chris8042002

Newbie to this forum, but have been lurking for a while. Here goes: Almost every morning when I start my Pathfinder, I get a check engine light almost immediately, and some rough idling......take off down the road, and experience surging, hesitation, stumbling, near stalling at stop lights......after 5 minutes or so, the light goes out, the rig runs perfectly, like a top, and will do so for the rest of the day. Pulled codes today, got a 33: Exhaust Gas Sensor My confusion: After doing some research on this forum and others, it seems like the O2 sensor doesn't come into play when the vehicle is first started and cold. Should I just trust the code reading and replace the O2 sensor, or try something else (cheaper!) first? Could someone please explain the function of the O2 sensor in a cold engine? Thanks in advance, and I'll be sure to provide updates on this issue when I tackle it. Chris