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Worcester Bosch Boiler Fault Codes

In the unfortunate event that your Worcester Bosch boiler develops an issue, it will enter a locking or blocking condition, and fault code information will be presented on the boiler display.

So that you can quickly identify what the fault code means and what should be your next course of action, we have developed the below fault code finders for the Worcester Bosch range.

Just click on your boiler from the below list and then search for the fault code that your boiler is displaying or view fault codes that aren’t displayed.

For a quick search, use your browser’s ‘find’ function. Use the below command and type the fault code to find it on the page:

  • Mac – CMD + F
  • Windows – CTRL + F

Learn more about Worcester Bosch

Worcester Bosch Greenstar i System/Combi Boiler Fault Codes

Typically most Worcester Bosch errors on this boiler model is accompanied by a blue flashing light to notify of an issue.

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

Worcester Bosch Greenstar Si Compact Boiler Fault Codes

A more compact version of the ‘i’ range.

Error Codes and Descriptions
9U Problem with the code plug.
9A Incorrect HCM fitted.
A1 The pump is stuck or running with air in the system.
B7 Potential control board problem.
C1 Airflow stopped during the operation.
C4 Airflow present during last 24 hour.
C6 Fan running too fast.
C7 Fan not running.
D1 Return sensor short circuit.
D1 Return Sensor Error: The sensor may be wet or damaged.
D4 The temperature difference between the flow and safety sensor exceeds the limit.
E2 Flow sensor short circuit error.
E5 The flow temperature is too high.
E9 Sensor Fault: Temperature too high, sensor short circuit or open circuit.
E9 Max Stat: The flue gas thermostat is overheating.
Error Codes and Descriptions
EA No flame detected or flame signal loss during operation. No ionisation was detected after the ignition.
EF Boiler operating at minimum burner load with temperature difference greater than 18°C between flow and return.
F0 Internal error.
F7 False flame or flame detected before burner started.
FA (306) Flame detected after burner stop.
FA (364) Gas valve leak.
FB The gas valve EV1 leak test failed.
FD Electrical power interruption.
212 Safety or flow temperature rising too fast.
T1 Ignition test.
T2 Fan test.
T3 Pump test.
T4 Three-way valve test.
T6 Ionisation oscillator test.
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Worcester Bosch Greenstar 8000 Gas Boiler Fault Codes

One of Worcester Bosch’s latest models in the Greenstar range. Available from 30kW to 45kW.

Error Codes and Descriptions
200 Boiler in heating mode.
201 The fan is not running on start-up.
202 Boiler in anti-cycle mode.
203 Boiler in standby mode.
204 The current primary water temperature is higher than the set value.
224 The safety temperature limiter has tripped – top-up water until the reset pressure is reached.
227 No flame signal after ignition burner control unit could need replacing.
228 Flame signal without flame present – air/gas ratio control valve may need replacing as might the ionisation cable.
305 Boiler in hot water anti-cycle mode.
306 Flame signal after closing the fuel supply – the air/gas ratio control valve might need replacing.
360 Incompatible code plug – check if a correct code plug is installed and exchange or reconnect the code plug.
815 Low loss header temperature sensor faulty – check the sensor port or check the differential sensor for incorrect installation position or breakage.
1010 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 Water pressure too low – the water needs topping up and the vent system or pressure sensor may need replacing.
1021 The hot water temperature sensor is defective – the plug to the hot water temperature sensor may not be connected.
1022 The hot water storage temperature sensor is defective – connect the plug to the hot water temperature sensor correctly.
1037 The outside temperature sensor is defective – check the connecting lead.
1065 Pressure sensor defective or not connected – check pressure sensor connection as it may need replacing.
Error Codes and Descriptions
1068 The outside temperature sensor is defective.
1073 Short circuit of the flow temperature sensor.
1074 No signal from the flow temperature sensor is available.
1075 Short circuit of the temperature sensor at the heating block.
1076 No signal from the temperature sensor at the heating block is available.
2920 Error with the flame monitoring.
2924 No feedback from the modulating gas valve. The gas valve relay is defective.
2925 Feedback from the modulating gas valve is too low.
2927 The flame failed during the burner operation.
2946 Incorrect code plug detected – the code plug needs to be replaced.
2948 No flame signal.
2963 Signal from flow and heat exchanger temperature sensor outside the permissible range.
2964 The flow rate in the heat exchanger is too low.
2965 The flow temperature is too high.
2966 Flow temperature rises in the heat exchanger too rapidly.
2967 The flow/heat exchanger temperature sensor differential is too great.
2970 The pressure drop in the heating system is too rapid.
2971 System pressure too low – the heating system needs venting or water needs topping up.

