 28kW, 32kW, and 34kW outputs, ideal for small to medium-sized homes. Its compact design fits neatly into a standard kitchen cupboard, making it a great choice for space-conscious installations.

	Fault code description
T1	Ignition test
T2	Fan test
T3	Pump test
T4	Three-way valve test
T6	Ionisation oscillator test
9A 362 error	Incorrect HCM fitted
9U 233 Heat Control Module (HCM) error	Problem with code plug
B7 257 Internal error	Potential control board problem
C6 215 Fan problem	Fan running too fast
C7 214 Fan problem	Fan not running
D1 240 Return sensor error	Sensor may be wet or damaged
E2 222	Flow sensor short circuit error
E5 218	Flow temperature too high
E9 219 Safety sensor fault	Temperature too high, sensor short circuit or open circuit
EA 227	No flame detected or flame signal loss during operation
F0 237	Internal error
F7 228 Flame error	False flame or flame detected before burner started
FA 306 False flame fault	Flame detected after burner stop
FD 231 Mains power fault	Electrical power interruption
FA 364 Gas valve EV2 leak test failed	Gas valve leak
FB 365	Gas valve EV1 leak test failed
A1 281	Pump stuck or running with air in the system
C1 264	Airflow stopped during operation
C4 273	Airflow present during last 24 hour
D4 271	Temperature difference between flow and safety sensor exceeds limit
D1 240	Return sensor short circuit
E9 224 Max thermostat activated	Flue gas thermostat overheat
EA 227	No ionisation detected after ignition
EF 349 Central heating boil detected	Boiler operating at minimum burner load with temperature difference greater than 18°C between Flow & Return
NO CODE 212	Safety or flow temperature rising too fast

Worcester Bosch CDi Classic Combi Boiler Error Codes

Launched in 2012, the Worcester Bosch CDi Classic was the brand’s top-tier combi boiler, offering outputs from 29kW to 42kW. Ideal for homes with high hot water demand, it has since been replaced by the Worcester Bosch 8000 range.

	Fault code description
A1	Water is leaking or the pump needs replacing or freeing due to the pump running dry
A7	The hot water negative thermistor coefficient (NTC) sensor is defective – the hot water sensor or connecting leads need checking
A8	Break communication to FX sensor controls electrical connections need checking
B1	Code plug not detected
C6	Fan speed too low – the fan lead and connector need checking and may need replacing
E2	Central heating (CH) water flow negative thermistor coefficient (NTC) sensor defective check CH flow NTC sensor and connection leads
E9	Safety temperature limiter in central heating (CH) flow has tripped the system pressure or safety temperature limiter needs checking
EA	Flame not detected due to a gas issue – the gas supply, power supply or igniter, electrode and lead need checking – contact a Gas Safe installer
FO	Electrical connector contacts or programmer interface module – check ignition leads are not loose
F7	Flame detected even though the appliance is switched off – check the electrode assembly is dry and the pcb and flue are clear
FA	Flame detected after gas shut off – the gas valve needs checking – hire a Gas Safe installer
FD	Reset button pressed by mistake – press reset button again

Worcester Bosch CDi Highflow Error Codes

The Worcester Bosch CDi Highflow is a powerful floor-standing combi boiler with exceptional hot water flow rates, ideal for large homes with multiple bathrooms. It doesn’t require a separate water tank and can even be installed beneath a kitchen counter.

	Fault code description
A1	Pump dry run detection or pump is running in the air the system pressure needs checking
A5	Tank negative thermistor coefficient (NTC) defect
A7	Domestic hot water (DHW) negative thermistor coefficient (NTC) defect
A8	Energy Management System (EMS) communication error
B1	Code plug not detected
B2, B3, B4, B5, B6	Data error
B7	Burner control error
C6	Fan defect
D3	External temperature limiter
D5	Condensate pump failure
D6	Internal heat bank overflow
EA	Flame not detected
E2	Primary negative thermistor coefficient (NTC) defect
E9	Safety temperature limiter in central heating (CH) flow stat tripped
F0	Internal error
F1	ROM fault
F7	Flame detected after appliance has been shut off
FA	Flame detected after gas shut off
FD	Reset button pressed – try pressing the reset button again

Worcester Bosch 8000 Style Error Codes

The Worcester Bosch 8000 Life is a premium, high-performance combi boiler with outputs from 30kW to 45kW — ideal for large homes or those with high hot water demand. With a sleek design available in black or white, it combines top-tier efficiency with standout aesthetics.

