do I need to reset the ECU or clear codes when replacing O2 sensor?

sunliner

Original poster
Member
Mar 25, 2012
365
I was discussing my P0171 woes today and someone mentioned that after I replaced the O2 sensor ( and maybe some of the other stuff I replaced..) that I should clear the codes with a my scanner and/or unplug the battery to reset the computer. logic was that the truck has 'learned' to compensate for the problems and may still be adjusting fuel trim on the high side. I guess in other words...maybe the issues have been fixed but I'm still showing high trims, until a certain number of miles or drive cycles has happened and the computer re learns everything.

does that make sense?.....
 

BlazingTrails

Member
Apr 27, 2014
19,409
sunliner said:
I was discussing my P0171 woes today and someone mentioned that after I replaced the O2 sensor ( and maybe some of the other stuff I replaced..) that I should clear the codes with a my scanner and/or unplug the battery to reset the computer. logic was that the truck has 'learned' to compensate for the problems and may still be adjusting fuel trim on the high side. I guess in other words...maybe the issues have been fixed but I'm still showing high trims, until a certain number of miles or drive cycles has happened and the computer re learns everything.

does that make sense?.....
I would definitely recommend doing a pcm reset. it will do a new case learn and run much better.
 

Mounce

Member
Mar 29, 2014
13,667
Tuscaloosa, AL
Most people clear them out just to get the service engine light outta their face. If you fixed the problem, it will go away on x amount of drive cycles but if you have access to a scanner, clear them if you want. I don't think the engine gets a memory for that and only compensates when it sees something wrong, not that it sees something wrong once or twice then compensates from there forward.
 

sunliner

Original poster
Member
Mar 25, 2012
365
after I connected the battery again, it shows a pending P0300 code (random misfire). Only other time I've ever seen this code is one time after I had pulled the fuel pump relay to check pressure, and it cleared within 2-3 drive cycles. hope it's something similar this time.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Quite possibly, with the bad O2 sensor, it couldn't detect the misfire. Now with a functioning sensor, it sees a misfire or the sensor you got is a dud. What brand sensor did you use?

And after resetting the PCM, you might need to clean the throttle body if it's been a while, although that would result in a bad idle or stalling when using the A/C.
 
  • Like
Reactions: BlazingTrails

sunliner

Original poster
Member
Mar 25, 2012
365
Mooseman said:
Quite possibly, with the bad O2 sensor, it couldn't detect the misfire. Now with a functioning sensor, it sees a misfire or the sensor you got is a dud. What brand sensor did you use?

And after resetting the PCM, you might need to clean the throttle body if it's been a while, although that would result in a bad idle or stalling when using the A/C.
I cleaned the throttlebody a few months ago. went back in there tonight as part of my hunt for the 0171 cause, and found the bolts holding the throttle body on were loose (but it was still shiny!)... I wonder if there was a leak from it being loose. my fuel trims look a little better but it's too soon to tell.
 

BlazingTrails

Member
Apr 27, 2014
19,409
sunliner said:
I cleaned the throttlebody a few months ago. went back in there tonight as part of my hunt for the 0171 cause, and found the bolts holding the throttle body on were loose (but it was still shiny!)... I wonder if there was a leak from it being loose. my fuel trims look a little better but it's too soon to tell.
In that case i would go ahead and clear the codes. how loose were they? enough to be sucking air around the throttle body?
 

sunliner

Original poster
Member
Mar 25, 2012
365
BlazingTrails said:
Just don't clear the p0300 code. If you keep driving it you may get a code for a specific cylinder or 2. Probably need new coils.....
that would be awesome, because heaven knows I don't put nearly enough money into this crapcan. Looks like those are about $42 a piece...do I have to do all six at once, or can I get by just doing the ones that are misfiring?
 

sunliner

Original poster
Member
Mar 25, 2012
365
BlazingTrails said:
In that case i would go ahead and clear the codes. how loose were they? enough to be sucking air around the throttle body?
I feel like it's a possibility-at least two were loose enough that they moved under the weight of the wrench even before I got my hand on good. I just hope it's that easy.
 

