Coroutine not working correctly?

I am trying to get the closest target to the unit using Physics2D.OverlapCircleAll, but I do not think that the coroutine is actually running and I don't know how to fix it. Please help.

 using UnityEngine;
 using System.Collections;
 
 public class Target : MonoBehaviour
 {
 
     public LayerMask layer;
     public float targetRange = 2f;
     public Transform target;
 
     public float currentDist;
 
     public Transform currentTarget;
 
     public Collider2D[] cols;
 
     private RaycastHit2D hit;
 
     private Vector2 thisPos;
 
     // Use this for initialization
     void Start ()
     {
         StartCoroutine(ScanForTargetAllAround());
     }
     
     // Update is called once per frame
     void Update ()
     {
         thisPos = new Vector2(transform.position.x, transform.position.y);
 
         cols = Physics2D.OverlapCircleAll(thisPos, targetRange, 1 << LayerMask.NameToLayer("Unit"));
 
         if (!target)
         {
             hit = Physics2D.Raycast(transform.position, Vector2.up);
             Debug.DrawRay(transform.position, Vector2.up);
         }
 
         if (target)
         {
             var direction = target.position - transform.position;
 
             hit = Physics2D.Raycast(transform.position, direction);
             Debug.DrawRay(transform.position, direction);
 
             float angle = Mathf.Atan2(direction.y, direction.x)*Mathf.Rad2Deg;
             transform.rotation = Quaternion.AngleAxis(angle, Vector3.forward);
         }
     }
 
     IEnumerator ScanForTargetAllAround()
     {
         while (cols.Length > 1)
         {
             currentTarget = null;
 
             float distance = Mathf.Infinity;
 
             Collider2D currentCollider2D = cols[0];
 
             for (int i = 0; i < cols.Length; i++)
             {
                 currentDist = Vector2.Distance(thisPos, cols[i].transform.position);
 
                 if (currentDist < distance)
                 {
                     currentCollider2D = cols[i];
 
                     distance = currentDist;
                 }
             }
 
             currentTarget = currentCollider2D.gameObject.transform;
     
             target = currentTarget;
 
 
         }
         
         yield return null;
     }
 
 }