Symptom: 
 
  
 
 
  
  
   
   
    
    
    
   
   
    
    
    
     Diagnostic help
    
   
   
    To compare the customer's car with another almost identical car, which you know is in full working order, helps to establish whether the customer's problem can be a characteristic of the car's design. To reach a valid conclusion, the comparison should be performed under the same conditions and with the same criteria on a car that has the same optional equipment as the customer's car.