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Discussion Starter · #21 · (Edited)
Day-7 Monday
Truck sat over night; fuel dash gauge at 3/8th full.
Started engine, shifted to neutral, A/C & Fan off, 4 incomplete I/Ms

MinuteECT (F)IAT (F)Fuel SystCodesA/C & [email protected]
04843Opennoneoff
49448Opennoneoff
611348Opennoneoff
712248Opennoneoff
812948ClosedP0161off
10

Was expecting a P0141, not a P0161.

So for the computer to evaluate the function of the two O2 sensor heater circuits, it doesn't seem necessary to run an idle test (2).

I had removed/labeled/swapped bank 1 and bank 2 post-cat sensors two days ago, but got the same code today, P0161. This is consistent with sensor heaters testing fine, both with a resistance of 4.9 ohms.

But with key-on, there is battery voltage across the pink & black harness wires at the disconnected harness connector. And there is also battery voltage between the battery post B+ and the inner terminal of O2B at the fuse box in the engine compartment.

There have never been codes related to the other three sensors. Since the wiring diagram shows the 4 sensor heaters sharing a common ground, the ground connection is probably in good condition.

Since the bank 2 precat sensor has always been code-free, the P0161 could be caused by a poor connection in bank 2 postcat sensor heater circuit. There is single fuse for both sensor heaters on bank 2. There is single fuse for both postcat sensors. Somewhere after the fuse the wires must fork, one going to the precat sensor heater and one to the postcat sensor heater. Somewhere after the fuse the wires must fork, one going to the bank 1 postcat sensor heater and one to the bank 2 postcat sensor heater. The fork could be in the harness where the harness, or could be at the fuse box, which is where the wiring diagram shows them forking, but it's just a schematic, not a blueprint. Anyway, the possible poor connection would be somewhere between where the wires fork to the two sensors, and the harness connector for the post-cat sensor.
 

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2021 Trail Boss LT l84 10L90
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I read the wrong code number when I stated P0155, you are right, should have been P0141.

Mitchell shows 02A fuse feeds the upstream and 02B feeds the downstream, connected together right at the underhood fuse box. Both wires will be crimped in the same terminal.

The ground shows as B2S2 is first in the circuit from the ground on G104 (back of the driver's side head). You should physically look at the eyelet and tug on the wires to make sure one isn't corroded or broken.

Any exhaust leaks?
 

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Discussion Starter · #23 · (Edited)
Mitchell shows 02A fuse feeds the upstream and 02B feeds the downstream, connected together right at the underhood fuse box. Both wires will be crimped in the same terminal.
Thanks, on closer look at the Haynes/Chilton schematic, it shows what you said. I'll go back later and correct my mistkes in previous posts.

The ground shows as B2S2 is first in the circuit from the ground on G104 (back of the driver's side head). You should physically look at the eyelet and tug on the wires to make sure one isn't corroded or broken.
Thanks, the Haynes/Chilton schematic doesn't identify which/where the ground is, at least not that I can see.

Any exhaust leaks?
No leaks that I can hear. But there is a downstream exhaust leak, at the joint where the muffler pipe connects to the Y-Pipe. The location from that joint to the P0161 sensor measures 13 inches. I only know that there is a leak at that joint because it drips water, so I was thinking the leak is minor, would not matter, and was going to take care of it after the DEQ inspoection. Let me know if you think the exhaust leak is a likely cause.

When I had the y-pipe off 6 or so weeks ago, it didn't look like that joint has a replaceable gasket. Does it? (V6 2WD Ext Cab Manual Trans). Short story is that when I had the y-pipe off due to a cracked weld/flange on the y-pipe where it bolts to the driver's side bank1 exhaust manifold. I replaced the two gaskets where the y-pipe bolts with the exhaust manifolds.
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Day-7 cont Monday
I'm working on some voltage-drop tests at the moment in hopes of identifying a poor connection. I've never run a voltage-drop test, but have heard about them. So I rewatched Schrodingers video here:

I draped a ten foot 14 gauge extension cord from B+ to the passenger side. It is a cord I use regularly without issues with power tools. I pinch-clamped one of the cord's flat terminals at the B+ post.

