Angular 2 observable error testing with jasmine

I want to test this function:

register(): void {
let user: User = new User();
user.username = this.username.value;
user.email = this.email.value;
user.password = this.password.value;
this._authService.register(user)
  .map(rsp => rsp.json())
  .subscribe((response) => { // 
    this._router.parent.navigate(["Login"]); // 
  }, (error) => {
    this.responseError = JSON.parse(error._body).message; 
  }, () => {
    this._authService.login(user)
      .map(rsp => rsp.json())
      .subscribe((data: any) => { // 
        this._authService.handleSuccessLogin(data, user);
        this._router.parent.navigate(["../Game"]);
      });
  });
}

My _authService using http but I want to fake that call. I have tried to call through in it and mocking the http, but even if my response was 4xx it ran on the success part. Is it possible to test the error part somehow?