	Fault code description
200 O	Boiler in heating mode
201 O	Fan not running on start-up
202 O	Boiler in anti-cycle mode
203 O	Boiler in standby mode
204 O	Current primary water temperature higher than set value
208 O	Chimney sweep demand
224 V	Safety temperature limiter has tripped – top-up water until the reset pressure is reached
227 V	No flame signal after ignition burner control unit could need replacing
228 V	Flame signal without flame present – air/gas ratio control valve may need replacing as might the ionisation cable
305 O	Boiler in hot water anti-cycle mode
306 V	Flame signal after closing the fuel supply – the air/gas ratio control valve might need replacing
360 V	Incompatible code plug – check if a correct code plug is installed and exchange or reconnect code plug
815 W/B	Low loss header temperature sensor faulty – check the sensor port or check the differential sensor for incorrect installation position or breakage
1010 O	No BUS communication cable, EMS-Bus isn’t connected or damaged – connect the cable for the EMS-Bus or replace the damaged cable exchange fuse
1017 W	Water pressure too low – the water needs topping up and the vent system or pressure sensor may need replacing
1021 B	Hot water temperature sensor is defective – the plug to the hot water temperature sensor may not be connected
1022 B	Hot water storage temperature sensor is defective – connect the plug to the hot water temperature sensor correctly, mount the hot water temperature sensor correctly or replace the hot water temperature sensor
1037 W	Outside temperature sensor is defective – check the connecting lead
1065 B	Pressure sensor defective or not connected – check pressure sensor connection as it may need replacing
1068 W	Outside temperature sensor defective – check the connecting lead
1073 W	Short circuit of the flow temperature sensor – the temperature sensor may need replacing
1074 W	No signal from the flow temperature sensor available – connect the plug to the flow temperature sensor correctly or temperature sensor could need replacing
1075 W	Short circuit of the temperature sensor at the heating block – temperature sensor may need replacing
1076 W	No signal from the temperature sensor at the heating block available – replace the temperature sensor, the connecting lead or the control unit
2920 V	Error with the flame monitoring
2924 V	No feedback from the modulating gas valve. The gas valve relay is defective – press the reset button and apply burner start, wait to see if fault reoccurs, if it does the gas valve needs replacing
2925 V	Feedback from the modulating gas valve is too low
2927 B	Flame failed during burner operation. Open the main shut off valve, shut down the appliance and check gas line, replace the ionisation electrode, set burner correctly and minimum rated load, check the integrity flue system and if the interconnected room air supply is too small, or the size of the ventilation opening is too small, clean the heating block on the flue gas side or replace the control unit/burner control unit
2946 V	Incorrect code plug detected – the code plug needs replacing
2948 B	No flame signal. With low output the burner starts automatically after cleaning. If the fault occurs repeatedly, the setting of the CO2 valves need checking
2963 B	Signal from flow and heat exchanger temperature sensor outside the permissible range – the connecting lead needs connecting correctly
2964 B	Flow rate in heat exchanger is too low
2965 B	Flow temperature too high
2966 B	Flow temperature rise in heat exchanger too rapid
2967 B	Flow / heat exchanger temperature sensor differential too great
2970 B	Pressure drop in heating system too rapid
2971 B	System pressure too low – the heating system needs venting or water needs topping up until the preset pressure is reached. Replace the cable to the pressure sensor replace the pressure sensor

What Should You Do If You See an Error Code?

Some minor issues can be sorted by resetting the boiler or topping up the pressure (check your boiler manual for instructions).
However, if you’re unsure — or if the error code keeps coming back — it’s important to call a qualified technician.

Safety first: Never attempt to open or repair a gas boiler yourself.
Gas appliances must always be handled by trained, RGII-registered professionals.

At TP Gas, we’ll: