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Bad oxygen sensor(s)?

kponhlo

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City, State
Niskayuna, NY
Year, Model & Trim Level
'97 XLT
Hi. I have a '97 4.0L SOHC, 102k, XLT, that has set code P0420 only. I replaced the leaking intake gaskets a few months ago, but the code P0420 waited a few weeks before appearing. The truck also has a rough idle. I have cleared the codes and it comes back the same. i also hooked up a scan tool to view the O2 sensor data - all four of the sensors read 0.00 mV AND the fuel trims are at 100-percent.

From reviewing other posts here, it seems that a P0420 is probably the post-cat O2 sensor on bank 1, but could also be the cat itself. It also seems that the rough idle could be caused by an O2 sensor (or sensors) that have failed or it could simply need a good fuel system cleaning, although I do run good fuel additives on a fairly regular basis. I replaced one of the pre-cat O2 sensors when the truck had about 60k, but the others I suspect are original.


I am wondering why none of the O2 sensors are reading a voltage, and why the fuel trims appear to be maxed out. Could running with a bad intake gasket for a while have toasted all four O2 sensors, and if so shouldn't at least one of their resepctive codes be set?

Thanks in advance as always,

-Keith
 


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dogfriend

Human-Animal Hybrid
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Carmichael, CA
Year, Model & Trim Level
'97 Sport 4x4
I would check the wiring and connectors for the O2 sensors. The two sensors in front (upstream) from the cat(s) should be switching back and forth around .45 mV. If you are seeing 0 mV all the time, I'm thinking that you have a wiring problem.
 












kponhlo

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'97 XLT
Thank you both for the suggestions, and Drew for the helpful link (first link seems to be to same thread as second link?).

Drew, which fuse in particular are you thinking of, or where? I've been known to overlook simple things in the past... :rolleyes: Also, one of those links showed an O2 sensor access hatch for a 5.0L truck - I assume it is the same for a 4.0L truck? Could it be that "easy"?

I'm also trying to remember if i ever got anything with silicone in it near the engine...I don't think so, but it seems like that would be one way to lose all the sensors at the same time.
 




kponhlo

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'97 XLT
Well, its defintely not the main HEGO fuse, although that was a great suggestion.

I'm thinking of plugging in one new sensor to see if that one starts to read or not. That should tell me if the problem is the sensors or wiring/other shouldnt it?

Thanks again.
 




kponhlo

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'97 XLT
UPDATE and IMPORTANT SCAN TOOL INFO:

I got access to a Snap-On scanner and checked to vehicle data that way. The O2 sensors and fuel trims appear to be responding normally. This still leaves me with a P0420 code - perhaps it is the converter after all? It did run like garbage for a while before i changed the intake gaskets...

Also, I bought an Actron CP9145 scanner just to help with this problem. It incorrectly read the O2 values as zeroes. Upon correspondence with Actron, I got this reply:

Actron Customer Service -
"I spoke with engineering today again and have discovered that Ford in 96-97 had an issue with their software on light duty trucks/SUV's. If a Generic OBD II tool is used or a generic OBD II request was made by a tool such as your friend's Snap-On tool (you would have to pick OBD II instead of the vehicle application before scanning), the vehicle will not report O2 voltages. This should be able to be reproduced with your friend's tool by picking Global OBD II instead of the vehicle application, if you wanted to verify this. The CP9145 will only give a generic request for trouble codes, it will not request manufacturer specific data like the Snap-On tool or our Kal Equip 9640 tool will. I totally forgot about this problem until one of the engineers overhead my conversation with the head engineer and re-informed us of this problem. We do not believe Ford corrected this problem in their software for the vehicle. You would need a tool with Enhanced capabilities such as the OTC Nemisys or Kal Equip 9640 to get these PIDS."

So there you have it. If you have a '96 or '97 Ford truck, apparently a global OBDII tool will not work for you.

I hope that info saves someone troubleshooting time and/or a $300 bill for an unusable (in this application) tool.

Anyone want a barely used CP9145 scanner?
 




kponhlo

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'97 XLT
One more tidbit....I just found this link (vcertt.org/news/03-03.pdf) to an interesting article about P0420 and OEM vs aftermarket converters. In it, P0420 is described as a reliable way for determining bad converters vs. bad O2 sensors.
 








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