GM Truck Club Forum banner
1 - 20 of 72 Posts

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #1 ·
Please Please Help..

1999 Suburban K1500 LT 5.7 Vortec with the newer Spider style. Completely Stock other than added a second battery with a relay.

Background: Fresh Rebuild after previous head gasket and 2 cracked heads.

Issue: P0134 P0154 and seem to be in Open Loop all the time.

Diagnostics via Foxwell N510 Elite with GM.
Showing Multiple I/M Not ready, including
1) Catalyst Monitor,
2) Evap System Monitor
3) Oxygen Sensor Monitor
4) Oxygen Sensor Heater Monitor

Sensors appear to be working: Specifically MAF, ECT, ICT and Baro which feed into I/M Readiness

Manual Testing: Disconnected O2 sensors and measured back towards PCM with key on. 0.37 volts on the sensor side, 12.2 on the heater circuit side. All O2 Sensors have been swapped, no change.

I am at a complete loss and smog is due soon... what could it possibly be??
 

·
Registered
Joined
·
89 Posts
That looks to me like the ECM isn't seeing either front sensor (B1S1 or B2S1). On the Foxwell, if you go to Live data what are you seeing for the O2 sensors? I'm thinking that perhaps there is no reading at all for the front sensors, but perhaps not for the rear either???

I have a Foxwell and the same truck so can compare what you get with mine, if that will help.
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #3 ·
That looks to me like the ECM isn't seeing either front sensor (B1S1 or B2S1). On the Foxwell, if you go to Live data what are you seeing for the O2 sensors? I'm thinking that perhaps there is no reading at all for the front sensors, but perhaps not for the rear either???

I have a Foxwell and the same truck so can compare what you get with mine, if that will help.
Correct...there is no activity on any of the O2 sensors
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #4 ·
I jut took it out for a spin to see if I could somehow get the I/Ms ready, but after 10-15 minutes of driving, it throws the codes again, it never seems to close loop, and the same I/M are still not ready, including EGR this time..
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #5 ·
One thing I didnt understand was the scan tool indicates that B1S1, B1S2, B2S1, B2S2 all have 0.445V. But my physical measurements (across the connector pins) were 0.37 across the board. Arent these supposed to be the same measurement?? Is this just the resistive drop of the wiring? Seems like quite a bit.
 

·
Registered
Joined
·
16,031 Posts
I'm guessing your going into closed loop; your going into closed loop and the pcm seeing no O2 activity sets the codes.

Both O2s appear to be inactive, so what is common?

When you say you see .37 volt (0.5volt reference voltage) at the connector pins, are you talking about the PCM connector?

If you are, I'd look very closely at the wiring going to the O2s.
If they both died together, it has to be something common, perhaps a common wire, perhaps a shared ground.
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #7 · (Edited)
So the 0.37v is measured right at the open connector, across the 2 pins for the sensor (O2 sensor disconnected) and thus looking all the way back towards the PCM (as opposed to looking towards the O2 sensor itself which - which was disconnected. I did this on EACH of the 4 O2 sensor connectors, pre and post cat (under the vehicle). I was trying to prove out that the wiring from the PCM all the way to the sensor connector was good through this method. Similarly, I measured the other 2 pins for each connector to validate that power was being given to each sensor heater element. (Key on, Engine off during measurement)

In contrast, the scan tool is telling me 0.445v (rock solid) for the O2 sensor voltages. Shouldnt these be the exact same values? (Even irrespective of a bad ground, since these wires go directly to the PCM)? Or perhaps there is some ground issue right at or within the PCM that is somehow causing difference between what I am measuring and what the scan tool indicates?

The apparent lack of any response for ALL the O2 sensors on the scan tool did seem to me to indicate a wiring issue, and a common wiring issue at that. I initially checked the fuse (ENG1) it looked fine.

It may well have gone into closed loop momentarily..before opening up again, but I have not been able to confirm this. As far as I can tell, it is always in open loop.
 

·
Registered
Joined
·
16,031 Posts
I suspect your right, it goes into closed loop, sees no activity, throws the 2 codes and goes back to open loop.

The voltage your seeing could be due to the meter, what do you have for a meter?

I recall you saying you swapped the sensors, are any of them new?
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #9 ·
So the old sensors were effectively new (those were added just before the rebuild to try to debug the performance problem at the time. When the BarsHG1 no longer bandaged the head gasket, and the O2 sensor swap didnt either, it was time to tear it down. (gasket and 2 cracked heads). I didnt think it was the sensors, but changed them out to new ones again..and ..of course...nothing changed.

