Hego/pin 29

[Dunderberg]

I didn't see a "Computer/fault codes" forum, so here I post!

I've been getting a fault code of 41, suggesting that it running lean.
BUT it is actually running quite rich. This suggested that the HEGO or O2 sensor was bad. Testing the three leads to the sensor, the power and ground wires are within 0.15v of the battery value.
The O2 signal wire is what is baffling me:

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Ignition off: 0 volts
Ignition on, not running: 4.06 volts (!)
Engine running: initially 4.00 volts, slowly decreasing to 1.60 volts
at a couple of instances, voltage has 'twitched' to 0.55 volts
Kill engine with ignition off: 1.09 volts decreasing slowly to 0 volts.

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HEGO disconnected, testing computer side of connector (pin 29), ignition on:
4.3 volts. ignition off: 0 volts

The engine actually runs fairly well- but I need to smog it. I only get a fault of 41 on the running test, I get an 11 on the KOEO test.
What is baffling me is why the computer is "giving" 4.06 volts to the HEGO sensor on pin 29, when the sensor generates its own voltage? Is the computer fried in this small spot? The sensor is 3-wire, and there is no pin 49 connection or an orange ground wire as I have read in some posts.

Ideas? Thanks!

 

[RonD]

You need to test voltage difference between Dark Green/purple(Grey/light blue at PCM) and the Orange wire(PCM Ground)

Rapid shifting between 0.01-1.1 volts it what is expected with engine running.

Code 41 is usually not a bad O2 sensor, it is a good sensor reporting a problem, usually is the key word, O2 sensors do start to fail, recommended replacement is 100,000 miles, mine lasted close to 250,000 miles.
If you have an exhaust manifold leak, that leak will suck in air causing O2 to report too much Oxygen in the exhaust, a lean exhaust.
A vacuum leak can also cause it.
Computer sets the code because it has added more fuel(opened injectors longer) and has reach a preset "open time" that is above preset parameters so it is warning the driver that there is a problem.
Engine can run fine, it is just beyond preset parameters for air/fuel mix as calculated by the computer.

Low fuel pressure can also set this code, computer can't monitor fuel pressure, it assumes 35-40psi.
If fuel pressure was 20psi then it would have to open injectors longer to get the needed fuel to richen exhaust out of the O2 reported Lean state.

Remember O2 sensor only reads Oxygen level in exhaust not fuel level, too much Oxygen is Lean, too little Oxygen is Rich.
And computer sets Lean or Rich code based on O2 sensor's Oxygen level report(voltage) and the time the injectors are open(pulse width).

 

[Dunderberg]

Thank you for your reply!

I had heard from others that the Orange wire (pin 49) didn't exist on 1987 3-wire O2 models, but I accessed the computer pins and voila, an orange
wire at 49! I tested and it gave me 4.02 volts. Turned off engine.
Digging through the wiring near the Distributor I came across a Tan wire unconnected. Thinking this was actually an Orange wire, I grounded it and retested the beast. Got an 11! Been looking for that # for awhile.

Thanks again, JT

 

[RonD]

Good Work