4vdc on O2 Sensor wire?

[DarrellinCeres]

I have a strange problem that started with my truck running like crap.
I took it in to a garage and they told me that the computer is putting 4vdc on the O2 sensing wire causing the computer to lean the truck out. They got a reman computer from Napa and when they installed it it was the same. I had to pick the truck back up, unfixed, and will have to take it back later. Has anyone seen this problem before? The truck lacks power and hesitates when feathering the throttle while driving. It also idles ruff at times.

Thanks

 

[SVO42]

Well, your O2 sensor is heated. It has 4 wires: 2 for the heating element (power & ground) and 2 for the actual sensor--ground and the voltage for the computer. An O2 sensor generates its own fluctuating voltage from 0 to 1 volt. The heater circuit (an O2 sensor has to be hot to work--heating elements were added to speed up the transition to closed loop operation). The heater circuit gets either 5 or 12V, I can't remember right now. Did the shop test the right circuit? Go to an AutoZone or something and have them scan your truck for codes. A bad O2 sensor could cause the problems you're seeing, but you should have a check engine light on if that were the case (as long as your truck is OBD II, which everything 1996+ is). To test the O2 sensor output voltage to the computer, you have to use an analog voltmeter, NOT digital. You'll see the needle on the gauge bounce back and forth from 0 and 1 after it's warmed up and operating properly.

 

[DarrellinCeres]

The shop said they backprobed the ECM with the O2 sensor unhooked and found the voltage there. They checked to see if the sensor heater was leaking voltage from the heater circuit and the sensor operation as well. They also said they got a couple of O2 sensor errors, No O2 sensor switching detected and O2 sensor always rich, something like that. I'll go buy autozone and get them to pull the codes. The shop said it was causing the engine to run lean.
I was wondering if there could be voltage somewhere else where there shouldn't be causing the voltage to blead through to the sensor circuit.
I'm probably going to end up taking it to another shop. I just find it hard to believe that my computer has the same fault as the remanufactured one from Napa.
And my truck is an 1986 w/ 2.3L, Manual transmition, 4x4 so I think it's EEC-4

 

[SVO42]

IIRC, an '86 would have a computer-controlled carburetor and a 1- or 2-wire O2 sensor. It sounds more like a bad sensor to me. Take a look sometime and let me know what you really have under the hood and in the exhaust pipe.

 

[DarrellinCeres]

It's a 2.3L Fuel Injected with a 3 wire O2 sensor

 

[DarrellinCeres]

Sorry for leaving this tread open for so long. You may close it.
Problem was bad O2 sensor it failed open. When you mentioned using an analog meter I remembered the mechanic using a DVM. In a solid state system a DVM could detect the detection circuit in the ECM if there was no connected load. An analog meter showed 0.
The real problem wasn't the O2 sensor although that was bad. It also had a failing in tank fuel pump and the SPOUT circuit was intermitantly going open. About 6 years ago the wiring harness fell into the stearing collum linkage and ripped it to shreds. Well my repairs held up this long.
I'm looking for another harness.