Error code P0171, loss of power, low gas milage [Archive] - Forum


View Full Version : Error code P0171, loss of power, low gas milage

05-30-2008, 01:16 PM
My SES light came on the other day while i was driving. I ran it by Autozone to have them read the code and it came out P0171 - Bank 1 - System too lean.

I've changed my Fuel and Air Filters, checked for a vacuum leak, and fuel pump and pressure seems good.
The guy at autozone seemed to think that if it was a bad MAF, I would be getting that code on both banks, but I'm not sure.
I was thinking about replacing the O2 sensor (the one before the catalytic converter that sends the signal to the PCM) but not sure.
I hope nothing is wrong with my injectors. I dumped some injector cleaner in but that hasn't had any effect.
It idles and starts fine. Engine seems to run good, but there is most definitely a loss of power when stepping on the gas and I'm only getting about half the usual gas mileage. I've got 142,000 miles on the car. I replaced the LIM at 80,000 and the water "pump" at 90,000. I gave the engine a tune up when doing the LIM. New plugs, wires, serpent belt, etc.

So what's the next step? New O2 sensor? New MAF? Take it to someone and have them inspect the injectors?

05-30-2008, 01:51 PM
The MAF (Mass Air Flow) Sensor could be dirty or faulty. I would start with it first. Check the connection and get some MAF cleaner at the parts store. There could be a vacuum leak downstream of the MAF sensor as well. What type of air filter do you have?

05-30-2008, 04:42 PM
At that mileage it wouldnt be a bad idea to service the o2 sensor especially if you dont know the last time it was changed. It could be reporting slowly to the PCM and sense its still getting a signal the PCM isnt setting a code for it.

05-30-2008, 04:53 PM
I used a voltage meter to check the connector to the MAF and it was getting the proper voltage. Guess I'll make a trip to auto store and get some cleaner.

I have an STP air filter.

I bought the car with 47,000 on it and I know the O2 sensor hasn't been changed since I've had it. :)

05-30-2008, 05:03 PM
while everyone earlier has good input you all missed something obvious...there is no second bank o2 sensor. itll say bank 1, but thats the only code that can pop up. Gm has had alot of problems with their catalytic converters...hollow it out have some fun, lol

also the MAF sensor doesnt use voltage it produces frequency, so checking for voltage isnt necessary

05-30-2008, 05:17 PM
check the fuel regulator as well. mine went out because it became a little clogged, also change out the fuel filter.

06-01-2008, 10:25 AM
I used a voltage meter to check the connector to the MAF and it was getting the proper voltage. Guess I'll make a trip to auto store and get some cleaner.

I have an STP air filter.

I bought the car with 47,000 on it and I know the O2 sensor hasn't been changed since I've had it. :)

Found anything yet?

06-01-2008, 01:52 PM
Cleaned the MAF today.
Still have the SES light. I took the Neg cable loose from the battery for about a minute before starting the car to reset the code, but as soon as it started, the SES was back on.

So what's my next step? Replace the O2 sensor? Replace the MAF?

06-01-2008, 03:48 PM
Talked to a buddy of mine who has a MAF off a 96 GrandAm that I may go get tomorrow and slap on mine just to see what happens.

06-03-2008, 08:59 AM
Put on the MAF from the 96 GrandAm and still getting the error code.

So, should I try a new O2 sensor?
Speaking of which, why are there 2 sensors? One before and one after the catalytic converter? Should I replace both or just the one before the converter?

06-03-2008, 01:33 PM
instead of throwing parts at it, go to a qualified mechanic and have them look at it

sugar ray
06-03-2008, 09:26 PM

hey dude! Very glad i found these forums. I have a 2002 Grand-Am. Just a couple days ago it too has began doing the EXACT same thing that yours is. HUGE power loss under acceleration and horrible gas millage. Yup "SES" light has come on. It went out for a day but came back on and has stayed on

I purchased it a year used in 2002 up here in O'Canada. Back then it only had 20,000 KM's (think metric NOT miles in canada) the odometer now reads only 90,000 clicks. Hardly broken in!

My plan was to always Garage keep it in the Winter to AVOID costly repairs "BEFORE" it was paid off. Not to mention that nasty winters destroy many nice vehicles up here. Lone FINAL payment on the car is due the end of NEXT month so i ALMOST reached my goal.

Anyways.........your problem matches mine word for word. I have a GOOD friend that is a retired mechanic. Tomorrow we will be using his lap-top to see what Error code spits out. I "refuse" paying the dealership unless absolutely necessary. However, saying that i realize that there will come a time where it may me the only option. My friend suspects a sensor of some sort. He has alot of cool modern tools up his sleeve not available to the general public. Such as GM's all-data program. Im sure if someone looked hard enough they could find a boot leg version of it. Using "all-data" as an excellent cross reference program saves alot of guess work. My guess is the O2 sensor. Atleast, im hoping as that would be a painless and fairly inexpensive fix.

