Service Engine Soon Light On

Have a 1997 TP, 3.4l, with 147k miles, but a rebuilt engine with around 40k miles. The SES light is on and AZ's Code Reader gave me 2 codes, P0141 and P0142, Heater oxygen sensor, Bank 1, sensor 2. I'm considering purchasing a new O2 sensor, but they're $70 for a Delco brand. My gas mileage doesn't seem right, it's between 18-19 mpg for city/highway driving. I'm putting the van on the lift this weekend and hope to find a loose wire or something on the downstream sensor. My question: where does the downstream sensor plug in? Is this a difficult thing to replace?

..after the catalyser
http://i250.photobucket.com/albums/gg246/avigex/HO2S_97.jpg

Thanks, LMP, you always offer your experience whenever someone asks a question, that's great! Unfortunately, I replaced the downstream o2 sensor and no change, went AZ to clear the code, and within 35 miles or so - pop! Came back on. Same code before the clear - could it have been the upstream? Mileage doesn't seem right, 18-19 mpg.

Mileage doesn't seem right, 18-19 mpg.
That mileage is pretty much on par for what I was getting in the winter with my Venture. You have to remember that most vehicle's mileages drop off a bit in the winter due to many factors such as winter blended fuel, extra idling times, winter driving habits,(even in Virginia.) Mine always went up when the weather turned warmer in the spring. If they are using a higher ethanol blend in your fuel then your mileage will suffer also.

P0141 - Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 2
P0142 - Heated Oxygen Sensor (HO2S) Circuit Bank 1 Sensor 3

.... As for mileage, I agree with ventureowner, specially if it is city driving, even in part..but if the code keeps the engine computer in open loop, it may have also an impact...
...notice that 141 is about the O2 heater circuit..check if you have 12V feed at ....pink and black.. of the sensor plug

don't know why they label "sensor 2" and "sensor 3"...?? there are more than 2 dozen codes for O2 sensors, some for bank 1, bank 2....and sensors 1,2,3 in each bank....that is more than we need..:grinyes:

A mechanic advised me to change the upstream sensor - is this difficult? The downstream wasn't difficult once I got the old one loose, that was a pain! This guy's a crack mechanic and he also advised that any brand of o2 sensor will work as long as it's pre-wired with the plug. I can use a lift if necessary, but is this doable while the van is on the ground?

When you go to replace the upstream O2 sensor ask if the auto parts has the removal tool. I got mine from autozone. Just leave a deposit and you'll be glad you got it as you already know its in a helluva spot. I also used a rachet wrench with the wrist pin. I did mine from underneath and it was a PITA.