Starting yesterday my CEL came on so I decided to bring it to Autozone for a scan. He showed me the scanner and it showed p0171 as a current problem and showed up in the logs two other times with no other codes. He thinks its the o2 sensor but said it could be something else. Upon googling this it looks like it could be one of many things... o2, injector, vacuum leak, maf, intake gasket, and probably a few others.
The car is a 2008 with about 320,000 miles (guessing I've driven it about 20k past the OD stopping). The car has been running fine since I picked it up 6 months ago. Not a single issue and drives like its new then this happened. I've done nothing other than having the codes checked since it's about 15 degrees in Michigan where I live. Also... having an o2 replaced seems really expensive from what some people have said online ($350 parts and labor) at shop. I'm also concerned that the o2 could be the thing they want to replace judging by the code (as I would) but might not fix the issue. I don't want to start just replacing parts here. Would a shop decide not to replace the o2 by using other diagnostic tools or would they do what I'd do and just start with the o2? I can do the work if I know what the real problem is.
Let me know what you'd do. Thanks!
Joe
CEL p0171... But runs perfectly fine? o2?
CEL p0171... But runs perfectly fine? o2?
2008 Pontiac Vibe - 299,999 miles since 2012 - Just won’t die
Re: CEL p0171... But runs perfectly fine? o2?
Of the lot the easiest to check is the mass air flow sensor. viewtopic.php?f=28&t=29887
Re: CEL p0171... But runs perfectly fine? o2?
I'll try this today. Since the battery cable is pulled the codes will go away and the CEL will reset... If theres another problem when will the CEL come back? My idle has been higher lately but I thought that was just due to the extremely cold temperatures. Keep you posted!
2008 Pontiac Vibe - 299,999 miles since 2012 - Just won’t die
Re: CEL p0171... But runs perfectly fine? o2?
If your intake manifold gasket hasn't been replaced then this is very likely to be the problem. See the attached pdf.
The redesigned gaskets are orange in color and less than $10 when I bought one at the local Toyota dealer.
cold start DTCs.pdf- (128.39 KiB) Downloaded 697 times
Re: CEL p0171... But runs perfectly fine? o2?
I've made a few quick trips around this weekend and the light was on every time. Today I ran to Autozone and picked up some MAF cleaner and also grabbed some Lucas Oil engine treatment. I've never put something like this in my tank but this time I gave it a shot. After running to a couple more stores on the last start the CEL didn't come on. I didn't get to the MAF yet but will when and if it comes back.
2008 Pontiac Vibe - 299,999 miles since 2012 - Just won’t die
Re: CEL p0171... But runs perfectly fine? o2?
The problem with the intake manifold gasket happens intermittently at cold start. You may have the DTC recur in a few days. It is a design issue with the original gaskets so it is likely to occur for everyone. Mine went when the car was 5 years old so an '08 is about due now. It is pretty cheap and easy to do yourself.
Re: CEL p0171... But runs perfectly fine? o2?
p0171 indicates a lean running condition from bank 1 sensor 1.
it is registering excessive oxygen in the exhaust gases. a leaking intake manifold will introduce excessive air charge into the combustion chamber and will throw off the stoichiometric ratio. there is far more air going into the chamber than what the maf is reading (because the leak is after the maf; the intake gasket). fuel amount is determined based on how much air volume is entering the intake, so if excess air is entering the engine with not enough fuel to reach stoichiometric ratio then it will run lean and this code will be thrown.
it is registering excessive oxygen in the exhaust gases. a leaking intake manifold will introduce excessive air charge into the combustion chamber and will throw off the stoichiometric ratio. there is far more air going into the chamber than what the maf is reading (because the leak is after the maf; the intake gasket). fuel amount is determined based on how much air volume is entering the intake, so if excess air is entering the engine with not enough fuel to reach stoichiometric ratio then it will run lean and this code will be thrown.
Re: CEL p0171... But runs perfectly fine? o2?
So, it's been a few days now and the light hasn't come back on. All I did was add some Lucas Oil stuff to my 3/4 full tank. After 2 quick trips with restarts the problem went away. I've had at least 5 or 6 fresh starts since then with no issues. The idle seems slightly high but that could be because it hasn't been above 25 degrees (even for a high) for a week. Other than those couple days with the engine light it hasn't happened to me and driveability hasn't been a problem. I'm going to just wait and see and act once something comes up.