I was using a DT9205a.. some chinese meter, nothing special, stock probes on it. I will validate the probe resistance itself as I have another meter, but I really dont think they are that resistive to explain a 0.075 volt difference. Do you agree that should have measured what the scan tool indicated though..conceptually? Or am I missing something here..
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #10 ·
The resistance of the probes themselves is at most 0.5 ohm..and may be as low as 0.2 ohm..

Also, one other point. The CEL doesnt come on until after some time. It typically comes on after about 15 minutes of driving and then it may correlate with higher rpm or acceleration, but of that Im not sure. Limited data set to support that. . But it is clearly in open loop the entire time, even before the light comes on. It was the CEL initially led me to the codes...and then to the scan tool..and then to the inactivity of the O2 and then noticing being in open loop (as I didnt believe it was an O2 sensor issues, particularly when none of them had any activity).
 

·
Registered
Joined
·
16,031 Posts
If you unplug the sensors and stick the meter leads on the connector pins, you will see the same voltage that you would see at the PCM (assuming the O2 wires are not damaged).
When there is no load, there is no lose of voltage, the voltage is the same at both ends of the wire.
However, the old analog meters would add a small load and read lower voltages.
A good digital meter will add no load and will read source voltage.
Leave the O2 sensors plugged in and you introduce a load, the load will reduce the voltage the meter reads.

Yours is an $8 meter on Amazon I would worry about about .07 volts
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #12 ·
On thursday, I am heading over to someone who may have a better meter. But in any case, even if the meter introduces a little load and pulls down the measurement voltage, this doesnt seem to really impact the issue. The question is just weather the meter is responsible for the voltage delta, or some other issue is causing it.

Looking at the wiring diagram, the heater circuit could have a shared power bus as well as a shared ground loop segment. (from what I see in an old Chilton's Manual). And since the open circuit supply voltage is 11.2, it seems this should be high enough to heat the sensor properly. But perhaps not???

But the same diagram shows discrete wires for the sensor side for all for sensors, (no shared bus outside the PCM) on both sides of the sensor. The scan tool shows 0.447 V with the O2 sensors plugged in (and I think this was with engine on). But direct probing (without sensors) shows 0.37 V at the PCM. Maybe I should disconnect one sensor and validate the voltage for that sensor at the PCM via the scan tool?

My direct probing of the sensor showed them to be effectively open circuit. My understanding of the O2 sensors themselves are effectively high impedance, with the equivalent circuit of just a battery. I dont know what that voltage is when in the circuit and engine off. I assume its negligible until it heads up and is exposed to the exhaust gas. But I'm not really seeing how the issue could be the sensor side.

What was the comment about the 5v reference voltage...I didnt follow that.

But everything Im hearing so far is.... O2 sensor circuit..someplace...
 

·
Registered
Joined
·
16,031 Posts
5 volt reference, sorry, should have been 0.5 volts (I fixed the post).

I think it's strange that both sides have the same problem, there must be something common.
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #14 · (Edited)
yup..agreed... the 0.447 at the scan tool vs the 0.37 at the sensor connector really bothers me and potentially the 11.2 volts on the heater side seems potentially low (vs battery which I will check and document) also bothers me. I will update after I learn more...thks
 

·
Registered
Joined
·
16,031 Posts
Check your battery voltage, if it's low, I'd verify the accuracy of the meter. The meter could just be reading low.

Or, you could have a failing battery; but with the reading from the scan tool, I'd suspect the meter.
 

·
Registered
Joined
·
89 Posts
I'm a little confused. The codes you are getting indicate that neither S1 is being seen.
From your 1st post:
Manual Testing: Disconnected O2 sensors and measured back towards PCM with key on. 0.37 volts on the sensor side, 12.2 on the heater circuit side. All O2 Sensors have been swapped, no change.

It also sounds like you are seeing the PCM reference voltage (447mV in your case) on the Foxwell. As well, there are no codes for a failed Heater Circuit so that doesn't appear to be the issue, or at least not the primary one. Barring no codes possible on our trucks for failed heater circuits, this seems to lead to the wiring to the sensors from the PCM. It seems strange that both banks aren't reading. Is there another connector in the circuit that is open?

Are you reading anything on S2 on either side? I would expect to see codes if neither of them was being recognised, but as you show none, it seems that S2 is being seen on both sides?.