I jammed a T-pin in the harness connector for postcat sensor to back probe the pink wire in the harness. With O2 sensor connected to the harness and the key on, the voltage-drop was only 0.2V. I believe this means that the section of the circuit from the battery, to the ignition switch, to the fuse box, to harness connector is fine relative to the amount of current drawn by heater.

I jammed another T-pin into the back of O2 sensor pigtail connector at the wire corresponding to ground (black wire on the harness. With sensor connected and key-off, the voltage-drop to the frame was 0.00V. With key-on, the voltage-drop increased a fraction but still less than 0.1V. I believe this means that the section of the circuit from the sensor pigtail, through the connector, through the harness, to the ground bolt, to the chassis is fine relative to the amount of current drawn by heater.

In hind sight I should have back probed the pigtail for both voltage-drop tests.
 

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Discussion Starter · #24 · (Edited)
Yesterday, Monday, I finished up by moving the pink wire T-pin from the harness side of the connection to the pigtail side so that the voltage-drop measurement would include the connection itself. The key-on voltage-drop increased by about 1/2 volt, so no real change.

So my conclusion was that the heaters of both sensors in the post-cat bungs were fine, and that all the wiring and connections in the post-cat circuits was fine. I thought the exhaust leak might be a possible cause of the P0161, but unlikely. The Y-pipe, cats, intermediate pipe, muffler, and tail pipe are the factory originals and are 99.9% rust free. The y-pipe/manifold joints have new gaskets. The only leak I'm aware of is small, located at the Y and intermediate pipe connection. I was planning on fixing that as the next step in solving the P0161, but didn't get around to it yesterday. So last night I erased the codes so that for next day's (today's) cold start, I could repeat the tests I had run yesterday morning.

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Day-8 Tuesday
I'm dejected to have to say that after the cold-start P0161 yesterday, this morning the cold-start produced a P0141.
Truck sat overnight with the dash fuel gauge showing 3/8ths full.
Started engine, shifted in neutral, A/C and Fan off.
MinuteECT (F)IAT (F)Fuel SysCodesA/C & Fan
04343Opennoneoff
711848Opennoneoff
813048Closednoneoff
1216048noneoff
1316748P0141off
1517648P0141ON
22P0141off
26P0141

I'm at loss for any explanation what so ever. I've been taking meticulous notes on a clip board. I'm 99.9% certain that yesterday's code was P0161, and today's is P0141.

They are AC Delcos. I'd have to find my notes from several years ago to know if I replaced them then. I've never had to do work to pass DEQ. If I did replace them, it would have been in June of that year (DEQ comes up in Oct) when I was doing a ton of maintence prior to a long road trip. If I didn't replace them at that time, then they are factory originals.

Since time is running out for DEQ inspection, I'm ordering two new post-cat sensors today in case I end up wanting to replace them. But I really do not think the current ones are bad.
 

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Discussion Starter · #25 · (Edited)
Day-8 cont Tuesday
Drove the truck front end up onto ramps.

With key-on, and using yesterday's T-pins still back probed in the pigtail connector in sockets C and D, I measured the voltage across the T-pins. Voltage-drop, as expected was close to battery voltage, 11.81V. This on the passenger side which has the swapped in O2 sensor originally installed on the driver's side.

I grabbed two more T-pins and back-probed the driver's side sensor pigtail, finding 11.77V with key-on. With key-off, the resistance across the T-pins to be 2.5 ohms, which is what one would expect since the resistance path is for the two heaters in parallel, since the the two sensor heaters are connected via their common hot wires.

