Oxygen Sensors

I have a 96 taurus with just over 117,000 miles. It runs great but the service engine soon light has come on. It came on awhile ago and I have had the computer scanned multiple times. My mechanic said it was a bad oxygen sensor. I replaced two of them and had the light turned off but it came back on. Then he said it was the egr backpressure sensor so I replaced that and had the light turned off again. It stayed off until the next morning. I had the computer scanned again. Now it was another oxygen sensor. My mechanic said that my car has only 3 sensors, but a lot of place say that all tauruses have 4. I can only see the two that I replaced when the hood is up. There are diagrams that show 2 other sensors below the engine. So i don't know who is right. My mechanic said that he is going to fix it next week but i don't know if he is going to fix only the one that he believes is there, one of the two that may be there or both. P.S. The car is a GL. It is the 3.0L OHV V6? It is the bottom of the line, after the G. Thanks Rod.

Which engine does your Taurus have? If it's not a SHO, you will have either a 3.0L 12V V6, 3.0L 12V V6 FFV, or a 3.0L 24V V6. If the 24V, there will likely be badges on the front fenders that say "24V DOHC." If the FFV, there will be FFV badges on the fenders and or trunk lid.

According to the factory service manual, it looks like both regular gasoline engines have 4 oxygen sensors. It would appear the FFV engine only has 2 HO2 sensors (no post-catalyst sensors), but that seems odd.

-Rod