A Worcester Bosch boiler will display the E9 fault code if the heat exchanger or the thermostats and sensors thermostats connected to it are faulty.
The E9 fault code should come with a cause code. There are several ones, such as
This will be one of the following: 219, 220, 221, 224.
The fault code refers to the general issue concerning the heat exchanger, while the cause code pinpoints the exact issue.
Most likely, the heat exchanger is overheating. It is more likely to happen when a sensor on the exchanger detects a temperature higher than 105°C. In this case, there will be the 219 cause code next to the abovementioned fault code.
If there is 220, most likely, the temperature sensor on the heat exchanger is down
If your Worcester Bosch boiler is showing the 221 cause code, the PCB is not recognizing the sensor. This may be due to a problem in the sensor itself or wiring.
The 224 cause code indicates that the flue gas thermostat has heated up to the maximum temperature.