Does the “Kyrie Eleison, Christi Eleison, Kyrie Eleison” at the beginning of the mass have any power to forgive sin, or is it more of a formulaic part of the mass as a petition, but without any sacramental power in itself?
Yes, praying it will absolve you of venial sins.
I didn’t know that, I thought t was the confetier?
There are several penitential rites possible in the Roman Rite; the Confiteor is just one of them.
There is the rite of sprinkling with holy water; the Kyrie is omitted with this rite; then the words of absolution (“May almighty God have mercy on us, forgive us our sins,and bring us to everlasting life. Amen”).
The Confiteor, followed by the Kyrie Eleison, and then the words of absolution;
The Kyrie Eleison in the form of a trope (an invocation preceding the Kyrie Eleison and Christe Eleison parts), followed by the words of absolution.
The verses and responses:
V. Have mercy on us, O Lord.
R. For we have sinned against you.
V. Show us, O Lord, your mercy.
R. And grant us your salvation.
followed by the Kyrie Eleison and the words of absolution.
Note that is the formula of absolution (the conclusion of the penitential rite) said by the priest that grants us absolution from our venial sins, not the various formulae preceding those words; however those formulae are an integral part of the penitential rite and cannot be omitted; one or the other must be used.