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I have had a repetitive problem over the last several years with my 2000 DII fouling the same passenger side upper O2 sensor so I figure it is time to finally figure out the cause before I throw more parts at it.

I have an emissions test coming up and my truck is showing codes P0150, P0174, P1174 and P1590 (yes I have 3 amigos). I've done a lot of reading here and it appears the O2 sensor voltages are normal, but the bank 2 long term fuel trim is 25% (bank 1 is normal). I have interpreted this to mean I may have a vacuum leak that is causing my fuel trim to max out and run lean, throwing the P0150, P0174 and P1174 codes.

If any of the more experienced members can offer some advise on finding vacuum leaks, or most likely locations to check I would appreciate it. Also, if I am going the wrong direction here, feel free to straighten me out.
 

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Bad guess, replace both front 02 sensors and you'll be fine. You will probably need to fix your 3 amigo issues so you can get rid of the 1590 cause with it you won't pass.
 

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Bad guess, replace both front 02 sensors and you'll be fine. You will probably need to fix your 3 amigo issues so you can get rid of the 1590 cause with it you won't pass.
Thanks Mike - I replaced all four 02 sensors 17,000 miles ago (Bosch) and had to replace the front passenger side sensor 8,000 miles ago (Atlantic British sourced) to pass the last inspection. Now it is throwing codes again. Three sensors in the same location (bank 2, sensor 1) in the span of 17,000 miles seems excessive. Is there anything else I should be looking for so the new ones don't go bad?
 

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Have you checked and cleaned the sensor plugs, was there any oil in them?
Vacumn leak will not cause the 150 code, my money is on a dirty connector.
 

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Have you checked and cleaned the sensor plugs, was there any oil in them?
Vacumn leak will not cause the 150 code, my money is on a dirty connector.
If I recall, the connectors were a mess when I did the full 4 sensors a few years ago. I cleaned them as well as I could then with electrical contact cleaner. I'll order 2 new sensors as you recommend and be diligent about cleaning the connectors on install. If not the connectors, could it be the wiring? Any way to check other than visual inspection?
 

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Clean the front 02 sensors with electrical contact cleaner the fix the oil leak at the valve cover gaskets.
 

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Discussion Starter #7 (Edited)
Clean the front 02 sensors with electrical contact cleaner the fix the oil leak at the valve cover gaskets.
Update - I have been busy with a brake job and front driveshaft issues, so just yesterday got the new front oxygen sensors (Bosch 15175) installed as you recommended. No leaks from the valve covers as you suspect, but I still did a thorough cleaning of both connectors. On removal, the old passenger side sensor was fouled badly with black soot (indicating running rich) and the drivers side looked like new.

It took about 5 miles of driving for the P0150 and P0130 codes to pop up with the new sensors. My code reader shows 25% LT fuel trim on that side so I suspect the sensor(s) is not functioning at all. Given my history of going through 02 sensors, I also assume the root cause is not the sensor itself. What would be your recommendation of the next avenue to pursue?

Thanks in advance

EDIT: I am trying to learn more about how my OBD scanner works and have discovered I have significant differences in the voltage being delivered to the sensors on Bank 1 versus Bank 2. I did an 11 mile drive and the voltages for Bank 1 were hovering very close to 0.5 volts front and rear with 2.1% LT fuel trim. Bank 2 (where my codes are) is showing 0.24 volts front and 0.75 volts rear and 22.3% LT fuel trim. It looks like the sensors are working, but not getting the correct signals due to another issue. Advice?
 
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