angular2 - how to simulate error on http.post unit test

I m writing a Uni-test for a login Method with an HTTP.post call, like:

this.http.post( endpoint, creds, { headers: headers})
        .map(res => res.json())
        .subscribe(
        data => this.onLoginComplete(data.access_token, credentials),  
        err => this.onHttpLoginFailed(err),
        () => this.trace.debug(this.componentName, "Login completed.")
    );

The problem is that i'm not able to simulate the error branch; everytime is called the onLoginComplete Method;

here is my test:

it("check that  Atfer Login, console show an error ", inject(
  [TraceService, Http, MockBackend, WsiEndpointService],
  (traceService: TraceService, http: Http, 
   backend: MockBackend, wsiEndpoint: WsiEndpointService) => {

  let tokenTest: number =  404 ;

  let response: ResponseOptions = null {} // i think i have to modify this

  let connection: any;

  backend.connections.subscribe((c: any) => connection = c);

  let authService: AuthService = new AuthService(http, Service1, Service2);

  authenticationservice.login({ "username": "a", "password": "1" });

  connection.mockRespond(new Response(response));

  expect(ERROR);

}));

Thanks again to everyone.