Usually, yes. When a priest is formally excommunicated by the Church, you cannot participate in his sacraments. If you are unaware of this, you may participate, but once you know of his excommunication, you must avoid participating, as it would be a form of involvement in schism. It always depends on the situation.
However, if a priest has been unjustly excommunicated, for instance, simply for celebrating the Traditional Latin Mass and living a morally and spiritually good life as a priest, that is a different case. But such situations are very rare. In that case, the priest should not form his own separate community. Instead, he should seek a bishop or a good priest with whom he can collaborate, and work to clarify the issue of his excommunication. He should consider making an appeal to Rome, because excommunication is a very serious and significant punishment.
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