Regardless, testing with ignition on, engine off should give 400-450mV to each sensor, measured at the connector. It sounds like your are seeing 447mV on the Foxwell, and 370 mV on your meter, but I'm not sure where that measurement is being taken (At the ECM or at the connector?). If you are reading a little less than 447mV on your meter, but the same on all 4 at the connector, it is likely the meter calibration and not a wiring issue (ie not all 4 would have the same reading). If you are reading voltage at the connector, a 2nd check is to repeat the test with engine running and at temp. Now you should see 100-900mV oscillation on S1 and a relatively constant reading on S2 (key with S2 is it will not be the same as when cold & a digital meter should show swings of a few mV).

If you haven't got voltage at the connector end, the wiring from the ECM to O2 sensors should be be proved by shorting both leads and looking for continuity. A 2nd test should check each lead to ground to verify if there is a short. This will at least prove the wiring I think you have proved the leads this way, but not sure of the test to ground?).

The heater circuit should show battery voltage on the positive for all 4 sensors. 12.2 volts sounds like ignition on, engine off with a healthy battery and should be OK. As you aren't getting Heater related codes, I'm not sure the heater cct is an issue, esp if you can see both S2's and have no codes. As well, if you haven't already, the heater circuit ground lead should be tested to make sure there is connectivity from the connector to chassis ground.

If all looks OK with the wiring, a kluge way to verify things could be to connect another sensor to the leads without being inserted in the exhaust, but I think that only becomes of interest if you can see voltages as expected at the O2 sensor connector.......
 
  • Like
Reactions: RayVoy

·
Registered
Joined
·
89 Posts
And as another data point. This is what I see on my Foxwell with engine at temp. If you can see S2, they should read something like this and it sounds like you may get nothing or a flat line on the scope for both S1's?

167039



167040
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #18 ·
ok..some good info.. so maybe a little more insight now from your comments: Im heading over to my contact's place in a few hours...but..here is some of the foxwell data from my previous reviews and sorry for the long response..i am having a little trouble interpreting some of the language..

1) STFT B1/B2, LTFT B1/B2, STFTB1S1, STFTB2S1 all at 0
2) STFT B1S2 and STFT B2S2 both at 99.2%
3) Oxy Sens Output Voltage B1S1, B1S2, B2S1, B2S2 all at 0.445 V
and graphically I believe these were all rock solid horizontal lines, but I will confirm..


RE: Scott

1) "It sounds like your are seeing 447mV on the Foxwell, and 370 mV on your meter, but I'm not sure where that measurement is being taken (At the ECM or at the connector?). " 370 mV is down at the connector (under the auto, disconnected O2 sensor)

2) "Are you reading anything on S2 on either side? I would expect to see codes if neither of them was being recognized, but as you show none, it seems that S2 is being seen on both sides?."

STFT B1S2 and STFT B2S2 both at 99.2% - so how I interpret what you are saying is that S2's are effectively responding here from two perspectives. a) it is at 99.2%, albeit it is pinned at that value??? (but its still responding) b) there are no codes reflective of S2, my codes are only S1 related.. correct??.. this is pretty important as it will basically confirm the heater side is not the issue...and I can focus on the sensor side..

3) " If you are reading a little less than 447mV on your meter, but the same on all 4 at the connector, it is likely the meter calibration and not a wiring issue (ie not all 4 would have the same reading). "

So Im not sure I agree 100% here...if there WAS some common/shared issue to all for sensor lines..isnt it conceivable I would get the exact same voltage (assuming no sensor response). And from the wiring, it seems this would have to be basically inside the PCM...since when I look at the wiring diagram, there doesnt seem to be any shared external wiring for all 4 sensor lines and these lines seem to be floating (they do not seem to be grounded).. but otherwise, I think your comment is valid. What you are saying is that IF the sensors were providing signal that the PCM was actually seeing, they wouldnt be of the same magnitude. But my 0.37 v is with the engine off and sensors disconnected. (I vaguely recall watching a Pine Hollow Auto Diagnostics youtube video where the 04 Mitsubishi had an internal ground issue along these lines)

4) "If you are reading voltage at the connector, a 2nd check is to repeat the test with engine running and at temp. Now you should see 100-900mV oscillation on S1 and a relatively constant reading on S2 (key with S2 is it will not be the same as when cold & a digital meter should show swings of a few mV).

