p0300 random misfire and p030x

these are codes pulled from korean hyundai :


Address 0E: 6 Faults Found:
P0420 - Catalyst System; Bank 1: Efficiency Below Threshold
P0301 - Cylinder 1: Misfire Detected
P0302 - Cylinder 2: Misfire Detected
P0303 - Cylinder 3: Misfire Detected
P0304 - Cylinder 4: Misfire Detected
P0300 - Random/Multiple Cylinder Misfire Detected


· The downstrwam o2 sensor was as flactuating as upstream o2 sensor,one of possible solutions introduced by www.obd-codes.com is to Inspect the downstream heated oxygen sensor (HO2), and replace if necessary and other solution is to Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required. .
· M y question no. 1 : is why should replace downstream o2 sensor, I know it just only tell the ecu how catalytic converte is or otherwise is catalytic coverter good oood or not?
· Ouestion 2 : what is the effect of exhaust t leak on misfiring or catalytic concerter?
· Question 3 I think misfiring may be caused by pluged catalytic converter ( partial or fully) , is replacing it can solve the misfire problem?
Note : sparg pluges , wires coils have been changed .

If the downstream fluctuates as quickly as the upstream then it normally means the catalyst isn't buffering the pollutants out of it. When this happens it's because the converter has failled. A plugged up converter can also generate misifires. Im skeptical the downstream 02 will fix any of your issues.