P0171 CEL Code

Normally at half a tank im at 150 miles, now though at half a tank I was at 80, and at 1/3rd left, im at 130.

The CEL came on during hard acceleration up a hill. Sense this issue happened three weeks ago i discovered that my most rear exhaust hanger had come undone. Possible that the cat sensor wires snapped and is screwing my fuel efficiency?

Autozone tells me Fuel mixture system lean bank 1.

Ive had this issue on bank 2, and it turned out to be the coil, they're all stock, at 182k miles now.

Truck is still powerful, but at idle has a light chug, and during hard acceleration shudders a bit.

Causes Low fuel pressure, (pump/filter/blockage? I'd imagine id have a loss of power?)
Faulty MAF/VAF sensor (its stock and clean)
Cylinder misfire condition (this doesnt say a whole lot, coil?
Faulty HO2/AF sensor (is this the cat sensor?)
Large vacuum leak (ill spray starter/brake fluid, but doubt its this. i have all newish hoses.)

Also, this info is from autozone, when my parents had a cel on their 98 GMC, the dealer told them it happend a certain number of miles ago, which they calculated to a hill, it was a misfire and happened once and that was it. so the dealer reset the computer and theyve had no problems sense. Can a toyota dealer tell me anything more? Sure would be nice, autozones info seems.... universal and leaving some to be desired.

 

it's running lean so the computer is compensating the lean by dumping a lot of fuel thus your poor gas mileage. i know you said your maf is clean, but did you actually clean it or did it just look clean? do the usual tune up parts as well. it could be the a/f ratio sensor which is common to go out on these toyotas. you have a good list going so just start with the cheapest and easiest things first.

 

It could be your upstream O2 sensor, but doubt it.

PO171 is a common code for misfire, it's most likely a misfire.

I'd bet it's something far as spark. Coils, wires, or plugs.



You can try and resetting computer..pull NEG terminal on battery for 15-30 minutes.
Then put it back on..then start it, pull EFI 15A fuse under hood..it'll kill the truck. Then put fuse back in. Re-start it..let it idle, then drive. See if code comes back.

 

Hes right it is most likely the Upstream O2 sensor, i have the same light just came on,and I know every thing is good but i have not replaced that 02 sensor.

 

And still the fight continues!

 

I must know how you resolve this I think I have the same problem, Im at like a 100 miles at half a tank.

 

It is not the coil. Maaaaaaybe wires. I'm thinking the o2 sensor on cat, or maf. I should do seafoam through gas tank to clean injectors and other stuff.

 

Change out your mass air flow sensor, cleaning doesn't work as good as replacing. I had the same problem, except for the exhaust bracket. After I changed the MAF truck was running like new. On the first gen. the MAF is a common problem, don't know what year you got but the sensor is relatively cheap.

 

If you do decide to change out the MAF disconnect the battery while before you start so you computer will reset.

 

Did you get a new one or a rem? saw one with a core charge at autozone

 

got the one with core charge at autozone

 

Zombie i know, but did you (op or others with symptoms) ever figure this one out?
I got the same thing going on now in my 2004 tacoma

 

Im at 100 miles @ 1/2 talk no CEL light...

Granted alot of my driving is short low mileage trips...but didnt seem ALOT better when i drove to seattle (5 hrs)

 

I had this issue on a recent 700km road trip (1400 km total) and noticed on the way out around 10 MPG as well as the cel came on when i romped it on the highway...
Took it to a dealer (was 700 km from my tools and scanner )
They replaced pcv valve (aparently it had failed), replaced air filter, and cleaned maf. Truck ran okay for the first tank and then boom back to 10 mpg cel on. Next fill up cel back off and somewhere around 14 mpg? The intermitent nature of the porblem is the worst part, ordering a scangauge 2 to monitor faults and A/F real time.
Gonna do a tune up tomorow as i just noticed the plug wires are from 2004 and i have heard that misfires can cause a lean reading.

 

Replace wires and plugs. Then refer to FSM for coil testing.

Mine was coils. I ordered all new denso parts from RockAuto.

 

Check all your vacuum lines

 

Will do. Is NGK stuff okay? (for wires and plugs) Its all I can get locally. The local parts store wants nearly $80 for ngk plug wires...

Considering OEM from conicelli partznet is $55 that seems a little steep.
Id rather not wait the 1-2 weeks it would take to get stuff from rock or conicelli.

 

Shipping is like 3 or 4 days. And you know it'll be OEM quality cause denso makes Toyotas parts.

NGK I have zero experience.

 

Nvm, I see your from ca. Check dealer.

 

I fear the dealer prices, i will check tomorow but the parts i bought to do my waterpump and timing belt from conicelli, (around 400) was upwards of 1200 locally.