Not sure how effective I will be at backprobing the S1 connector here for a direct measurement at the connector (because I already know Im not seeing the variation at the PCM via the scan tool)...but can try... I believe that is what you are saying here. It also sounds like you are saying to check S2 with the scantool for engine off vs engine on at temp. My recollection is that the measurements were valid for both hot and cold engine, but I will confirm. Im not sure if you are saying compare it to key on engine off vs warm engine...but I can certainly check this. (This seems reasonable to determine if S2 is responding electrically at all)

5) "If you haven't got voltage at the connector end, the wiring from the ECM to O2 sensors should be be proved by shorting both leads and looking for continuity. A 2nd test should check each lead to ground to verify if there is a short. This will at least prove the wiring I think you have proved the leads this way, but not sure of the test to ground?).

Well, I do have 0.37 v at the connector (across the pins), Im wary about shorting the sensor wires, but I could measure voltage to ground. I get the impression that these sensor lines are floating..neither one is a ground?? The wiring diagram I have does not show either line as being grounded. But I can certainly measure each side to chassis.


6) "I'm a little confused. The codes you are getting indicate that neither S1 is being seen.
From your 1st post: Manual Testing: Disconnected O2 sensors and measured back towards PCM with key on. 0.37 volts on the sensor side, 12.2 on the heater circuit side. All O2 Sensors have been swapped, no change."

Yes, Im confused as well.because Im seeing voltage at the connector of 0.37v yet the PCM is telling me 0.447. So the way I interpret this is that there is cleary some discrepancy here in the voltage readings which needs to get resolved and is possibly the cause of the overall issue, but the PCM doesnt see ACTIVITY irrespective of whatever the actual (flat) voltage is...and really, Id have to say the PCM sees 0.447 v (via scan tool, sensor connectd) irrespective of what i measure at the connector (0.37v, granted the sensor is disconnect in this case). So while I can measure a voltage at the connector..I suspect that for some reason, the connected sensor response is not making it back to the PCM for either bank.

And I believe the 0.447v occurs both with and without the sensor connected (but I will confirm this). And if this is the case, it would further support that the sensor response is not making it back to the PCM.


I would also agree that the heater circuit is not the issue for a few reasons.. a) No specific codes, b) power is there with engine off (11.2 or 12.2v...even Im confused now,,but I think it was a typo and there was 12.2 v..but I will confirm) so with engine on (and alternator running) the voltage would be about 13.6v and thus there should be no power supply issue. c) The whole heater side circuit is not related to the sensor side circuit and things are pointing to the sensor side...

6) " kluge way to verify things could be to connect another sensor to the leads without being inserted in the exhaust, but I think that only becomes of interest if you can see voltages as expected at the O2 sensor connector......."

Didnt follow the logic for this one..didnt understand what test is being run here..

RE: Roy, so I had previously looked at the graphical output and it was straight line, rock solid. But I will try to capture images and post as well.

thank you both for help though...i will run various tests and post later today..
 

·
Registered
Joined
·
16,031 Posts
@oldandintheway , I think we can boil a lot of Scott's words down to a few if we said:
the scan tool was correctly reading the reference voltage of .450v (450mV) and that the scan tool was being used while engine is running and up to temp.
And that you read the ref voltage at 375mV; however you were with the key off.
Then 2 conclusions are possible:
1- the battery voltage is lower than normal when the key is off.
2- the meter is not calibrated.

My thoughts on the 2 conclusions are, I think the PCM would be designed to hold the reference volt pretty close to the 450mV spec.
Meaning if the battery voltage and the charging system was low, the PCM would not output the ref voltage to the sensors (the PCM would use something like a zener diode to ensure the ref voltage stayed at 450mV).

Therefore, if you see a steady 375mV and the meter is accurate, there is a voltage drop somewhere; however, there is no current flowing so a resistance will not cause a voltage drop.

I guess my suggestion is we need to have the accuracy of the meter checked.
 

·
Registered
1999 Suburban K1500 LT 5.7 completely stock - rebuilt engine
Joined
·
52 Posts
Discussion Starter · #20 · (Edited)
Fully understood...and very concise summary..thanks.. I just realized I have a digital power supply so will check it against that..

result: Power supply doesnt have the resolution...but I compared an Actron CP7672 with the DT9205A in the 0.7 volt range and they were 10 mV apart... thats all I have available at moment, but I wil see what my helper has available as well..my gut tells me the delta is real on the vehicle and not the meter, but will try to collect real data..

Also, with engine on..the supply is around 13.6 v, with key in and turned on (engine off), its lower, and with key just in (but not turned), its another value, and with key not in (direct to battery) its even lower, but I will quantify today as I charged the 2nd battery today (yes, I have a 2nd battery I installed with a relay which is powered only when key is on (I think))...
 
1 - 20 of 72 Posts
Top