BlazingTrails

Member
Apr 27, 2014
19,409
sunliner said:
that would be awesome, because heaven knows I don't put nearly enough money into this crapcan. Looks like those are about $42 a piece...do I have to do all six at once, or can I get by just doing the ones that are misfiring?
You can replace just the coils that are problematic.
sunliner said:
I feel like it's a possibility-at least two were loose enough that they moved under the weight of the wrench even before I got my hand on good. I just hope it's that easy.
I would go ahead and reset the codes again, and run it. see what happens.
 

sunliner

Original poster
Member
Mar 25, 2012
365
found this over on samarins.com...you guys may already know this but just in case.

Additional trouble codes can shed some light on the cause
Additional OBDII codes that are present along with misfire codes can shed some light on the cause. For example, if you have the P0300 code along with P0171 - System too lean, there is a good chance that the misfire is caused by the lean air/fuel mixture that initially triggered the code P0171. Once the problem caused the code P0171 is fixed, the misfiring will likely go away too.
 

BlazingTrails

Member
Apr 27, 2014
19,409
sunliner said:
found this over on samarins.com...you guys may already know this but just in case. Additional trouble codes can shed some light on the cause
Additional OBDII codes that are present along with misfire codes can shed some light on the cause. For example, if you have the P0300 code along with P0171 - System too lean, there is a good chance that the misfire is caused by the lean air/fuel mixture that initially triggered the code P0171. Once the problem caused the code P0171 is fixed, the misfiring will likely go away too.
Yes and quit cheating on us lol
 

sunliner

Original poster
Member
Mar 25, 2012
365
BlazingTrails said:
Yes and quit cheating on us lol
I just figured I was blowing up the boards over here a lot lately and driving people nuts...had to spread the love. Really though, I do appreciate the help...this truck has been kicking my a$$ lately. On top of work, kids, a school board made up of fools, wife, house and all that other stuff. I got plenty to do. thanks again.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
sunliner said:
On top of working for my wife, fools tearing down the house, a school board made up of kids, and all that other stuff. I got plenty to do. thanks again.
There, I fixed your post. :eek:
 

sunliner

Original poster
Member
Mar 25, 2012
365
but theriously folks....while it was probably a good thing I checked the throttle body (see previous post)...looks like I didn't really change anything.
took the ol' tub out for a drive. initially the LT trims were down around 4%. drove around town and then got on the interstate for about 15 miles...LT trim back up to ~12-14 %. got back onto the city roads at 30-45 mph and at one glance the trim was 17.5% ,as low as 5% but generally still hovering around 12-14% .

Currently no hard codes (I had unplugged the battery last night) My pending 0300 has cleared, but I do have the 0171 pending, not surprisingly. I've just about exhausted everything I can check or fix...much as I hate it....might be time to pay the man.
 

sunliner

Original poster
Member
Mar 25, 2012
365
and how about the MAF sensor? I cleaned it...but how would I tell if it needs replacing? Getting to the point where I can't afford to throw more parts at the truck...thanks
 

sunliner

Original poster
Member
Mar 25, 2012
365
finally thought to grab the freeze data when the code set...does look like I have a lot of air coming in , if I'm reading this correctly
CAM00962_zps92f985b7.jpg
 

sunliner

Original poster
Member
Mar 25, 2012
365
no....I'm afraid I might find one... :frown: ...but yeah...I need to look. would that manifest as a vacuum leak though? everytime I put a gauge on it, it looks normal.
 

IllogicTC

Member
Dec 30, 2013
3,452
sunliner said:
no....I'm afraid I might find one... :frown: ...but yeah...I need to look. would that manifest as a vacuum leak though? everytime I put a gauge on it, it looks normal.
No, not on the EXHAUST manifold, which would have positive pressure as the engine pushes it out (as compared to vacuum on the intake side). This could be the right direction. After all, you're seeing initially decent fuel trims, and after the motor warms up, then it goes bananas. Metal expands and contracts with heat and cold, perhaps there is a crack which is widening once the manifold is hot.
 
  • Like
Reactions: Mounce

IllogicTC

Member
Dec 30, 2013
3,452
Mounce said:
That seems like the thing to check after an o2 sensor change if the P0171 is still present and causing havoc. Quite common, too.
:yes: I've read threads where people are saying it's impossible, as the higher-than-atmospheric pressure in the exhaust manifold would push gases OUT not draw them in. But to that, I say it's acting on a sort of Venturi Effect, the draft of the exhaust gases passing down the manifold may be pulling some fresh air in, or not all the exhaust gas is getting accounted for if it is indeed escaping.
 
