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Discussion Starter #1
This has probably been covered before but I can't find it so please bear with me.

My daughter has a '05 Cobalt, bone stock, 2.2 with 103,000 miles on it. It just threw a P0171 code. I retrieved the freeze frame data which I will post below. The book says the O2 sensors should be reading 200-800mV. I was looking at what I believe to be the rear sensor and it was basically rock steady at 740-760mV. The reading for what I believe to be the front sensor was all over the place - near 0 to near 800mV and constantly changing so fast I couldn't really read the numbers. What should that front sensor be reading, how fast should it be fluctuating and what are the expected limits of said fluctuation?

I realize a lot of things can cause a lean mixture but with the history of the car I'm leaning towards bad O2 sensor(s). The history follows for those who care:

The car had a problem with random misfire for some months. I finally figured that out but in the meantime she had to keep driving the car. Immediately upon fixing the misfire problem it threw a code I don't remember and I changed the Cat (last Oct) and all has been fine. I probably should have also changed the O2 sensors but I didn't. Yesterday it threw the P0171 code for lean mixture. I was dumb enough to clear the code before grabbing the data but the car ran fine with no codes for about twenty minutes @ about 50-55mph. When running the car up on ramps to change the oil it threw the code again. The freeze frame data shows long term fuel trim at 19.5 and short term at 8.6. Should I suspect the O2 sensors?

Freeze Frame Data: (I don't even know what some of this stuff is lol)

TROUB CODE P0171
ABSLT TPS (%) 25.9
ENG SPEED (RPM) 779
BARO PRS (“HG) 28.9
CALC LOAD (%) 15.7
MAF (LB/M) 0.46
MAP (“HG) 11.8
OUT TEMP 126
COOLANT 187
IAT 127
IGN ADV 9.0
ST FTRM1 (%) 8.6
LT FTRM1 (%) 19.5
VEH SPEED 0
ABS LOAD 20.8
FUEL SYS 1 CLSD
FUEL SYS 2 N/A
REL TPS (%) 14.1
THROT CMD (%) 18.8
ABS TPS B (%) 26.7
ACC POS D (%) 20.0
ACC POS E (%) 9.4
EVAP PURGE (%) 15.7
CMD EQ RAT 0.999
FUEL LEVEL (%) 53.3
VPWR (V) 14.32
ENG RUN (m:s) 01:57
 

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Two possible issues. One, you could have a vaccum leak at the throttle body. Make sure the intake piping is securely on and not producing any leaks. Two, your bank 1 O2 sensor (front) has failed. If you replace the O2 sensor only use the AC Delco sensor, 90% of people have issues with the Bosch ones.

Sorry, dont know the specifics for the readings, but know that is your problem!
 

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Two possible issues. One, you could have a vaccum leak at the throttle body. Make sure the intake piping is securely on and not producing any leaks. Two, your bank 1 O2 sensor (front) has failed. If you replace the O2 sensor only use the AC Delco sensor, 90% of people have issues with the Bosch ones.

Sorry, dont know the specifics for the readings, but know that is your problem!
I appreciate the response. Can you explain how you know the front sensor has failed?

Do you know the GM part numbers for the O2 sensors?
 

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This has probably been covered before but I can't find it so please bear with me.

My daughter has a '05 Cobalt, bone stock, 2.2 with 103,000 miles on it. It just threw a P0171 code. I retrieved the freeze frame data which I will post below. The book says the O2 sensors should be reading 200-800mV. I was looking at what I believe to be the rear sensor and it was basically rock steady at 740-760mV. The reading for what I believe to be the front sensor was all over the place - near 0 to near 800mV and constantly changing so fast I couldn't really read the numbers. What should that front sensor be reading, how fast should it be fluctuating and what are the expected limits of said fluctuation?

I realize a lot of things can cause a lean mixture but with the history of the car I'm leaning towards bad O2 sensor(s). The history follows for those who care:

The car had a problem with random misfire for some months. I finally figured that out but in the meantime she had to keep driving the car. Immediately upon fixing the misfire problem it threw a code I don't remember and I changed the Cat (last Oct) and all has been fine. I probably should have also changed the O2 sensors but I didn't. Yesterday it threw the P0171 code for lean mixture. I was dumb enough to clear the code before grabbing the data but the car ran fine with no codes for about twenty minutes @ about 50-55mph. When running the car up on ramps to change the oil it threw the code again. The freeze frame data shows long term fuel trim at 19.5 and short term at 8.6. Should I suspect the O2 sensors?

Freeze Frame Data: (I don't even know what some of this stuff is lol)

TROUB CODE P0171
ABSLT TPS (%) 25.9
ENG SPEED (RPM) 779
BARO PRS (“HG) 28.9
CALC LOAD (%) 15.7
MAF (LB/M) 0.46
MAP (“HG) 11.8
OUT TEMP 126
COOLANT 187
IAT 127
IGN ADV 9.0
ST FTRM1 (%) 8.6
LT FTRM1 (%) 19.5
VEH SPEED 0
ABS LOAD 20.8
FUEL SYS 1 CLSD
FUEL SYS 2 N/A
REL TPS (%) 14.1
THROT CMD (%) 18.8
ABS TPS B (%) 26.7
ACC POS D (%) 20.0
ACC POS E (%) 9.4
EVAP PURGE (%) 15.7
CMD EQ RAT 0.999
FUEL LEVEL (%) 53.3
VPWR (V) 14.32
ENG RUN (m:s) 01:57
You do realize your title implies O2 sensors but you didn't post the HO2 readings. They are on the next page probably.
 

