An E20 fault on baxi and potterton boilers is caused by the failure of the primary temperature sensor in the boiler .
Resetting the boiler won’t fix this fault.
When the boiler starts up it checks the temperature sensors to make sure they are in the correct range.
If they are out of the expected range you get an E 20 fault code. (The boiler can only show two digits at a time so it’s split between E and 20).
Resetting the boiler won’t make the sensor go back in range so resetting is a waste of time.
How do the sensors work?
The sensors in most boilers are resistors that change resistance depending on their temperature.
A low temperature give a low resistance and a high temperature gives a high resistance. This relationship between temperature and resistance is consistent so the boiler knows what to expect. The boiler burns gas at higher or lower rates depending on the temperature it senses at the sensors.
Safety Warning
The sensor should only be changed by a gas safe registered engineer. This is not a DIY job. The boiler should be safely isolated from the power supply. The power isolation should be confirmed with appropriate test equipment. Every gas safe registered engineer knows they would never work on a live appliance. Once any part has been changed safety checks must be carried out on the boiler.
What happens with a faulty sensor?
If the sensor goes out of the expected range the boiler displays an e20 fault code.
If the sensor starts to develop a fault but stays inside the expected range the boiler can have other problems. If the sensor stays inside the expected range it will not show a fault. But the boiler will do strange things like not heating up enough or getting too hot.
How is the sensor changed?
It’s located in the combustion box of the boiler. To change this sensor you have to open the combustion box which can damage seals. This can be very dangerous.
The sensor sits in a dry pocket so I had no trouble from draining the boiler down.
There is always some heat sink compound on the sensor, I transfer as much as I can.
To make room I gently wiggled the overheat sensor harness off the over heat thermostat. The overheat sensors do get fragile and can break when touched. Then I released the harness from the sensor.
I turned the sensor with a little spanner. I use an adjustable 4″ like this one from amazon. They aren’t normally very tight as they aren’t holding any water in the pipework.
The new sensor gets as much heat sink compound transferred to it as possible. You won’t need to add extra. Then I gently screw it back into position. It doesn’t need to be very tight, just enough to hold it in position.
Once both the harness leads are back on the stats the boiler is ready to go.
There is one great thing about this type of fault. When the fault is there the boiler is off and there is no doubt about that. When the sensor is changed the fault is either still there or it’s gone. There is no doubt about that either.
Intermittent faults are the opposite of that. The fault happened a few days ago, and you can wait days for the fault to come back or not. You are never sure that you’ve fixed it.
What if it is not the sensor?
What if you have the sensor changed but the E20 fault is still there? Test the sensor with a multi meter to see if they are in the spec the manufacturer gives.
(This link is a useful list of various boiler sensor readings). If they aren’t then it will be the PCB. Your gas safe registered engineer will have to replace the PCB and carry out the required safety checks.
Conclusion
In this case when I turned the boiler back on the fault was gone. The heating and hot water were working again and every one was happy.
E20 faults are simple for a gas safe engineer to fix.
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