96/97 ECU O2 Sensor Circuit Issue?

KevinVarnes

KevinVarnes

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I tried searching, but I wasn't really sure what to search for. Can someone remind me what the issue and symptoms were on 96/97 MN12's that had issues with the O2 sensor circuit. I recall it was in the ECU and required replacement, but I don't know what the symptoms of the issue were (flooding/running rich??). Looking at a car with fuel issues and wondering if this is the culprit.
 
Over 15 years ago I recall having had a code for both front O2 heater circuits. After hours testing and troubleshooting the harness in various locations, I eventually found the issue was in the harness to the upstream passenger O2 sensor... hopefully this is not your problem!

IMG_2613.JPG
 
If it's setting a code for "o2 sensor heater circuit", it is very possible the ECM is bad. I went through two junkyard pulled ECMs that had this issue. The first was just on one bank, the second on both banks.

The car did run fine otherwise besides the codes being set and the check engine light. I suspect some of these cars ultimately end up in the junkyard because of this problem. They can't pass emission when the code is set and replacing the O2 sensors doesn't fix it. Some owners probably just give up at that point.
 
Not an issue on my car, but a car I am looking at. I thought there were drivability problems with the issue, but all it did was turn on the CEL?
 
My car ran normally with the bad ECMs and that code. I assume since it's just the heater circuit not working correctly, it won't affect the O2 readings.
 
I had 97 throw a code for all 4 O2 heater circuits that ended up being the ECM. What I remember from my research is that the MOSFETs that drive the heaters wear out from the amount of current draw.
 
Do you have an app that allows you to see O2 readings in real time?

When I had my O2 code, the problematic sensor had readings distinctly different from the other three.

If you look at my old thread, there are screen shots in there from before and after the replacement of the sensor.

1997ThunderbirdLXV6

Thread 'P0156 is back'

Ugh.

Alright, some of you may remember that I had this code last spring. Bank 2, sensor 2, oxygen sensor rear/left.

Back then, all I did was reposition the connector which kept hitting the underbody. The code never came back (till now), and I eliminated an annoying rattle in the process.


Fast forward to today:

Screenshot_20231122_121437.jpg

Looking at the connector, everything looks fine. I doubt I'll get away with any wire wiggling this time.

I went ahead and ordered two NTK sensors which are actually on sale for $35 a piece at Advance Auto Parts with free shipping. The wires on those sensors are...
 
I can stream data on my scanner, but the car is two states away. Trying to see if I can figure out what is wrong and if it is an easy fix or something else. The O2 sensor issue came to mind, but I couldn't recall what exactly it entailed. Doesn't sound like the issue with this car.
 
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