Worcester Bosch CDi Compact Boiler

Compact design, perfect for kitchen cupboards.

Error Codes and Descriptions
9U Problem with the code plug.
9A Incorrect HCM fitted.
A1 The pump is stuck or running with air in the system.
B7 Potential control board problem.
C1 Airflow stopped during the operation.
C4 Airflow present during last 24 hour.
C6 Fan running too fast.
C7 Fan not running.
D1 Return sensor short circuit.
D1 Return Sensor Error: The sensor may be wet or damaged.
D4 The temperature difference between the flow and safety sensor exceeds the limit.
E2 Flow sensor short circuit error.
E5 The flow temperature is too high.
E9 Sensor Fault: Temperature too high, sensor short circuit or open circuit.
E9 Max Stat: The flue gas thermostat is overheating.
Error Codes and Descriptions
EA No flame detected or flame signal loss during operation. No ionisation was detected after the ignition.
EF Boiler operating at minimum burner load with temperature difference greater than 18°C between flow and return.
F0 Internal error.
F7 False flame or flame detected before burner started.
FA (306) Flame detected after burner stop.
FA (364) Gas valve leak.
FB The gas valve EV1 leak test failed.
FD Electrical power interruption.
212 Safety or flow temperature rising too fast.
T1 Ignition test.
T2 Fan test.
T3 Pump test.
T4 Three-way valve test.
T6 Ionisation oscillator test.

Worcester Bosch CDi Classic Regular Boiler

Although no longer available, this was a very popular boiler. It’s now ben replaced by the 8000 Style boiler range.

Error Codes and Descriptions
B1 Code plug not detected.
C6 Fan speed too low or high.
E2 Central heating flow negative thermistor coefficient sensor defective.
E9 The safety temperature limiter in central heating flow has tripped.
EA Flame not detected.
Error Codes and Descriptions
F0 Internal error.
FA Flame detected after gas shut off.
F7 Flame detected even though appliance switched off.
FD Reset button pressed.

Worcester Bosch CDi Classic Combi Boiler

Although no longer available, this was a very popular boiler. It’s now ben replaced by the 8000 Style boiler range.

Error Codes and Descriptions
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.
A8 FX sensor controls electrical connections need checking.
B1 Code plug not detected.
C6 Fan speed too low.
E2 Central heating flow negative thermistor coefficient sensor defective.
Error Codes and Descriptions
E9 The safety temperature limiter in the central heating flow has tripped.
EA Flame not detected.
F0 Electrical connector contacts or programmer interface module.
F7 Flame detected even though the appliance is switched off.
FA Flame detected after gas shut off – the gas valve needs checking.
FD The reset button was pressed.

Worcester Bosch Faults That May Not Be Displayed

Appliance faults Remedy
Combustion noises too loud; rumbling noises ▶ Check the gas type.
▶ Check the gas supply pressure.
▶ Check the flue system; clean or repair if required.
▶ Check the gas/air ratio.
▶ Check the air/gas ratio control valve; replace if required.
Flow noises ▶ Set the pump rate or pump characteristic map correctly and match to the maximum
output.
Heat-up takes too long. ▶ Set the pump rate or pump characteristic map correctly and match to the maximum
output.
Flue gas values are incorrect; CO content too high. ▶ Check the gas type.
▶ Check the gas supply pressure.
▶ Check the flue system; clean or repair if required.
▶ Check the gas/air ratio.
▶ Check the air/gas ratio control valve; replace if required.
Violent ignition, poor ignition. ▶ Check the ignition transformer with service function t01 for misfiring, replace if required.
▶ Check the gas type.
▶ Check the gas supply pressure.
▶ Check the power supply.
▶ Check the electrodes with cable; replace if required.
▶ Check the flue system; clean or repair if required.
▶ Check the gas/air ratio.
▶ For natural gas: check the external gas flow monitor, and replace if required.
▶ Check the burner, and replace if required.
▶ Check the air/gas ratio control valve; replace if required.
Condensate in air box ▶ Check the non-return valve in the air-gas manifold, and replace if required.
Combi boilers: hot water outlet temperature is not reached. ▶ Check the turbine, and replace if required.
▶ Check the gas/air ratio.
Combi boilers: DHW volume is not reached. ▶ Check the plate heat exchanger.
No function, the display remains dark. ▶ Check the electrical wiring for damage.
▶ Replace defective cables.
▶ Check the fuse, and replace if required.
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