Help with 02 sensor confusion please!
 

My fuel mileage went in the toilet over the last couple weeks (20 average to around 12-13) and after reading the trouble codes I pulled P0137 - 02 sensor low voltage bank 1 sensor 2, and P0171 - System Too Lean (bank 1) and P0174 - System Too Lean (bank 2) which usually come up in conjunction with each other (I'm thinking due to the poor readings from the sensor that is throwing code P0137 or a dirty MAF). I found this image here on the forum, but I am confused because most I'm reading about code P0137 is that the offending sensor is the one BEHIND the cat B1S3, and not the one depicted in the image AHEAD of the cat. A little assistance in determining which sensor I should replace would be appreciated. Thanks guys.
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Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)

Read more at: OBD-II Trouble Code: P0137 Oxygen O2 Sensor Low Voltage (Bank1, Sensor2)
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Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)

Before you get too caught up in oxygen sensors, it would be best to check the fuel pressure and leak down first. Low fuel pressure will set P0171 and P0174 simultaneously. It can also cause fuel mileage to drop. Most auto parts stores rent or sell testers. Let us know when you have one available, and we can help you through the procedure.

EDIT: 1996 and 1997 are the only years that have 4 oxygen sensors. On 1998 and newer, B1S2 was eliminated, (unnecessary).

I changed all 4 02 sensors and I'm getting a po401 code, I don't understand. Help Please

P0401 means there is insufficient EGR flow. EGR lowers exhaust temperature. If exhaust temperature is too high, P0401 will set in memory. A lean fuel mixture will also cause exhaust temp to rise. Check fuel pressure and leak down;)

So I ended up changing S1B2 and the fuel filter and voila, drove it to Boston this weekend from CT and pulled 21 mpg. On the way home, she threw another code, PO420 "Converter Efficiency Below Threshold - Bank 1". Mileage back down to 14 mpg. Yuck. So I plugged in my scanner to watch the live feed with the engine at temperature. From my research, S1B2 (in front of the cat) should have fluctuating voltage, and S1B3 (behind the cat) should read relatively stable voltage. Mine is both fluctuating and mimicking the front sensors numbers. This indicates that either the S1B3 is bad or the catalyst is damaged. What do you all think the odds are that its S1B3 that has "gone lazy" or that the catalytic converter needs to be replaced? I'm going to replace the sensor either way because it is cheaper and has 170k on it, but I'm hopeful the sensor has gone stupid.

The sole purpose of B1S3 is to monitor catalytic converter performance. It has no affect on engine performance and can not affect fuel mileage. At operating temp, closed loop operation, B1S3 voltage should ~.4VDC and relatively steady. If B1S3 data is the same as B1S2, it means the converter is not doing its job.

Voltage is allover the place, and pretty much mimicking S1B2. Should I not bother replacing that sensor? Also will PO420 affect fuel mileage; because nothing else has changed within the 100 miles I drove it after changing out S1B2 and having good results with that aside from PO420 getting thrown.

You can try replacing B1S3, if it's lazy, it might cure the P0420. P0420 by itself will NOT affect fuel mileage. However, a restricted converter can cause P0420 and affect fuel mileage. You can check for a restricted converter at the B1S3 bung, but it requires a pressure gauge that is accurate under 5psi.

I really don't understand how I was at 21 mpg the first 100 miles and as soon as the MIL goes on its down to 13-14. It is only throwing the PO420 code. Is there any use in checking the temperatures with an infrared thermometer at the inlet to the cat and the outlet while at running temp to verify that the outlet temperature is higher to demonstrate that the cat is at least doing something? So frustrating. I REALLY don't want to take it to a shop and get hosed.

Infrared would give you an idea, but accurate O2 sensor readings would be far more accurate. Have you tried banging on the converter with a rubber mallet? See if you can hear it rattle inside.