Porsche mechanic Robert Schenker was found dead at his home in Peterborough, Cambridgeshire, in March 2006.
Three days earlier builder David Johnson had started reconstructing a chimney, Norwich Crown Court was told.
Mr Johnson, 56, also of Peterborough, denies manslaughter and exposing Mr Schenker to risk.
It is alleged Mr Johnson allowed mortar to drop and drip into the chimney, which was connected to the flue pipe on Mr Schenker's kitchen boiler, said prosecutor Deanna Herr.
The mortar solidified and blocked the chimney, preventing carbon monoxide from escaping into the atmosphere, she said.
The poisonous gas leaked into Mr Schenker's home and killed him, jurors were told.
The case continues.
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