Gents:
I am a newbie to this forum. In reading most of the previous posts, I am truly impressed with the quality and friendliness of the responses.

My vehicle has 220,000 km on the clock. Last week, the O2 light came on and has remained on, despite a battery disconnect.

The car is now very hard to start. When it does light up, it runs rough for a few minutes and then appears normal. The next day, the problem repeats except it is becoming increasingly difficult to start.

My question is:
Can I determine the cause with an OBD reader or do I replace the O2 sensor on spec? Or, God forbid, am I looking at catalytic converter replacement?