Not saying for sure that this is our problem but when i described it to him he cross referenced similar problems using all data. here is what the results read:

TSB Number: 01-07-30-009A Issued: Tue, May 1, 2001 and titled "Intermittent No Power And Diagnostic Trouble Code (DTC) P0731, Incorrect 1st Gear Ratio And-Or DTC P0732, Incorrect 2nd Gear Ratio (Replace The Transmission Fluid Pressure Switch Assembly)."

I still love my restored 79 Trans-Am. soooooo.......damn easy to work on. something goes there is NEVER any guess work. they just don't make em like they use to. these computers complicate the game and often cause consumers to spend money on unnecessary parts CHASING what COULD be the problem.

just hoping i can get my issues resolved within 2 weeks. Holidays coming up and there's no way in hell im leaving town with the car running like this!

I wish you luck! I will check back daily and post my findings. hopefully you can do the same??

06-04-2008, 08:08 AM
instead of throwing parts at it, go to a qualified mechanic and have them look at it

Not really throwing parts at it. The air filter and fuel filter needed to be replaced as part of normal maintenance. Putting some injector cleaner in it didn't hurt. Also the MAF from the 96 GrandAm was given to me and took 2 minutes to clean and swap out. The only part that I might be throwing at it would be the O2 sensor and what's to say that it doesn't need a new one even if that isn't my problem?

So far I've spent like $15.

If I do put a new O2 sensor in and still have the problem, then I probably will take it to a "qualified mechanic". But as of now, I see no reason to do that when I can do this basic stuff myself and save a million dollars in labor costs.

06-04-2008, 08:10 AM
Hey sugar ray!
The actual error code I'm getting is P0171 - Bank #1 - system too lean.

Not sure if that is the one you are getting. Let me know.

sugar ray
06-04-2008, 12:40 PM


should have read your post a little more clearly. off to my buddies now. will post my results in the next day or so.

06-04-2008, 01:00 PM
I hope you figure it out soon!

sugar ray
06-04-2008, 10:02 PM some good info today.
after plugging into the laptop the computer read 2 problem codes
P0300 & P0404

the laptop PC's seem to be the way to go after watching it in action for the first time. The software offers solutions to problems rather then just using one of those tiny hand held code readers from autozone. why i avoid the STEALERSHIP as much as possible you ask?? for S***Z and giggles i called my GM dealership and asked what they would charge to read my car's computer. stating that i wanted JUST the problem codes and i didn't want them to fix the problem. they quoted my $120 bucks. My retired mechanic friend plugged in and had the info in less then 5 minutes. kind of makes me ILL actually. The software he used (apparently avaliable online) even had the power to knock-out the S.E.S. light at the click of a mouse. naturally knocking out the light doesn't solve the problem but its nice to know that i have the power to do so once the problem has been resolved. MY situation (obviously your situation is different then mine as we have different problem codes)

P0300 & P0404 = a fault in the E.G.R. (exhaust gas circulation)

while still plugged into the laptop we drove and the software recognized random/multiple cylinder misfire

He has suggested we start with the possibility of a plugged Fuel filter and work our way down the E.G.R. system from there. I'll have a go at it tomorrow and post my results.

know this doesn't help you much as you have already changed your filter and the fact that our vehicles are reading different trouble codes.

Ill check back and see if you made any progress

06-04-2008, 10:37 PM
the egr being plugged open will cause random cyl misfire at idle and anything off cruise, so no need to look at the fuel filter. take the egr vavle off and make sure it isnt plugged open with carbon. then if that doesnt work, unplug the egr valve and see if you still have the same problem. then find something to block off the egr with(there are plates available from WOT TECH iirc, milzy if he still has them, and prob off ebay as well.) block it off and see if that fixes it. if those dont then look to other places, but what can also set an EGR code is a bad map sensor. When the comp commands the egr open, the computer looks for a drop in the map sensor reading, if there is a bad map, it can cause cyl misfires, loss of power, and inaccurate fueling and can cause a faulty egr code even tho there isnt anything wrong with the egr valve. just a few places to look first.
but also changing the fuel filter anyway isnt a bad idea

06-05-2008, 09:40 AM

What kind of setup does your friend have? How does he connect the laptop into the car? What program does he use to read the code and monitor the system?

I am very interested! I may try to duplicate his setup if its reasonable.

Picked up a new O2 sensor yesterday. I will be putting in on Sunday and will post back.

sugar ray
06-05-2008, 01:13 PM
gectek: thanks for the heads up! TRUST ME i will try ALL of your suggestions!

tenstrum: my buddy isn't running anything special! just enough to get what were after. He was using a basic lap-top running windows XP. the key component is a tool known as " Elm scan 5 " you will need the elm scan to get the info from your car's computer. Good news is the program we used (scan tool) is avaliable to download for FREE!

