Answer : You are one step closer. You need to attach your custom validator to the FormGroup instead, because it needs to know two FormControl ( categories and mealTypes ), so attaching to FormGroup will give the validator more broad view and access to the entire FormControl To achieve that, change your ngOnInit to ngOnInit() { this.findForm = new FormGroup({ mealTypes : new FormControl(null, Validators.Required), categories : new FormControl(null) // others form control here }, validateMealType); // <-- see here is your custom function } On above code, you actually have to use FormGroup constructor instead of FormBuilder , so you can attach your custom validation in the parameters. Also, move your custom validator outside the component class. Take a look at this Plunker to get more insight for your specific case here. The solution proposed by @Michael worked for me with a minor change for the Angular 4. In the validation function...