Notorious P0171 Code

Keep getting the P0171 code thrown at me. Changed the fuel filter, and MAF and Upstream O2 Senors. Still getting the code, think I might just have a burnt out cat. Anyone have some insight on this?

 

could be exhaust leak. sometimes people have changed rear o2 and fixed it. otherwise it needs a new cat and you should also be looking for the reason it went bad. cats often are killed by leaking/ burning oil. if it isn't burning oil and just had a lot of miles then just replace it.

 

P0171 is tricky because it’s caused by a lot of things. Try cleaning your MAF that cleared my code.

If you replaced it maybe it’s sometning else.

 

My P0171 was caused by a bad vacuum switching valve.

 

Probably gonna get a vacuum leak test done on it, then move to the cat if that doesn’t reveal anything. Thanks for the input!

 

Forgot to mention this, after replacing my fuel filter and upstream o2 sensor my truck started having little spurts of delay in power and completely stalled on me at one point. Was in a parking lot the the day after and it felt like it was misfiring. Any ideas on what might be causing that? I assumed it was some air I accidentally trapped in my fuel lines but it’s been over three days since I changed the filter. It’s also idling at a lot lower rpms now.

 

The biggest thing people forget to do when dealing with a P0171, is to disconnect the battery or pull the 20amp EFI fuse to reset the learned values for fuel trims in the ECM. I’ve seen tons at the dealer that someone cleaned the MAF and failed to reset the ECM and the code comes back.

 

The sensor’s a Denso.
I had the codes cleared after I replaced each of the parts I listed earlier. It usually took 3-4 drive cycles, but then the light would come back on.
More worried about the way the truck’s running now. Started happening the day after I replaced the fuel filter, I wouldn’t think that if I did trap air in the fuel lines it would still be posing a problem, but keep getting misfire symptoms. Especially when getting off the freeway, and driving at a slower speed, the idle’s been a little erratic also.

 

I am also having this problem. I have had this engine code for a while and I've slowly been replacing parts to fix it, but it has not gotten better. It has actually gotten much worse just recently and i experienced the same misfiring and extremely rough idle when I was at a drive-thru. I almost thought it was going to quit on me for a little bit because the bucking got so bad. It is definitely worse at low speed or stopped. It used to be it only did it when stopped and in drive so I would just put it in neutral, but now it is all the time no matter the transmissions position (Neutral, park, etc.). I idle around 700 rpms. It seems to have dropped just like you described yours did.

I have changed the fuel filter, cleaned the MAF, replaced an 02 sensor (the rear), and I am about to replace the PCV valve. I don't know if it will help much, I've heard it can, but that part is mostly because I know it has to be bad after 310k miles. I have about had it and am going to remove the throttle body and clean it here soon. TPS sensor and all to see if that helps. Ill let you know if I have any luck.

 

tps sensor is likely your issue Toyotajim

 

Had a buddy of mine run a voltage test on the new MAF sensor I installed. Turns out the one I bought was faulty, voltage was all over the place especially after slowing down and stopping to idle. Through a nicer one in and it seems to be running okay.....for now hahaha

A tps sensor probably wouldn’t be a bad idea for me to replace either though.

 

Your problem could be a lot of things. You’re describing a rough running condition. If you have a vacuum leak- your o2 sensor will read lean, resulting in the p0171 code. If you have a misfire, the o2 sensor picks up all of that fresh air being pumped into the exhaust, which it thinks is a lean condition, and will also pop a p0171 code. Fuel pressure, vacuum leak, dead cylinder, and the list goes on and on. The easiest way to check for a vacuum leak is to listen for a hissing sound, and also spray carb or brake cleaner around the engine bay with the engine running. If you notice the engine start to pick up and run better, you’re most likely spraying directly, or very close to your vacuum leak. P0420 is the catalyst efficiency code that will pop up when the catalytic converter has gone out. That’s also easy to see with a scan tool. If your up stream and downstream 02 sensor signals mirror each other, your cat is not working correctly. Because your engine isn’t running correctly, and you have a “lean condition” code... not an o2 sensor code, it doesn’t mean your o2 sensor is bad. It maybe telling you exactly what you need to know in order to find the root cause of your problem.

 

How often does a MAF need replaced? And also how did you test it? My code will go away at times and then it will come back. If i reset the ECU it will stay off for 150~ miles but always comes back with a lean code. Im gonna replace sparks and wires this weekend and see if that helps.

 

I've had to clean my MAF about every 30k. never had to replace it yet. it's 2 screws and i keep a can of electrical cleaner in the truck for that purpose. fixes it every time. if you still have the oem maf try cleaning it and putting out back in. you have to clean the wires down inside, not just the bulb looking thing. don't touch then with anything. they're delicate just spray with a forceful blast from the can of cleaner.

 

I’ve never cleaned my MAF. 253k miles so far.
Edit, since I’ve owned it. 98k - 253k

 

i had a k&n filter for awhile and it caused me to clean it more often due to the oil in the filter. if you've never cleaned it or replaced it if be surprised it hasn't caused issue yet. i used to really notice it going up hills if it was dirty. it would have trouble when hard on the throttle and would hesitate or stumble a little bit. if left too long it would throw that code p0171. bank 1 lean. cleaning the maf always fixed it.

 

Cleaned the throttle body good. Must have pulled a quarter pound of carbon build up out of it, but we are running better than ever now. Put in new sparks too so that helps. So far so good on no engine codes too.

 

I had this code when I first got my truck and the dealer tried everything to get it go away (was still under warranty), new O2, air/fuel and MAF sensors, new spark plugs/wires, new vacuum lines, etc. Finally a master tech figured out that there was a tiny leak on the back of the supercharger where the capped EGR valve was broken. Definitely make sure you look hard for vacuum leaks because this code usually results from that.

 

No codes thrown on me since I installed the new MAF sensor.
The truck’s definitely idling rougher than it did before and don’t feel like I have as much power, acceleration seems a bit delayed and/or weak. Think it might have to do with the throttle, but after researching, realized the ‘04 3.4 has a different throttle body assembly than other 1st Gens. I’m kind of at a loss at this point, anyone have any idea why I’d be idling rougher and having less power?

 

Right after you installed the new MAF sensor? Did you go OEM? Toyotas do not respond well to aftermarket sensors