As laid down in the Basic Law, electoral scrutiny is the responsibility of the German Bundestag. It has to check whether the established election result has been obtained in a lawful manner.
However, the Bundestag will act only when an objection has been lodged.
Article 41 of the Basic Law (GG)
Section 49 of the Federal Elections Act (BWG)
Law on the Scrutiny of Elections (WahlPrG)
Section 48 of the Federal Constitutional Court Act (BVerfGG)