1999 Nissan Quest O2 Sensor

Old 10-24-04, 05:30 PM
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1999 Nissan Quest O2 Sensor

The Service Engine Soon (also known as Check Engine Light) on my 1999 Nissan Quest recently came on. I picked up a "carchip", which allows me to download information about my car, including the codes, to my computer. The codes that came up are P0325 "Knock Sensor and or Circuit Fault" and P0141 "Downstread Heated O2 Sensor Heater Fault". Reviewing my Haynes manual I see other specific codes related to the O2 sensor, so I am wondering if this code is indicating that the heater in the O2 sensor has failed, or if something else is going on which would result with this code. I tried measuring some voltage on the O2 sensors and I found it odd that I didn't see an O2 sensor after the Catalytic Converter. I found what looks like an O2 sensor, right before the Catalytic converter and I found what looks like an O2 sensor inside the engine compartment on the exhaust manifold. The one right before the catalytic converter has 4 wires and the one on the exhaust manifold has 3 wires. As I mentioned earlier, I tried measuring voltage, but the diagram in my Haynes manual didn't match what I found. Which is part of why I am suspecting that maybe I wasn't looking at the right sensor.

1) I'm trying to figure out if the "Downstream" O2 sensor (I think the one that is supposed to be after the catalytic converter) is bad?
2) I'm trying to figure out if the two sensors I described above are indeed O2 sensors?

Any information anyone can provide would be appreciated.
Old 10-24-04, 05:52 PM
Join Date: Oct 2004
Location: Ontario
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the down stream O2 is after the Cat
and the O2 Sensors are usuly threaded tooo the exhyaust some ware in the pipes or manifold the up stream are before the cta and down stream art after
Old 10-24-04, 06:04 PM
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Join Date: Oct 2003
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Yes, it has to be after the cat and it will usually have 4 wires. Ground and signal return for the sensor side and power and ground for the heater. You should be able to test for power and ground at the plug with the key on and the sensor disconnected. The heater wires are usually both the same color and usually black or white. If you have 12v and ground, the sensor is bad.

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