All the electrical test seem consistent. The circuit wiring and various connection for the post-cat sensors on both banks match and are fine even under load. And the two heaters match and fine. Given ohms law, each heater draws heaters draws about 2.4 amps, so the 15 amp fuse carries about 5 amps.

I look online again for a gasket for the leaking Y-pipe / intermediate-pipe connection, but couldn't find any indication that the joint is designed for one. The two flanges there have an integral, non-replaceable donut type configuration similar to the replaceable donut on at passenger side Y-pipe / exhaust-manifold connection.

So to address the leak, I unbolted the leaking joint, wiped down the mating surfaces, applied a bead of Permatex silicone that they market as a sensor-safe exhaust sealant, then rebolted the joint.

There is easy access to the two y-pipe / exhaust-manifold joints. Since the engine and exhaust had cooled, I started the engine and felt around those two joints with my fingers but didn't feel any puffs/currents of warm air, or any puffs at all.
 

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Discussion Starter · #26 · (Edited)
Day-9 Wednesday
Truck sat overnight with the dash fuel gauge showing 3/8ths full.
Was up at 2:30 am, went outside to try to sneak in an extra early cold start.
Started engine, shifted in neutral, A/C and Fan off.
ECT IAT were 57F and 48F, which barely meets the "within 11F" cold-start criteria

MinuteECT (F)IAT (F)Fuel SysCodesA/C & Fan
05748Opennoneoff
510848Opennoneoff
6118Opennoneoff
7127Opennoneoff
813552Closednoneoff
914552noneoff
1015652noneoff
11165P0141off
15P0141off

Turned off engine. Erased codes.

Truck then sat for 8 hours until I went back out at 11am for another cold start.
Dash fuel gauge showing 3/8ths full.
Started engine, shifted in neutral, A/C and Fan off.
ECT IAT were again 57F and 48F, which barely meets the "within 11F" cold-start criteria.

MinuteECT (F)IAT (F)Fuel SysCodesA/C & Fan
05748Opennoneoff
39752Opennoneoff
511852Opennoneoff
613054Opennoneoff
713854Closednoneoff
8noneoff
915454P0141off
1418754P0141off

Turned off engine. Erased codes
 

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Discussion Starter · #27 ·
Found this ground distribution diagram for O2 sensors in a 9 year old photo collection from a library book:

Black Font Schematic Parallel Rectangle
 

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Why are you still chasing the heater circuit? There is no problem with it.

The problem is a lazy O2 sensor, taking too long to start switching. You moved the sensor and the code moved so replace it!
 

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Discussion Starter · #29 · (Edited)
I don't think it's that certain, but yep, could very well be that half of the sensor. Or something else. I doubt it's the small downstream exhaust leak (fixed yesterday), or the G104/G114 ground (since it's shared by the oil level circuit and the two precat sensors, which haven't thrown any codes).

One test I haven't done is monitor the voltage drop across the heater during the 8 minutes or so of idling following a cold start. But given the momentary measurements that I've taken of the key-on voltage-drops across the heater elements, I doubt it's the heater circuitry.

Another test I have not done is back-probe monitor the O2 sensor signal. I don't have an appropriate tool for that, but think they can be had for under $100, such as the PS100 by Autel.

I've several apple boxes full of nearly all the old parts I've pulled from the truck. I rummaged through them today, but found only two oxygen sensors, and they were precat, replaced precautionarily a few years ago when I was preping for a long road trip. So the postcat O2 sensors that are currently on the truck are factory origianl, 21 years old with nearly 300k miles on them. They are AC Delco AFS 106. As I wrote above, I've two AC Delco AFS 106's on order.
 

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Discussion Starter · #30 · (Edited)
Day-10 Thursday
Truck then sat overnight. Dash fuel gauge showing 3/8ths full.
Started engine, shifted in neutral, A/C and Fan off for the duration below.

