SE means that the boiler service interval is up, if it's been serviced recently then the the timer needs re-set, it shouldn't affect the day to day running. get british gas out to re-set it for you, or if you like you can download the instruction here....
http://www.worcester-bosch.co.uk/cache/file/239/guide-to-re-setting-the--se--code-on-greenstar-cdi-boilers.pdf
Ian
January 2010
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