  • Like
Reactions: Mounce

BlazingTrails

Member
Apr 27, 2014
19,409
IllogicTC said:
No, not on the EXHAUST manifold, which would have positive pressure as the engine pushes it out (as compared to vacuum on the intake side). This could be the right direction. After all, you're seeing initially decent fuel trims, and after the motor warms up, then it goes bananas. Metal expands and contracts with heat and cold, perhaps there is a crack which is widening once the manifold is hot.
So are you thinking clogged cat?
 

IllogicTC

Member
Dec 30, 2013
3,452
BlazingTrails said:
So are you thinking clogged cat?
I'm thinking a crack in the exhaust manifold. A hairline one may not do much but as the metal changes shape from heat it may open the crack up and cause issues.
 
  • Like
Reactions: Mounce

sunliner

Original poster
Member
Mar 25, 2012
365
could be a hairline crack. My good ol' Blazer had a cracked exhaust manifold but that thing was loud as hell. I heard it before it ever cause a code to set. Drove that money pit for 15 years...

Looks like the exhaust manifold is another common weak point on this platform, but I see they're also famous for the bolts backing out (as are the intakes) I know I've been on a rant lately, but does GM ever fix anything?
 

Mounce

Member
Mar 29, 2014
13,667
Tuscaloosa, AL
I hadn't heard of the bolts backing out but I've heard many cases of breaking them when trying to take the manifold off. :eek:

I guess your first attack should be to pull the heat shield off and inspect for cracks.
 

wbduval

Member
Aug 24, 2014
10
Guys - I'm new on here and wanted to know if there's a place I can go to get a primer on reading and interpreting the spec values on the OBDII, i.e., the ones in Sunliner's post # 24 above? I've been working on cars for a long time, but the computerized stuff is fairly new - beyond reading the codes. I'm in the process of working through and P0171/P0172. Currently replacing both O2 sensors and contemplating replacing all coils. I have 189K on my 04 4.2L XUV.
Thanks in advance.
 

sunliner

Original poster
Member
Mar 25, 2012
365
Mounce said:
I hadn't heard of the bolts backing out but I've heard many cases of breaking them when trying to take the manifold off. :eek:

I guess your first attack should be to pull the heat shield off and inspect for cracks.
Agreed. Gonna go get me a can of PB Blaster...sounds like I might need it.
 

wbduval

Member
Aug 24, 2014
10
BlazingTrails said:
well i have 2 scanners but i still do a pcm reset. it resets all of the fuel and timing maps. i use the scanner to clear stored codes.

We speaks tha treuth :coffee:
Another newbie question - how do you do a PCM reset? Seems appropriate for my situation. Just disconnect the battery for 30 or do I use the scanner?

Thanks - came over here this morning from TrailVoy when I read that most experts there had come over here. This site is a lot more active, for sure!
 

BlazingTrails

Member
Apr 27, 2014
19,409
wbduval said:
Another newbie question - how do you do a PCM reset? Seems appropriate for my situation. Just disconnect the battery for 30 or do I use the scanner?

Thanks - came over here this morning from TrailVoy when I read that most experts there had come over here. This site is a lot more active, for sure!
Welcome to the Nation!

OK so here's the scoop on a PCM reset, beyond clearing the trouble codes it also reset fuel and timing maps.

Here's how to do it:

1. Make sure all your HVAC controls are in the off position.
2. Disconnect the neg battery terminal.
3. Touch the brake pedal, and then turn the key like you are going to start the truck. This will ensure that the power has been completely drained.
4. Reconnect the battery terminal.
5. Turn the key to the run position for 2 min.
6. Start your truck and let it run for 10 min, do not pit it in gear, do not rev it, do not turn on a/c.

This is so it can program the idle.

That's it.

Do not do a PCM reset unless you have thoroughly cleaned the throttle body. Or it will stall out a lot while trying to drive it
 

wbduval

Member
Aug 24, 2014
10
Anyone have an opinion on whether I should just bite the bullet and replace all the coils (189k)? I've got 'em here and as long as I'm taking everything off to clean the TB, it'd save time.
 

Forum Statistics

Threads
23,272
Posts
637,481
Members
18,472
Latest member
MissCrutcher

Members Online