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Discussion Starter #5
You do realize your title implies O2 sensors but you didn't post the HO2 readings. They are on the next page probably.
I appreciate your response and yes, I'm asking about O2 sensors. I double checked the freeze frame data for O2 sensor readings and I don't see any. There is no "next page" as one just arrows down until the end of the list.

As far as the live readings go, as I posted, "the rear sensor [is] basically rock steady at 740-760mV. The reading for...the front sensor was all over the place - near 0 to near 800mV and constantly changing so fast I couldn't really read the numbers."

I don't have an oscilloscope or any kind of graphing capability on the code reader so I can't say exactly what the front sensor was doing.
 

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I know this is the issue only because it's a common code and those are the common problems associated.

ACDELCO Part # 2131693
 

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I know this is the issue only because it's a common code and those are the common problems associated.

ACDELCO Part # 2131693
I very much appreciate the information, thank you. I figured the culprit ws probably the front sensor but I think I'm just going to replace both of them while I'm under there. I'll check the intake as well. I'm sure it could use a new air filter anyway.

From what I have researched this afternoon, the front sensor changes from 3 to 7 times a second, with a corresponding change in the short term fuel trim. Without a graphing capability to see actual amplitude and frequency, the readings are fairly worthless from what I gather.

Once again, thank you.
 

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Front HO2 is always more active with action especially if your throttle body is dirty, because throttle actuator has a hard time to follow "PCM" control signals and the first HO2 gets rich and lean situations in a short time. If at this time your engine is idling all over the place I would change air filter if dirty, clean throttle body is a must if you haven't already done so, and inspect your "MAF" sensor for dust,hair, lint or oil. Then clear your code and retest your HO2's. Second HO2 is always calmer because it's after the "CAT".

---------- Post added at 06:44 PM ---------- Previous post was at 06:39 PM ----------

Where do you live to have intake air at 127°F? Might want to look at your intake air temperature sensor. That could explain your lean code. It is coupled with the "MAF"
 

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Front HO2 is always more active with action especially if your throttle body is dirty, because throttle actuator has a hard time to follow "PCM" control signals and the first HO2 gets rich and lean situations in a short time. If at this time your engine is idling all over the place I would change air filter if dirty, clean throttle body is a must if you haven't already done so, and inspect your "MAF" sensor for dust,hair, lint or oil. Then clear your code and retest your HO2's. Second HO2 is always calmer because it's after the "CAT".
The amount of action on the front sensor is what had concerned me but apparently that's normal, as far as I can tell, which to be honest, doesn't seem to mean much. Whether the engine light is on or off the car seems to be running just fine but that's hardly unusual for CELs. When I get my hands on the car again this weekend I'll check out the intake system and see what I can find. I know CRC makes a Mass Air Flow Sensor Cleaner. I thought I would pick some up and give it a try along with a new air filter.

Where do you live to have intake air at 127°F? Might want to look at your intake air temperature sensor. That could explain your lean code. It is coupled with the "MAF"
I believe that reading has a lot more to do with underhood temp than anything else. The ambient temp was in the mid-80s. I had just run the car at 50-55mph for about 20 minutes and no codes were sent. The car then sat (not running) in direct sunlight for a few minutes while I adjusted the ramps. I then restarted the car and as I was driving up the ramps the light came on again. That also explains the ~2 minute run time.
 

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Do you have a "SRI" on the car or OEM intake. OEM intakes the air from just in front of the passenger front tire almost at the lower grill level. Air should be ambient or close to it not 50°F higher. Next time you check out the readings keep an eye on it. I'm going to go check mine and I'll be right back.
 

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Do you have a "SRI" on the car or OEM intake. OEM intakes the air from just in front of the passenger front tire almost at the lower grill level. Air should be ambient or close to it not 50°F higher. Next time you check out the readings keep an eye on it. I'm going to go check mine and I'll be right back.
OEM Intake. Everything on the car is factory stock - standard 2.2L engine.
 

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Just pulled the 4 that we were talking about. Like yours my first HO2 reads .1v to .8V screenshot happened to catch it at .1. Second HO2 is solid @ .7V. Intake and outside temperature as you can see are the same.



---------- Post added at 08:30 PM ---------- Previous post was at 08:27 PM ----------

OEM Intake. Everything on the car is factory stock - standard 2.2L engine.
Then my friend I would tend to believe that there is something not quite right with your temperature sensor.
 

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Then my friend I would tend to believe that there is something not quite right with your temperature sensor.
Well, the Car is fixed. I pulled the entire air intake system off the car - what a pain in the butt - made sure everything was hunky dory and put it back together. I changed the air filter and cleaned the MAF sensor. The gasket between the two halves of the air intake was essentially missing so I put that back and sealed up the gaps with RTV. When I put the air intake back on the throttle body it seemed to sit a lot lower than when I took it off. I suspect that it had slipped up somehow and was leaking air which is why I got the lean code.

I took it for a thirty mile test drive with mixed city/highway driving. The intake air temp is now only 4-6 degrees above ambient instead of thirty. I think that joint was letting in a lot of hot engine compartment air.

The long term fuel trim is running 3-5 instead of 19.5. The short term fuel trim was running -2 to about 3 so that looks much better.

The rear O2 sensor is putting out about .660V instead of .750V and the front, although still jumping around quite a bit isn't going bat guano crazy like it was.

The GM O2 sensors are already ordered but I guess I'll just refuse them when they get here.

All in all a pretty simple fix and I thank all of you for helping me out. :grouphug
 

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We love a success story.....
 
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