Where are the oxygen sensors for 1999 malibu 3.1

[mamachipy]

I have a 1999 malibu with the 3.1 V-6 and it has 107,000+ miles on it. It was hesitating durring accelration and I decided to replace the O2 sensors. Every site I checked said i needed 3 sensors, 1 post converter and 2 fronts. I got the sensors and replaced the post converter sensor and the one at the firewall at the Y in the exhaust manifold. I would have guessed the other one would have been in the front but I can not seem to fine it. I have looked eveywhere I could think and even felt around the exhaust manifold, nothing. I anyone knows the secret location let me know. If not I will have to go and buy a manual that will tell me it does not exist, ha,ha.