The external temperature sensor will be outside the property - the installer will very probably put the sensor relatively close to the boiler position but on an external wall.
The sensor connects to terminals 3 & 4 as shown on page 17 of the manual here...
https://tinyurl.com/npuwv9r
You might try disconnecting the sensor as a diagnostic check.
It may have been damaged.
Good Luck...
September 2015
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