O2 voltages, PCM codes, and scanners

[Antonkov]

Long story, will try to make it short: I bought my ’97 Explorer with no clues of previous service history, everything worked more or less fine but gas mileage could have been better. I thought the car is 10y.o. and it wouldn’t hurt to change the spark plugs and wires and to replace the front O2 sensors. Changed, didn’t get any noticeable improvement neither in power not in mileage, then, in a month or so my MIL lamp went off.

The codes were:
P1130 - Lack of Switching – Bank 1, sensor 1
P1131 - Lack of Switching - Sensor Indicates Lean, Bank 1, sensor 1

Then I started noticing some engine vibration, which kinda comes and goes from trip to trip. The power went slightly down but the mileage increased. I checked the passenger side sensor, it looked fine (no any buildups or greenish stuff from the coolant, if it leaked).

Then the MIL lamp turned off by itself and several trips later came back on….

I bought LM237 interface and tried to check what goes on inside - all four sensors read 0V, both fuel trims are at -100%, fuel mode is in “Closed loop with a sensor failure”.

I remember someone said on this forum that if one got two sensors died at the same time, he should not leave his house coz of high chances of being struck by lightening, I had four silent sensors, scary….

I tapped into the signal wires of bank2 sensor and checked the voltage with a digital voltmeter; it jumped from 0.3V to 0.7V on the voltmeter but still 0V on the software scanner ( I used ProScan).

My questions are:
1) Even if my B1S1 sensor IS dead (going to play with it tonight) the other one MUST be alive - first, because I saw it breathing on the voltmeter, and second, because how would it enter the closed loop with both sensors reading 0V ……so if B2S1 is still good why can’t I read it with a scanner?

2) Is it possible that PCM failed some high level test and doesn’t report out O2 reading although uses them internally?

3) If the sensor IS dead, why would MIL lamp go on and off from trip to trip? Is it like it is “almost” dead but still reports something under certain modes?

Any ideas are much appreciated!

 

[Antonkov]

Tapped into the troublesome sensor, same thing, see the signal on the voltmeter but all software scanners (tried at least 4 different programs) all read 0V

Is there any fuse or something that could cut the incoming O2 signal from the ECM? I don't even know what to test know

 

[SuperEx91]

never heard of that. if you have voltage on your dvom, sensors are not the problem. the fact that all four are reading 0v pretty much eliminates them from being the problem. lean code originally made me think vacuum leak. not sure now though.

are the codes always the same when it comes back on?

if the light will go off, that means that the computer passed all of its tests which most likely means intermittent problem.
i hate intermittent problems. they are the worse ones to diagnose.

pay attention to when the light comes on. does it happen in similar or random conditions?(weather, load, brand of gasoline, length of trip)

first impressions say pcm but never condemn on first impressions.
i know there is no fuse between any sensors and the pcm.

 

[Antonkov]

When I took out B1S1 sensor out it was all black in soot. So if 0V reading is legitimate then it should mean lean, but it doesn’t add up with the soot. But if ECM *thinks* it’s 0V ,while it’s not, it would trick it into going super rich with a black smoke.

Also, even if I had a leak, or slow switching, it still would show up as *something* on the scanner but not as a flat 0, right? Especially when I see it on a voltmeter.

Hm… makes me think something is fishy with the way the 02 signals are collected, processed and reported via OBD…

 

[jah81592]

I used to use a handheld scanner to diagnose problems but sometimes the pids wouldn't report properly giving a "false reading". Now I use a real time datalogging software and it will report the pids properly. One thing to note is that the heaters in the wiring circuit are in parallel but the sensor switching itself will report individually to the computer. I would say that you have one dead sensor. The switch time should be on as much as off. I wouldn't recommend replacing just one sensor as you will have a failure of bank2 shortly after the initial failure. Best case scenario the sensor will be lazy. Fueling the one bank differently than the other. Always replace in pairs.
PO171&174 are lean codes the codes you mentioned are definately referenced to the o2's. Replace and enjoy your ride!