King City Police Officer Jose David Hernandez was found guilty April 2 of brandishing a firearm, a misdemeanor.
The case is unrelated to a series of arrests in February 2014, when about a third of King City's police department was arrested on various charges.
Hernandez had been renting a room in the same house as his ex-girlfriend, and just before midnight on Halloween last fall, overheard her being intimate with another man, according to sheriff's deputies.
He said he just “couldn’t take it anymore,” according to prosecutors, and armed himself with a loaded semi-automatic pistol then pushed open his ex's bedroom door.
He then interrogated the couple, gun in hand, for about 15 minutes about their relationship, and refused to back down despite at least 10 requests by the woman to leave the bedroom.
The verdict was found by Monterey County Superior Court Judge Andrew Liu in a court trial.
Hernandez will be sentenced on April 24, and faces a minimum of three months in jail. He'll also be prohibited from possessing a firearm for 10 years.