Secondary 02 Sensor

Service engine soon light came on and took 2000 4.3l to auto zone to have code pulled and they told me it was a secondary 02 sensor. i asked if they had one and the guy said the truck has a couple secondary sensors and he can't tell which one it is with the machine. does this sound right or is he just blowing smoke up my ass. also my heater fan only works in 3 and 4. any help would be appreciated.

the o2 senor is on the exhaust manifold output or by the catalytic converter just ask for the o2 sensor at the part store

For the Heater fan. I had a similar problem, mine worked on 1-3, but no 4. then completely.
1) change the speed selector switch. about $10 from autozone.
2) the fan selector switch (used to chose Def, vent, heater, a/c, ect) could also be bad.

Try the speed selector switch first. Then take it from there.
to fix mine I had to chance the fan selector switch, which suprised me, but it's worked great ever since. Oh, and when mine went, it was 5-7 degress Farenheit, this past Feb. It sucked.