Help!! Intermittent signal loss from all 4 O2 sensors

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eicca

Member
Messages
75
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.
 
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361UZ

Member
Messages
167
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
75
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
5,465
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.
 

eicca

Member
Messages
75
Location
Utah
I can't believe that I forgot to update this thread. It seems like replacing that fuse fixed not only the original problem but also a host of little things that I didn't like about my car's performance.

Not only is the ECU working properly again, but the transmission is shifting much more solidly. It used to be pretty slushy between first and second. I thought it was just wear. But now there's discernible shift shock, even at WOT. It also used to "cut out" when engine braking, like it would slip into neutral for a split second when the revs got down to 1200. Doesn't do that anymore. Downshift response is also much quicker.

I'm still a bit boggled that a seemingly-ok fuse could have done all of this, but over the past four years of owning this car I've pulled that fuse and reset the ECU dozens of times. I even cleaned the fuse blades just for good measure. Nothing has ever made a lick of difference, until I put in the new fuse. Now it's a whole new car.

It's running so well I gave it a Christmas oil change and upgraded to Mobil1 full synthetic.
 

361UZ

Member
Messages
167
Location
Canberra ACT
I can't believe that I forgot to update this thread. It seems like replacing that fuse fixed not only the original problem but also a host of little things that I didn't like about my car's performance.

Not only is the ECU working properly again, but the transmission is shifting much more solidly. It used to be pretty slushy between first and second. I thought it was just wear. But now there's discernible shift shock, even at WOT. It also used to "cut out" when engine braking, like it would slip into neutral for a split second when the revs got down to 1200. Doesn't do that anymore. Downshift response is also much quicker.

I'm still a bit boggled that a seemingly-ok fuse could have done all of this, but over the past four years of owning this car I've pulled that fuse and reset the ECU dozens of times. I even cleaned the fuse blades just for good measure. Nothing has ever made a lick of difference, until I put in the new fuse. Now it's a whole new car.

It's running so well I gave it a Christmas oil change and upgraded to Mobil1 full synthetic.
Good to hear, and great that you remembered to let us know the outcome.
As I said above, I have had "worn" fuses, especially the blade type. Not sure if its the link that goes high resistance under load, or the female socket that loses tension over the years. The Crown wiring I have in my Chev has (had...) a few like that, and as you say, I was surprised by what else worked batter than it did before, that I hadn't noticed.
 

eicca

Member
Messages
75
Location
Utah
Ok so it's starting to happen again. First symptom is the engine will fire really fast when cranking on cold start. When it's working properly it cranks for about 1 full second before firing. When the issue starts happening it fires after only a half second.

Transmission is starting to feel a little slushy again, and my fuel trims are creeping back into the negative double digits. O2 voltages seem to be oscillating more slowly but I haven't seen any flatlining yet. It's coming though, if this keeps up.

Now the fuse I put in that made a big difference was an old one, meaning a spare that had been sitting in that fuse box for the life of the car. Maybe it means that fuse is starting to go "stale" as well, or maybe the fuse block itself has corrosion in the contacts. The fuse blocks in the 2UZ Land Cruisers of the same era have been known to go crusty and cause some issues with the immobilizer. A new fuse block is cheap enough I'm willing to give it a shot. I plan to install that as well as dielectric grease on all the connections from the fuse block to the ECU.

And if that doesn't fix it, I don't know...
 
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