Just had a car come in the other day with a lazy post O2 sensor. The other tech was going to quote the customer with a cat replalcement (Sensor 2 O2sensor is only used to check Catalyst effeciency) until I looked at a very slow and erratic waveform.
Your pre o2 sensor could also be the culprit as the PCM simply compares the pre and post O2 readings to determine cat effeciency. But then again you would have gotten a cat code-
Repalce them all I say.
It is possible to have 4 O2 sensors with a vehicle that has a single exhaust as the O2 sensors are placed as close to the exhaust manifold as possible to aid in bringing them quickly up to tempreture, then after the cat the exhaust is collicted into 1 pipe. Infact I think its OBD2 mandated on all vehicles with more than 4 cylinders to have 4 O2 sensors.
Perhaps your right- Ive seen alot of OBD2 (1996 and up)V engined cars with 4 O2 sensors (one for each bank) but Im not completely sure if its OBD2 mandated or not. I guess if you look at your emmissions decal under the hood it will probably have it written as 2HO2S (2 heated O2sensors) 2TWC (2 three way cats) AIR ( air injection reaction) EGR (exhaust gas recirculation) SFI (sequential fuel injection) and then say wether the vehicle conforms to Federal or California emissions standards.