Help!! Intermittent signal loss from all 4 O2 sensors

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eicca

Member
Messages
73
Location
Utah
Been chasing down a running rich issue for a while now and I made a scary breakthrough tonight. All four O2 sensors are randomly and simultaneously flatlining. When it’s running normal, upstream sensors will oscillate between 0.1 and 0.7 ish volts and the downstream sensors will hold relatively steady around 0.7 ish. Out of the blue all four will drop to 0.000 to 0.030 ish and the car will run like crap, backfire, read -35% fuel trim, etc.

This really worries me. I can imagine one of them konking out, but all four at the same time? This smells electrical to me and I don’t even know where to start.
 
Last edited:

361UZ

Member
Messages
158
Location
Canberra ACT
What model car? I would agree, it sounds like the supply to the sensors (or heaters?) is intermittent. If you can get the make/model info, you can find wiring diagrams or even what fuse supplies the sensors. Someone on tis forum will know how the sensors are wired.
I was chasing an intermittent problem on my ex-Crown 1UZ. Turned out to be in intermittent fuse.....
 

eicca

Member
Messages
73
Location
Utah
What model car? I would agree, it sounds like the supply to the sensors (or heaters?) is intermittent. If you can get the make/model info, you can find wiring diagrams or even what fuse supplies the sensors. Someone on tis forum will know how the sensors are wired.
I was chasing an intermittent problem on my ex-Crown 1UZ. Turned out to be in intermittent fuse.....
1998 GS400. I would love if someone could confirm what I found in the EWD. It looks like the O2 sensors and heaters are powered directly from the ECU and they all terminate at ground point EC. I checked all the connections and everything is solid.

I pulled the negative terminal to wipe the ECU and replaced the 25A EFI fuse. The old fuse looked a little discolored but read zero impedance.

My Techstream doesn’t refresh fast enough to effectively monitor multiple data points, so I’m relying on my basic OBD tool to watch graphs and numbers. The flatlining seems to occur when I hit the accelerator.

I’m typing this as I sit in my car waiting for the relearn cycle to finish. We’ll see what a fresh fuse and a test drive reveal.
 

Zuffen

Super Moderator
Staff member
Messages
5,461
Location
Sydney, Australia
I had a similar problem with my LS3 powered toy.

Accelerate hard and it would lose the Crank position by going hi or low impedence at is will.

Found a chaffed wire in the loom that was causing it.

Your problem sounds like a broken wire that pulls apart on acceleration or a bad ground.

I'd try putting meter (or test light) on the sensor wiring and start wriggling and pushing and pulling the loom. If it drops out you know roughly where the problem is.
 
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