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All four O2's are reading lean rich lean rich and showing circuit fault

Tiny
BSHEPPARD84
  • MEMBER
  • 1999 CHEVROLET 3500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 200,000 MILES
I have 5.04 reference volts at all 4. I was thinking ground but have no idea where to look. Anyone have a diagram or input would help?

Truck is a C3500
Tuesday, August 20th, 2019 AT 4:51 PM

21 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,348 POSTS
Welcome to 2CarPros.

First, all sensor have a common ground. Additionally, there are two splices to tie them together. I attached a pic to help you locate the ground which is on the right front of the engine block.

Are you sure there isn't a vacuum leak or fuel pressure issue causing the computer to over compensate resulting in the fluctuation?

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know if this helps.

Take care,
Joe

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Tuesday, August 20th, 2019 AT 6:39 PM
Tiny
BSHEPPARD84
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Is that right front when you looking at the front of the motor?
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Tuesday, August 20th, 2019 AT 6:45 PM
Tiny
BSHEPPARD84
  • MEMBER
  • 14 POSTS
I just glanced at the readings on the live data. It had said all o2's were saying circuit fault. I inspected the harnesses and they looked okay, but I was more worried about the voltages. I may inspect them again, but as all 4 say it. It must be something else. Do you have a picture of where the ground should be?
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Tuesday, August 20th, 2019 AT 6:51 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,348 POSTS
Welcome back:

It's the right front of the engine block. Take a look at the picture I attached earlier.

Let me know. Also, what trouble code are you getting? Also, check the ground. I have seen the connectors fail and end up with a really weak ground or none at all.

Take care,
Joe
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Tuesday, August 20th, 2019 AT 8:15 PM
Tiny
BSHEPPARD84
  • MEMBER
  • 14 POSTS
P0161
P0141
P0155

i guess I don't understand the right front. Is that right front as in passenger side right front as if your sitting in the truck or standing in front of the truck which would make it driver side front? Sorry
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Tuesday, August 20th, 2019 AT 8:22 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,348 POSTS
Welcome back:

Passenger side and no need to be sorry. Someone had to tell me once too. In automotive manuals, they give the side as if you are sitting in the driver's seat.

Now, the only code I can look up is the 155. Here is why:

Change Vehicle Bookmarks Library Request Conversion Calculator Technician's Reference - Collision

Search vehicle information

1999 Chevy Truck C 3500 Truck 2WD V8-5.7L VIN R
P0161
Vehicle ALL Diagnostic Trouble Codes ( DTC ) Testing and Inspection P Code Charts P0161
P0161

Under 8600 GVW Without C6P
Over 8600 GVW With C6P

I need the GVW

Now, I'm sorry,

Take care and let me know.

Joe
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Tuesday, August 20th, 2019 AT 8:28 PM
Tiny
BSHEPPARD84
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Okay, passenger side front of the block and it will be on the block itself or on the frame? Mine is below 8600.
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Tuesday, August 20th, 2019 AT 8:34 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,348 POSTS
It should be on the block. At least that is what the schematic indicates.

Let me know.
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Tuesday, August 20th, 2019 AT 8:40 PM
Tiny
BSHEPPARD84
  • MEMBER
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Okay, so I think I found the ground right below the mechanical pump block off plate. There is a battery ground there and right next to that on another ground post is a smaller ground. I followed it up and it has 4 black wires tje purple and I think redish pink wires. It goes into a big loom under the intake hat and looks like it goes into the PCM.
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Wednesday, August 21st, 2019 AT 9:01 AM
Tiny
BSHEPPARD84
  • MEMBER
  • 14 POSTS
Either that ground is corroded. Didn't look too bad or I lost connection ground circuit inside the PCM.
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Wednesday, August 21st, 2019 AT 9:04 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Check the connector. They sometimes fall apart when touched.
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Wednesday, August 21st, 2019 AT 5:48 PM
Tiny
BSHEPPARD84
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The connector at the pcm or all the connectors themselves.I'll look again at the ground on the block
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Wednesday, August 21st, 2019 AT 6:00 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Welcome back:

I would check them all. I had a vehicle like this lose spark. I went crazy looking for the problem I even checked continuity to ground which checked good. Finally, I traced the wiring down to the ground on the engine. When I wiggled the connector, it fell apart. It was showing continuity to ground with a DMM, but when a load was placed on it, the ground failed, so check that too and any of the splices you mentioned.

If they are all good, then I would check the pins on the PCM. Remember, if you are going to disconnect the PCM harness, disconnect the battery first. Then, reconnect the PCM connector before the battery is reconnected. Damage to the PCM can result if the battery is left connected during this procedure. You may have corrosion or a damaged pin in the connector. Wiring can be difficult.

Let me know.

Joe
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Wednesday, August 21st, 2019 AT 6:15 PM
Tiny
BSHEPPARD84
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I don't think I had any splices. I can't remember what I said, lol. So could I take the ground off the block for the o2's use it to check a positive wire and see if my test light lights up? And that would confirm a good ground from the PCM? I'm only assuming that the wires go to the PCM. When they go into that big loom I can't tell where they go but to the PCM is my logical guess.
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Wednesday, August 21st, 2019 AT 7:17 PM
Tiny
BSHEPPARD84
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Also coming from the ground on the block the diagram shows them going to each o2. Are they going from the block ground directly to the PCM and then to the o2 sensors or going from the block ground to the o2 sensors then to the PCM?
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Wednesday, August 21st, 2019 AT 7:23 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Welcome back:

Checking grounds that way usually works. However, in the case I described earlier, when higher voltage was applied to the ground, it failed. Do it the way you suggested but make sure the condition of the wiring is good.

If I recall correctly, power runs from the PCM to the sensor, then to the ground.
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Wednesday, August 21st, 2019 AT 8:29 PM
Tiny
BSHEPPARD84
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Would you happen to know or have a diagram of the which harness on the PCM has the grounds? Then I could run a continuity test from the PCM to the o2 grounds and see if there are any open circulates from the PCM to the o2's. Then go from the o2's grounds to block ground continuity.
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Wednesday, August 21st, 2019 AT 8:38 PM
Tiny
BSHEPPARD84
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I meant open circuits.
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Wednesday, August 21st, 2019 AT 8:38 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,348 POSTS
Welcome back:

I attached several pics below of the control module pin-out. I overlapped them so you can follow them.
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Thursday, August 22nd, 2019 AT 8:48 PM
Tiny
BSHEPPARD84
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  • 14 POSTS
I don't really know what you mean by overlapping them or which connector it is top or bottom plugs on the PCM on the fender, but I'll do my best to locate the wires for the o2's and see if they are corroded or something.
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Thursday, August 22nd, 2019 AT 8:54 PM

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