Causing a fault?

[yorkie67]

On my car the engine diagnostic lamp has lit up twice recently,the cars still under warranty so on the first visit they tol me it was a misfire on a cylinder and had changed the plugs,on the second time in they told me it was a loose wire.Now im not a trusting sole and these lads in the service center im getting bad reviews about them from other customers.What im asking for her is there any way i can make the light come on and go off just to see if its picking faults up and not just been pulled.
thanks