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How my Modis caused an O2 problem - With video!

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7 years 4 months ago #3302 by Tyler
Had a strange case of a scanner problem that ended up causing a driveability problem :blink: How? Read on.

First, some background. Late on a Friday afternoon, I try connecting to a '10 Grand Caravan to read the codes, doing some standard CYA. No comm with most modules. I think nothing of it - the van runs just fine, the CEL was on beforehand, and none of this has to do with the heater hoses I replaced. Van leaves, I move on.

Saturday, someone on here has questions about some Global OBD data PIDs. I've got some errands to run, so I figure I'll just grab the data while I'm out, no problem. A few miles down the road, I take a glance at the data... The short term is pegged lean (30%), and the upstream O2 is stuck at around 150mV. WTF? The trims are always fine on this thing, maybe the Global side is lying? Switch over to OEM, and it's saying the exact same thing. Snap throttle, WOT, nothing gets a reaction.

Jeez, how long have I been driving around like this? :huh: And no code? I pull over and unplug the upstream sensor, hoping to force open loop until I can figure this out. No luck there, still stuck at 150mV!

Now I'm worried. I drive to work, and the car FINALLY sets an O2 code, about 20 miles later :lol: I go for the good ol' O2 circuit integrity test, figuring I'll see no reaction, and proceed to look for an open wire. Nope, perfect reaction from the upstream O2 PID, over 1V. Not open... where the heck is this voltage coming from? And why won't the O2 go high on it's own? :angry:

Then I notice that the test light bulb is dimly lighting when connected to the O2 signal circuit... Not bright, but lit. Bingo, here's our problem :woohoo: There's some kind of resistive short pulling the O2 signal down, and it's carrying enough current to light the bulb. I pull the wiring diagram before ripping the PCM out of the dash. I've highlighted the O2 signal circuit.



Now that's NOT what I was expecting - the signal runs to the DLC, on Pin 1. I'm trying to imagine why anyone would do that when it all goes *CLICK* in my head. The problem is the scanner itself :blink: Somehow, some way, it's pulling the O2 signal to ground. It explains the new symptoms, the Grand Caravan, and the test light. To prove it, I grab the shops Autel and hook up. Perfect O2 switching, fuel trims are countering each other, and the O2 voltage goes to 0V when unplugged.

Gotta be the cable, from all those times I shut it in car doors, right? Grab my spare, same thing. Here's when I play ScannerDanner and take some video on my phone:

drive.google.com/file/d/0BwIz-f3J815JTGd...dmM/view?usp=sharing

I had scan data captures of all this, but forgot to get them off the Modis before sending it in for repair :( Anyway, it's fixed, back to working normally.
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7 years 4 months ago #3308 by Noah
Man, that's wild! Great find.
At first I was thinking maybe it was an issue with a personality key, but you got the fancy one.
I would pass that along to your Snap-On rep. Seems like a design flaw.
I wonder if a unit that uses the keys would have done that. I never understood how they could just configure the umpteen keys out of the design of the tool and not have issues.

Good video, thanks for sharing!

"Ground cannot be checked with a 10mm socket"

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7 years 4 months ago #3309 by Ro-longo
I don't think I have ever seen an o2 sensor wired to the DLC.

Great find and Great video.

"Silver bullets are for killing Werewolves, not fixing Cars." -Rob Longoria-

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7 years 4 months ago #3353 by Doc
Awesome - thanks for posting

Youtuber specialising in Diagnostic techniques, sharing skills and helping technicians be the best they can be #Picoflu

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7 years 4 months ago #3354 by Dylan
Just read your post and saw your video :huh: Scratching my head....

Thanks for the video Tyler, awesome.

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