I found out that the Actron CP9580 allows one to live-graph one data item at a time, so I took some pics of the postcat O2 sensor signals during this mornings trial. I'm not sure if this is the raw voltage data streaming from the sensor, though I doubt it, or if it is what the computer is reporting. Anyway, it includes whatever filtering is coming from the wiring.

The main difference I saw between the P0141 sensor signal and the other sensor signal is that the P0141 sensor signal has a visible amount of high frequency noise, though it might not be too obvious in the cell-phone photos due to blurring of a slow shutter speed. The amount of noise in the P0141 sensor signal decreased somewhat as the minutes passed. Also, it looks like the sensor signals are analog, so I'm not sure what is meant by switching in "too long to start switching".

MinuteEngineECT (F)IAT (F)Fuel SysCodesO2S12O2S22
0start5448Opennone
4 Rectangle Font Line Slope Parallel Rectangle Slope Font Parallel Pattern
6idle12254Opennone
7idle13554Closednone
8idlenone Rectangle Font Slope Parallel Pattern Rectangle Slope Font Line Parallel
9idle16554P0141
10 Rectangle Font Slope Parallel Pattern Rectangle Font Line Parallel Pattern
14off
15erased codes
16start18357none
17off18957none

Turned off engine. Erased codes
 

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Discussion Starter · #32 ·
Day-10 cont Thursday
The pair of new sensors that I ordered (ACDelco AFS106) arrived last night. Unfortunately, or not, they are not ACDelco AFS's 106s. My truck has the factory original sensors,, and they are stamped ACDelco AFS106. The ones that arrived are GM Genuine 19355306, and their heater's have 3 times the resistance, 17 ohms compared to 5ohms. With a resistance being 3 greater, the current will be 1/3rd, or 12/17=0.7 amps. Given the reduced current, the resistive heating will be reduced to 1/9th. The sensor looks completely redesigned, so maybe they reduced the thermal mass accordingly.
Some other aspects match, such as length of the pigtail, and a single divider in the connector.

Anyway, drove the front end up on ramps, pulled the P0141 sensor and installed the new one. The engine had not cooled enough to qualify as a cold-start, but I let it idle into closed loop mode on the off chance the computer would give it a pass.


MinuteECT (F)IAT (F)Fuel SysCodesA/C & Fan
09164Opennoneoff
412964Opennoneoff
513564Closednoneoff
14noneoff

So, no codes and all 5 I/Ms incomplete: cat / evap / egr / O2 Mon / O2 Htr

Dash fuel gauge was getting close to 1/4, so drove to the station to add a few gallons.
 

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So are you done yet?
 

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Discussion Starter · #34 ·
Day-11 Friday
Truck sat overnight with the dash fuel gauge 1/2 full.
Started engine, shifted in neutral, A/C and Fan off.
ECT IAT were 59F and 48F, which either meets or misses the "within 11F" cold-start criteria

MinuteECT (F)IAT (F)Fuel SysCodesI/MsA/C & Fan
05948Opennone5 incoff
712254Opennoneoff
812754Opennoneoff
913354Closednoneoff
1013854noneoff
1114454noneO2Htr OK; 4 incoff
1916754noneO2Htr OK; 4 incoff

Took it for a drive. The other O2 I/M completed OK, at which point there were no codes and 3 incomplete I/Ms: cat/evap/egr

Did the (345) drive cycle test, then the (678), then turned around and headed home driving regularly. Still no codes and 3 incompletes.
 

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Discussion Starter · #36 ·
Day-11 cont Friday
Sat for a couple hours.
Took it out on a one-stop errand, 40 minute, surface streets.
Sat for an hour.
Took it out on some errands. Firstly, 35 min, mostly highway at 55mph. Sat for a half hour. Then 3 more stops. Then 45 min, mostly highway, mostly 30-40 mph.
Three I/Ms have not completed: cat/evap/egr.
Both I/Ms for the O2 sensors are completed, OK'd.
No codes.
 
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