Unable to disable checkbox in Ionic 3.4.2

Ionic Framework ionic-v3
1

Hello,

I 'm trying to disable the checkbox untill the User enters the text in text field, but the Checkbox is never disable. Can some one help me on this? Appreciate your time

   <ion-list *ngFor="let fc of queFailCodes; let i = index;">
 <ion-item class="textbox">

                            <ion-label>
                                <ion-icon name="information-circle" (click)="showMoreInfoForAns('que', questn)"></ion-icon>
                                <strong>{{fc.failCode}}</strong>{{fc.failDesc}}
                            </ion-label>
                            <ion-checkbox [(ngModel)]="fc.failCodeSelected" [disabled]="fc.failCodeSelected ? 'true' : 'false' " (ionChange)="failCodeSelectedChanged(fc)"></ion-checkbox>
                        </ion-item>
   </ion-list>
//Function called 
failCodeSelectedChanged(item){
    if(item.assignee.name !== "" && item.reviewer.name !== "" && item.assigneeCompletionDate !== "" && item.reviewerCompletionDate !== "" && item.severity !== 0 && item.failureCompletionDate !== ""){
          item.failCodeSelected =  true;
          this.isChecked = true;
      } else{
          this.isChecked = false;
         item.failCodeSelected =  false;
      }
  }

Let me know for further information.
Thanks,
Sandeep