Please help with P0171
 

Hello everyone,
I have an 05 cobalt and the engine light has been going on and off randomly.

Code is P0171 system too lean and only symptom is HORRIBLE milage lately. Like 186 miles per tank bad.

Im thinking the MAf or the O2 sensor is faulty. I've posted all the freeze frame info for the OBD2 reader. Long term fuel trim is almost at 20%, thats not right is it ? anyone who knows how to read that info see anything else out of whack ???

Looking for help thanks !!

Here are the pictures

http://i50.tinypic.com/vuwqv.jpg http://i46.tinypic.com/23tl202.jpg http://i50.tinypic.com/18j3mf.jpg
http://i45.tinypic.com/dqrhw0.jpg http://i45.tinypic.com/24faf06.jpg http://i45.tinypic.com/vxno1i.jpg

Welcome to the forum

is the air filter clean or changed regularly?
Have you tried cleaning the MAF sensor ?
Make sure the air intake snorkel is properly attached and all the clamps snug, if you are sucking air in somewhere it can cause problems...

Air filter is changed regularly.
Everything is attached properly and tight and i cannot find any cracks or leaks.

Have not cleaned the maf sensor yet, i am planning on doing it but i dont think that is the problem, and here is why. Someone please correct me if i am wrong.

p0171 is activated when the LTFT becomes way to elevated. ie 20%. If the MAF sensor was dirty, then MAf would be detecting less airflow than there was and the car would run lean, o2 sensor would detect lean mixture and long term fuel trim would go up to compensate. It would cause a p0171 but the car would still be getting good milage, because the trim would be compensating and sending the right amout of fuel for the right air/fuel mixture. the only reason the code would go on was because of the high trim. is this right ?

However, if the o2 sensor is faulty and not reading the right o2/fuel mixture. it could be thinking im running lean and cranking up the fuel trim even though car isnt running lean. That would cause mileage to go way down cuz im really running rich but the sensor isnt picking it up. and then the code comes on from the trim being so high... can anyone confirm this before i buy a $160 02 sensor???

thanks guys!

Hmm. Lean condition to me says air leak, inlet air not being measured acrosss the maf. The O2 sensor could be telling the pcm to compensate for that, however if its running rich the O2 sensor would see that. I'd suggest clean the maf with approved cleaner. I understand there has been no mods to the inlet air plenum?

Here is a troubleshooting guide for the P0171, however keep in mind that the P0171 may not be the root, but a subsequent failure caused by something else...

thanks guys,

will give the above things a try.

@kevinkpk
yes no mods, completely stock 05 cobalt LS with all appropriate maintenance done myself.

@in2pro
very useful pdf, great info, but i couldnt get the links in it to work

That was in2pro. The pdf link worked for me.

The links within the PDF file itself won't work because the document is a standalone. Let me know what particular link you were wanting to follow and I can see if I can post it;)

still working on this issue. Would it be possible to upload the link for "Intermittent conditons" under the heading "Diagnostic aids" on page 2.

In the context on that page, intermittent condition would refer to it being intermittent right ? the CEL comes on for a week or 2 then clears itself for a week before coming back on. is this what that what they are refering to ?

thank you very much.

Intermittent Conditions
 

Here you go, give it a try :)