Elm scan 5 is a little Gizmo scan tool that supports all OBD-II protocols. its about the size of a pack of smokes and plugs into your USB or serial port. the going rate on the device ranges from $99 - $150 depending where you shop online. He has and uses several DIFFERENT software applications. We used the FREE scantool program to retrieve my codes. the interface is very straight forward. Then he opened up a couple of DIFFERENT programs that he actually purchased. they were more advanced then the FREE scantool. However, the more advanced programs are just bells and whistles. He claims the FREE scantool program is all anyone needs to be pointed in the right direction.

Here are a couple of links. im hoping we are allowed to post these???
as im NOT endorsing a product, just trying to help a brother out! Especially since the software for the device is FREE

the device you would need for your computer to read your car

you can download the software for free here:

hope thats the info you needed? im off to the garage to hopefully solve my issues

ill check back

sugar ray
06-09-2008, 05:23 PM

after attempting your suggestion of UN-plugging the EGR valve i made a strange discovery. the car ran BETTER like signifigantly BETTER with the power cord UN-plugged at the EGR valve. the mis-fire at LOWER RPM went away all together!
as a test...... i plugged it back in and lone behold.......low rpm misfire. Unplug and low rpm misfire went away. Conducting this test down the highway about 3 miles from home both ways.

However, at higher rpm range (4500) especially up-hill the car is still a dog. Up-hill its a slug, to the point where you have your foot to the floor but aren't going very fast at all! This is the same with or without the valve plugged in.

so what gives??

i have NOT completely taken the valve off to inspect for carbon. just tried the un-plug trick. I don't imagine that replacing the valve or purchasing a blocker plate will improve my situation??

My mechanic friend has been quite busy so i haven't had a chance to see him since last post.

06-09-2008, 05:29 PM
well the EGR is only supposed to work during low load cruise conditions, it will still act sluggish at any speed really esp higher rpms, also if your egr valve has been causing the prob for a while it could have contaminated your 02 sensor and/or catalytic converter with unburned fuel, which is bad. its prob a good idea to change the 02 sensor anyway and have a person that knows what they are doing to perform a backpressure test on your converter i e put a pressure gauge where your UPSTREAM O2 sensor is and raise the rpm to 2k, you should see less than 1.75 psi, any more than that and ull need to either clean the cat out or get a new one, or but the zzp downpipe...

sugar ray
06-09-2008, 10:44 PM
thanks a zillion for all your sugestions!

I think i will just bite the bullet and book my car in at the STEALERSHIP. Old school small block chevy is more my level. As much as i hate to bring my car in anywhere.
I spoke with a GM mechanic today that is familiar with these cars. He's figured that i have already saved myslef $$$$ by retrieving the trouble codes. He explained that as nice as the "scantool" program is, its fairly basic. Where GM has more advanced programs that are capable of diagnosing systems even further.

I just hate throwing my hard earned $$$$ at dealerships but due to the fact that this is my family's daily driver this time of year, what can ya do??

perhaps i will get off easy and a faulty o2 sensor is the issue. I certinly don't mind spending money to cure the problem, but as you pointed out earlier in this post "why throw money at it guessing?" I hope to be certain before purchasing un-necessary parts.

hopefully they will grant my request regarding after market parts. I realize GM dealerships are eager to throw in original GM parts that usually cost 2 to 3 times the cost of an aftermarket.

I'll post back with my results. Hopefully the other gentlemen got his problems resolved.

thanks again, your input is much appreciated:applause:

06-10-2008, 08:13 AM
gm doesnt have any more special tools than you can buy off the tool truck, you just gotta have the money for it. they like to say that, but its :thumbsdow

sugar ray
06-11-2008, 12:09 AM
yup.....i realize that they will say just about ANYTHING to gain your buisness and $$$$. what manufacture wouldn't? however, probably cheaper for me to get it repaired there rather then to spend the $$$$ for those fancy tools and go through the long learning process of the wonderfull world of O.B.D. II

did you ever see that news clip done on CNN about a year ago? This lady owned a 1974 Olds. The motor lived to see 1 MILLION miles. needless to say the body was in poor shape (not UN-safe rusty poor just massive paint fade) the rest of the vehicle had usual repairs. Apparently it went through a couple of trannies and timing chains as well. Needless to say, Olds. agreed to give this women a brand new vehicle in exchange for her workhorse. The news clip ended with a GM rep. on record stating that they wanted to strip down the block and figure out what went wrong with this car? why would an engine ever go this long, this just doesn't make sense etc.....

you would think GM would take that oppurtunity to brag about longevity? nope....they are only content if were bringing in our cars for weekly repairs to their dealerships.

