Any hopes Stoffel Vandoorne may have had of achieving at Monza his second top-ten finish of the year have likely been scrapped thanks to a 25-place grid penalty.
The Belgian was all set to start P8 this afternoon after reaching Q3 in yesterday's delayed and disrupted qualifying session.
But a loss of power late in the day and a subsequent analysis by Honda has forced a power unit change on the McLaren driver and hefty grid drop.
Vandoorne takes on a seventh Internal Combustion Engine, seventh MGU-K, 10th MGU-H and 10th Turbocharger.
"After the issue we had with Stoffel's PU during yesterday's Q3 session, the decisions was made to replace his PU this morning," said a Honda.
"The problem was with the MGU-K shaft.
"Although it is possible to replace the MGU-K shaft without having penalties, due to parc ferme conditions we judged we would not have enough time to do so."
Honda also confirmed that Vandoorne's replacement PU will be a 'Spec 3.7' unit.
Vandoorne should be demoted to P18 on this afternoon's grid, where he'll line up just in front of team mate Fernando Alonso.