P0171 on 2002 Echo - Page 2 - Toyota Nation Forum : Toyota Car and Truck Forums
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post #16 of 43 Old 08-22-2017, 09:16 PM
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sure thing. i forgot which one..but bank x is precat bank y is post cat..just double check to see which is which if replacing just one so you do the right one.good luck.try to stick with denso.
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post #17 of 43 Old 08-22-2017, 09:42 PM Thread Starter
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sure thing. i forgot which one..but bank x is precat bank y is post cat..just double check to see which is which if replacing just one so you do the right one.good luck.try to stick with denso.
Thanks...one more question, if you don't mind. If I find that my first O2 sensor numbers were misread, and that the new O2 readings do fluctuate in idle, do you think it still makes sense to tackle the O2 sensor(s) first, or should I replace the MAF sensor first?
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post #18 of 43 Old 08-23-2017, 11:40 PM
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Yeah I would tackle the 02 sensor first. They generally are less expersive than the MAF by $20 or so. Buyautoparts(dot)com has them, they are usually are re-boxed denso, at least their fuel injectors are. Plus, you cleaned the MAF, so unless it's gone, id check less expensive things first. Worst case in the end (semi-optimally) would be you ended up replacing the o2 sensor and eventually the MAF.

This is another thing you can do to eliminate if MAF or not:
If you know someone with one of the following vehicles, when they aren't looking steal their keys, and pop the hood and steal their MAF.....Or you could just ask them....(I suppose that's better..I guess..)

LEXUS ES300 (2002 - 2003)
LEXUS ES330 (2004 - 2006)
LEXUS GS300 (2001 - 2006)
LEXUS GS430 (2006 - 2007)
LEXUS GS450H (2007 - 2011)
LEXUS IS300 (2001 - 2005)
LEXUS LS430 (2001 - 2006)
LEXUS RX330 (2004 - 2006)
LEXUS RX400H (2006 - 2008)
PONTIAC VIBE (2003 - 2008)
SCION XA (2004 - 2006)
SCION XB (2004 - 2006)
TOYOTA 4RUNNER (2003 - 2009)
TOYOTA CAMRY (2002 - 2006)
TOYOTA CELICA (2000 - 2005)
TOYOTA COROLLA (2003 - 2008)
TOYOTA ECHO (2000 - 2005)
TOYOTA FJ CRUISER (2007 - 2009)
TOYOTA HIGHLANDER (2001 - 2010)
TOYOTA MATRIX (2003 - 2008)
TOYOTA MR2 SPYDER (2000 - 2005)
TOYOTA PRIUS (2004 - 2009)
TOYOTA RAV4 (2001 - 2005)
TOYOTA SIENNA (2004 - 2006)
TOYOTA SOLARA (2004 - 2008)
TOYOTA TACOMA (2005 - 2014)
TOYOTA TUNDRA (2005 - 2011)
TOYOTA YARIS (2007 - 2014)

Run the car and see if check engine light goes off/stays off with their MAF or not. If not, then it's the O2 sensor (unless if ultra-rare chance it's both).
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post #19 of 43 Old 10-05-2017, 10:00 PM Thread Starter
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Quick update:

My son ended up buying the MAF sensor anyway and not surprisingly, the P0171 code came up again. Ran this through the scanner afterwards. After reconnecting the battery terminal, we got the temporary rough gas pedal conditions again, with the P0171 code as well as codes for misfires in cylinders 1, 2 and 4. After the gas pedal issue disappeared, we reset the codes and the P0171 returned, but not the misfires. So it seems the response issues from the gas pedal are just from the engine relearning its timing patterns after a battery reconnect. Since the misfires do not come back, I'm assuming it's unlikely a tune-up is needed at present.

As for the P0171 code, some slight differences from the prior readings after the MAF replacement. Whereas previously the STFT1 readings were at -3.1% when idling and at 1.6% at 2500 RPM, they now read 14.8% at idle and 0.0% at 2500 RPM. So they are now dropping considerably instead of rising slightly. The LTFT1 readings are slightly higher than previous (now 44.5% at idle, 41.14% at 2500 RPM); however, it's still a consistent 4% drop when run the engine at 2500RPM . MAF readings are very similar to previous (between 3.0 g/s and 5.9 g/s). O2 1x1 is reading with normal fluctuations; 02 1x2 is still hovering in the lower range (around 0.1V regardless of RPMs).

Since the readings are still low for the O2 1x2 sensor, I will likely tackle that next. Safe to assume that is the post-cat sensor and that means ripping out the passenger seat and carpet to access it?

