How to find the root cause for any quality issues and how to take corrective action and preventive action?

Currently I am working in automotive field of filter manufacturing for trucks,passenger car and heavy duty equipments to all kinds of filters like spin on oil,fuel ,air filters and turbine gas system filters and heavy pressures filters and moulded filters,also panel filters producing and quality checking by using some ISO standard testing.My previous experience in nickel,tin and acid zinc plating at M/S Needle industry India ltd at ooty, Tamil nadu, there my job roll was chemists checking of all incoming material for process means chemical analysis and physical testing like bursting strength,tearing strength,pull strength,tensile strength for all types of wires like PB wire and high Corbon steel , brass wires and final product testing CASS test etc.Nearly four years in lab testing after that I was worked as a production chemist same industry my work was vat analysis and vat treatment and also effluent treatment . and small experience in Anodizing process and gold plating also.and iso related work.