Also, how can the o2 detect a lean condition in just part of the engine (a cylinder or two) if there is only one o2 in the exhaust manifold? Wouldn't it just detect a lean condition for the engine in general?
Also, how can the o2 detect a lean condition in just part of the engine (a cylinder or two) if there is only one o2 in the exhaust manifold? Wouldn't it just detect a lean condition for the engine in general?
2008 Pontiac Vibe - 299,999 miles since 2012 - Just won’t die
Re: CEL p0171... But runs perfectly fine? o2?
The oxygen sensor detects a lean condition that could be on one or more of the cylinders. The engine controller uses two methods to compute the length of time to open the fuel injectors (and thus control the mixture). One is by looking at the temperature, rpms and MAF to compute a number (called "open loop"). The other is by using the oxygen sensor to actually measure the mixture by way of the amount of oxygen in the exhaust (called "closed loop" since there is a feedback mechanism). If there is more than a 20% difference between these numbers then the p0171 is set. It is telling you that the engine controller has to add more gas to the engine than it is expecting based on the measured operating conditions. So the problem could be an air leak so that not all the air getting into the engine goes through the MAF or it could be a problem with the MAF itself. The MAF should be cleaned whenever the air filter is replaced as a normal maintenance item.
Re: CEL p0171... But runs perfectly fine? o2?
By what you said... it sounds impossible for the engine to know what cylinder / injector / bank is having the trouble unless it's individually adding more fuel to only one injector and remeasuring at the 02 and doing this for all cylinders.
Anyway, I'm concerned how difficult this will be to do. I have a lot of experience working on 90's mustangs and older carb'd cars. Not so much the vibes. I have plenty of tools and have pulled engines and rebuilt engines but looking at this thing gets me a little worried. Maybe I'm just old school. How long would it take someone who hasn't done it before? Any major gotchas that might make we want to take it in?
Thanks!
Joe
Anyway, I'm concerned how difficult this will be to do. I have a lot of experience working on 90's mustangs and older carb'd cars. Not so much the vibes. I have plenty of tools and have pulled engines and rebuilt engines but looking at this thing gets me a little worried. Maybe I'm just old school. How long would it take someone who hasn't done it before? Any major gotchas that might make we want to take it in?
Thanks!
Joe
2008 Pontiac Vibe - 299,999 miles since 2012 - Just won’t die
Re: CEL p0171... But runs perfectly fine? o2?
On a 4 cylinder engine, "bank 1" refers to the entire engine. "V" engines can have 2 banks. For the Vibe, "bank 2" is not used.
If you change oil and coolant yourself and can do brakes, this will be no problem. Look at the document I attached earlier for instructions ("Repair Procedure"). You don't need to drain the coolant like the instructions say. I just disconnected the hoses to the throttle body and plugged them with a piece of wooden dowel. Since the hoses are up high on the engine you will loose only a little bit of coolant. Leave the throttle body attached to the manifold. Be sure to reconnect all the vacuum lines. I think it took me a couple of hours.
If you change oil and coolant yourself and can do brakes, this will be no problem. Look at the document I attached earlier for instructions ("Repair Procedure"). You don't need to drain the coolant like the instructions say. I just disconnected the hoses to the throttle body and plugged them with a piece of wooden dowel. Since the hoses are up high on the engine you will loose only a little bit of coolant. Leave the throttle body attached to the manifold. Be sure to reconnect all the vacuum lines. I think it took me a couple of hours.
Re: CEL p0171... But runs perfectly fine? o2?
Cool! Thanks for the bank clarification. Makes sense. If that CEL comes back I'll do the gasket. If not... I'll keep driving it 
So far so good all day today. If something comes up about it I'll update the thread. Thanks!
So far so good all day today. If something comes up about it I'll update the thread. Thanks!2008 Pontiac Vibe - 299,999 miles since 2012 - Just won’t die
Re: CEL p0171... But runs perfectly fine? o2?
under warranty that technical service bulletin will pay the technician 1 hour for the job. since you're mechanically inclined and have adequate tools then you will have no problem doing the gasket.