Ok, I'll give you the facts. My car NEVER had any kind of a hesitation in the 50K miles that I've owned her until 10 seconds after pulling away from the muffler shop that installed my y pipe.

1. It will stumble/hesitate after starting to drive the car. It will do this if the car is ice cold or if I ran it 30 mins ago, but no matter the temp of the car, it will only do it for 5-10 seconds. So even if the car is ice cold, it goes through its hissy fit, and then is fine.

2. Once this stumbling stops, the car is every bit as strong as it ever was, even stronger with the warpspeed over my old cattman!

3. My gas mileage is still wonderful. I just drove a 100 mile treck on Monday morning and again last evening and both times it used around the first 1/4 tank(actually, a bit less).

So is it possible for an O2 sensor to work only once it gets a bit warm? My MIL light is not on.

 

If you never yourself replaced the o2 sensor I think it's the o2 sensor, you can buy one for $64.00 plus shipping from foreignautoparts.cc Also whoever installed the y-pipe could of damaged the o2 sensor when removing it or installing it.

 

Check the wiring. Maybe the shop accidentally tore a wire, unplugged it...