Title states my situation.

Symptoms:

• Car idles mildly rough
  
• switching from reverse to drive on cold start sometimes triggers the overdrive light to flash on and off forcing me to turn the car off and turn it back on to temporarily fix it
  
• Going into 3rd gear, my RPMs drop significantly and performance takes a hit
  
• Shifting into drive in general takes about 2-3 seconds and before that, the car stalls
  

I've replaced the ignition coil packs and thrown in some NGK spark plugs and even replaced the VICS solenoid but I've read that a P0421 might involve the cat converter, bad spark plugs or an o2 sensor. What do you guys think?

 

1st step is to carry out diagnosis on the O2 sensor.