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Hi all, I Just bought a 2001 Nissan Frontier that was showing a code P0141, 02 sensor heater problem. I replaced the the downstream sensor but code still comes up. I also noticed a third sensor in front of the rear Catalytic that has no where to plug in. Is there supposed to be a third sensor or is it supposed to be capped and someone screwed in a sensor ? HELP 馃
 

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2004 Nissan Frontier XE KC KA24DE
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Hi all, I Just bought a 2001 Nissan Frontier that was showing a code P0141, 02 sensor heater problem. I replaced the the downstream sensor but code still comes up. I also noticed a third sensor in front of the rear Catalytic that has no where to plug in. Is there supposed to be a third sensor or is it supposed to be capped and someone screwed in a sensor ? HELP
If the one that you're talking about is located underneath the driver seat area of the truck...the sensor wire goes into the cab thru a rubber grommet.
 

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Discussion Starter #3
If the one that you're talking about is located underneath the driver seat area of the truck...the sensor wire goes into the cab thru a rubber grommet.
Hi yes, the 02 sensor I replaced is under the cab and goes through the floor to be plugged in inside the cab under seat. So after replacing it the code P0141 still appears. But also I see another sensor about 12 鈥 upstream just before the rear catalyst. It has a plug but no where to plug it into. Is this bung supposed to be capped ?
 

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Looking at your diagram, the third 02 in question would be 鈥淏鈥. But I don鈥檛 see where it plugs in ? This is why I鈥檓 thinking it should be capped. The truck has California emissions and was bought at puente hills Nissan in So.Cal. As for why the P0141 keeps coming on I鈥檓 wondering if it鈥檚 from this third sensor or is there a problem with the ECU ?
 

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Discussion Starter #6
Looking at your diagram, the third 02 in question would be 鈥淏鈥. But I don鈥檛 see where it plugs in ? This is why I鈥檓 thinking it should be capped. The truck has California emissions and was bought at puente hills Nissan in So.Cal. As for why the P0141 keeps coming on I鈥檓 wondering if it鈥檚 from this third sensor or is there a problem with the ECU ?
On the 4 wire plug for sensor 2 , do you know which color wires are for the heater ?
 

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On the 4 wire plug for sensor 2 , do you know which color wires are for the heater ?
Check the FSM for your year model.
Might wanna download the Alphabetical Index first to find out which section contains the info you seek.
 

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Discussion Starter #8
Check the FSM for your year model.
Might wanna download the Alphabetical Index first to find out which section contains the info you seek.

Thank you brother. This repair manual will be a big help to diagnose error code. But I don鈥檛 see a third 02 sensor for this truck so I鈥檓 just going to cap it. Looks like a third bung was welded on pipe. I don鈥檛 know why???
 

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Is the sensor you're referring to before or after the cat? Here's a pic of mine.
 

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Tge diagram shows both the B and C are after the cat. Are you 4wd?
 

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Yes it's a cat.
If the 3rd sensor you speak of isn't plugged into anything....isn't that the same as the bung being plugged?
 

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Where exactly is the 02 sensor that you replaced?
Did you clear the codes after replacement and the SES/CEL light turns on again?
My truck only has two sensors and they are located exactly as this diagram shows.
 

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Discussion Starter #15
Where exactly is the 02 sensor that you replaced?
Did you clear the codes after replacement and the SES/CEL light turns on again?
My truck only has two sensors and they are located exactly as this diagram shows.
Ok so I finally figured this out. So the original code i saw was a P0141 , heater on sensor 2 defective. I had replaced the O2 but the same code came on. When I was probing the rear sensor plug I noticed a wire pin was pushed out of place. Corrected that problem and code is gone. So I drove the truck to re set the monitors to get ready to smog and another code popped up. P0420, catalytic problem. So what this guy did ( the one i got the truck from ). Instead of replacing the front catalytic he welded another 02 sensor bung behind the rear cat. So now there is a before and after 02 sensor bung on the rear cat. Then he installed 02 sensor #1 to the bung in front of the rear cat and made a long wire extension so it could be plugged in to the 02 plug near the engine. Then sensor # 2 was just moved to the the newly welded bung behind the rear cat. So now there鈥檚 an upstream and downstream 02 sensor on the rear cat. I supposed it worked to keep the check engine light off but can鈥檛 imagine it passing the smog test legally. So now I have to get a new front catalytic done, $700. I hope that鈥檚 it because with that repair I鈥檓 in over $1000 in repairs. Luckily I got the truck free but with these repairs and getting the registration up to date it鈥檚 costing me. Thank you for all your help.
 

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Discussion Starter #16
Ok so I finally figured this out. So the original code i saw was a P0141 , heater on sensor 2 defective. I had replaced the O2 but the same code came on. When I was probing the rear sensor plug I noticed a wire pin was pushed out of place. Corrected that problem and code is gone. So I drove the truck to re set the monitors to get ready to smog and another code popped up. P0420, catalytic problem. So what this guy did ( the one i got the truck from ). Instead of replacing the front catalytic he welded another 02 sensor bung behind the rear cat. So now there is a before and after 02 sensor bung on the rear cat. Then he installed 02 sensor #1 to the bung in front of the rear cat and made a long wire extension so it could be plugged in to the 02 plug near the engine. Then sensor # 2 was just moved to the the newly welded bung behind the rear cat. So now there鈥檚 an upstream and downstream 02 sensor on the rear cat. I supposed it worked to keep the check engine light off but can鈥檛 imagine it passing the smog test legally. So now I have to get a new front catalytic done, $700. I hope that鈥檚 it because with that repair I鈥檓 in over $1000 in repairs. Luckily I got the truck free but with these repairs and getting the registration up to date it鈥檚 costing me. Thank you for all your help.
Yes it's a cat.
If the 3rd sensor you speak of isn't plugged into anything....isn't that the same as the bung being plugged?
Yes it's a cat.
If the 3rd sensor you speak of isn't plugged into anything....isn't that the same as the bung being plugged?
 

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Discussion Starter #17
So yes to your question about the third sensor not plugged into anything is the same as being plugged. I鈥檓 going to install a cap Or screw in plug to that third bung , basically so the smog guy doesn鈥檛 make a big deal about an extra 02 sensor and may or may not let me smog it because it doesn鈥檛 pass his visual inspection. He may flag it for illegal modifications.
 

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Best of luck. I had the p0420 popping up on me a few years ago and I just bit the bullet and replaced both cats, both sensors and haven't seen the p0420 since.
Only the front cat was clogged on mine. But since I had already purchased both....I swapped em both out.
 

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Check your exhaust manifold for cracks before replacing the catalytic converter. Mine gave a P0420 and I took off the heat shield and found a good sized crack. Replaced the manifold and the code went away.
 

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Discussion Starter #20
Check your exhaust manifold for cracks before replacing the catalytic converter. Mine gave a P0420 and I took off the heat shield and found a good sized crack. Replaced the manifold and the code went away.
Roger that. My truck is at the muffler shop now. I鈥檒l have them take a look. Thank you
 
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