a little off topic......but a fun side note

06-11-2008, 06:47 AM
not much to learn about obd II really, its like this, if you know the phrase...GIGO
garbage in, garbage out. if you see a lazy sensor that should have a descent refresh rate or something that stays constant or throws back a funny number, that or the wiring or somthing in the subsystem or logic is messed up

06-11-2008, 07:41 AM
Haven't had a chance to to put in the new O2 sensor yet.
I was over at my parents house yesterday looking at it with my dad and it's going to take more effort than I realized to change that thing out. The upstream O2 sensor is located between the firewall and the manifold right one top of the exhaust pipe. Very small space and tight squeeze. My dad said that he could rig up some jumper wires and we should be able to test the O2 sensors with a digital multimeter. But it will have to be this weekend.
I'll report back again after that.

06-11-2008, 08:47 AM
no you cannot use a multimeter, the refresh rate on it will ONLY show an average, not nearly enough to test the sensor, srry, and your meter has to be very precise as well, because the operating voltage is between .100v to .900v. the only way you can test them without a scan tool is an oscilloscope

sugar ray
06-11-2008, 03:39 PM
BINGO guys!

found the problem, atleast i sure hope!

brought it to the dealership and explained the trouble codes i was getting. They hook it up and of course get the SAME trouble codes.

a plugged converter seems to be my issue. Im just hoping that the plugged converter is NOT plugged because of something else has failed? Would you expect a converter to go on its own with such low miles on the car? At anyrate, they call me back and explain the situation. This is the part that buggs me to no end........

"yes sir, its a plugged converter. all sensors, regulator, and the EGR valve are fine nothing serious but the part will cost $512"

cough....cough...excuse me! i reply... "$512! is that an aftermarket converter or original GM replacement??" "not sure sir....$512 was the price i was quoted plus 1 hr labor to put it in" i tell them at that price to JUST run a straight pipe and bypass the converter. naturally they won't due that due to emission even though up in my part of the world we don't even require emmission tests what so ever.

I direct them to do NOTHING and i will come pick up the car.
I call a local auto parts store and find out they have aftermarket converters in stock for $129. so whats the catch? i explain to the auto parts store that i was quoted $512 for this part at the dealership. after further research we discover that $512 is the price for a staright bolt in GM factory replacement. you gotta be kidding! gee....lets see here....what would you do?

long story short........ a local muffler shop will charge me $129 (for the after market converter) + $100 bucks for labor and new fittings and pipe as the aftermarket converter isn't a straight bolt in.

done deal! car goes in to the muffler shop tomorrow

what ripps me is the dealership didn't even give me that option of going with a MUCH CHEAPER aftermarket converter. they were ready to just install that fancy $512 replacement. I will be emailing that dealerships owner with the fact that i wasn't even presented with options. NOT That the complaint will do any good.

I ended up paying $65 at the dealership for their diagnosis time. IF the converter fixes my problem then it was money well spent.

06-11-2008, 03:44 PM
while everyone earlier has good input you all missed something obvious...there is no second bank o2 sensor. itll say bank 1, but thats the only code that can pop up. Gm has had alot of problems with their catalytic converters...hollow it out have some fun, lol

also the MAF sensor doesnt use voltage it produces frequency, so checking for voltage isnt necessary
^^^quoted to remind you, you dont need a new converter, just make a "high flow" one at home

06-15-2008, 05:15 PM
got in my car yesterday, started it and no SES light....?????
Started fine, drove fine, normal power, gas mileage seems good...
Same thing today....
Don't know what exactly cured, if it is fixed. Guess I'll hang onto that O2 sensor I bought for a bit to see if the problem comes back.

I wonder if the brand of gas I bought may have had something to do with it...
I'm totally confused at this point.

sugar ray
06-17-2008, 01:15 AM
on my end of things..........

after installing a new converter life is "GRAND" again (no pun intended)
the guy at the muffler shop said he had seen worse but NOT many.

Gectek: thanks again for all your input. hollowing out a converter to make it high flow is just waaaaaaaaaaaay to creative for me. I'd just rather bypass the sucker all together! But you know how air care quality rules go. Just curious, would a hollowed converter ever pass an air-care test? they can visiually inspect to see that a converter exists but a hollowed one?

great idea none the less!

06-17-2008, 08:06 AM
yes actually it wont put out alot more pollutants than running with a cat at lower rpms, itll pass the sniffer anywhere but california, and no unless they hit it with a hammer, you cannot tell it has been unrestricted. I know alot of people say to leave the cat together and just get another one, but when it not economical to do that, then removal of the material is an alternative. I had mine opened up for quite a while, ran it on the 5 gas at school and the emissions were still within spec, attribute that to a cool running engine for the low NOx, and a properly tuned well running engine for the rest. under WOT its a different story, but they dont do emissions test under WOT...just moderate load.