Before I do, however, in an earlier post you mentioned "if no intake or exhaust leaks, I would say 02 and poss MAF." With that in mind, should I check for vacuum leaks first, before tackling the O2 sensor, or is it unlikely a leak would cause one sensor to consistently read low? Please let me know your thoughts.

Many thanks!

Last edited by Mumford67; 10-05-2017 at 10:02 PM.
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post #20 of 43 Old 10-05-2017, 10:22 PM
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When I went through this, I remember buying a can of starter fluid to test for intake leaks. On a COLD car, start her up, and spray starter fluid around injectors and intake manifold. If RPMs go up when you do that, you got an intake leak. A cigarette *might* work too, seeing if the smoke sucks in or not--but I would prefer starter fluid method. As far as post cat 02, no it's easy. Disconnect Wiring from Under Seat.Take pass seat out (for access/room). Along/under center console where carpet goes up behind it, you will see the rubber gromet ,and wiring where it goes through to the oxygen sensor.


https://www.toyotanation.com/forum/26...due-po141.html

*High STFT and LTFT generally are "bad"...as it means there is excess air coming in that computer cannot compensate enough fuel for proper mixture as still to much air, and can't go any more rich since maxed out...I think? I seem to remember FT readings around 15-20 at 2k RPM or something, so something is still making it try to run richer--yeah I would check for intake leaks. There are some good videos showing how to spray it around. Take off plastic engine cover for better access to injectors. But faulty o2 can cause this by giving faulty readings, so buying cheap can of starter fluid could/would help rule out intake leaks. Use the straw supplied with the fluid to get around/in the intake->block mating area (tight/hard access..tricky).

Last edited by l33ch; 01-15-2018 at 11:50 PM.
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post #21 of 43 Old 01-15-2018, 03:27 PM Thread Starter
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Another update on this repair:

We first started last month with the intake manifold test with a can of starter fluid. After repeated attempts, there were no noticeable adjustments when spraying the fluid around the manifold area so we ruled that out.

Today we tested the heater resistance on the downstream oxygen sensor and received a reading between 15 and 16, which is within spec so we determined that sensor is functional. So then we moved to the upstream sensor, where we got a reading between 6 and 7. Since this should be in the 11-16 range, similar to the downstream, we are assuming this is faulty and is in need to replacement. So we are going to order the part through RockAuto and install it over the weekend. Looks simple enough and easily accessible from under the car.

One strange thing, though. Recently the CEL has just been intermittently been going on and off, with no rhyme or reason. Seems like one day it will go on and a few days later it will go off. Any thoughts as to why? Is there maybe still enough resistance in the upstream sensor to trigger inconsistent readings to the computer?

BTW, my apologies for the wide gaps between updates - given my son's schedule, we have been lucky to be able to look at the car for more than a couple of hours each month. Since we're hitting the end of his last extension for his emissions test, we are intent to wrap this up this coming weekend. Fingers crossed.
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post #22 of 43 Old 01-15-2018, 06:13 PM
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Could be any number of reasons. During a driving cycle(s) the computer checks its sensors if operating within spec, if not, the CEL comes up. If during x number of driving cycle(s) the computer determines the sensor(s) operating within spec, it will clear out that code.If a sensor isn't operating properly within spec but still between driving cycle(s), it will log a pending code and if it continues it will go to a full fledged CEL.

The reason for intermittent CEL can be many:heat, weather, humidity, wiring/connectors, degrading/faulty electronics (ecu, coils, resisters, boards, etc.), fuel related*, elevation, vibrations, goblins, possessed spirits, UFOs, mischievous people.

*I have an intermittent CEL random misfire code (p0300?) that occurs when my gas is 1/2 or less full. Car will start up the 2nd time, everytime, but will stumble and run rough..then the CEL will come on about 15 seconds later. If start car again within few hours with 1/2 or less of gas, car starts right up. If I keep the gas more than 1/2 full, it will go away after about a week of driving and the car starts and fires up after the 1x attempt within couple cranks. I've pretty much narrowed it down to fuel delivery (vs ignition), and one of these years will replace the fuel pump, and its pressure regulator (and filter). This is just an example of an intermittent CEL where in my case, if there is more gas (1/2+) in the tank--somehow-someway (gravity/pressure...The Force--I dunno..lol) I don't have this issue.

Last edited by l33ch; 01-15-2018 at 11:29 PM.
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post #23 of 43 Old 01-21-2018, 12:14 PM Thread Starter
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So I replaced the upstream sensor today. Actually removed pretty easily and it looks like it has seen better days:



That said, after driving for about 30 minutes, I got a pending P0171 fault code again on Torque. Hasn't triggered a CEL yet, but I'm guessing it's just a matter of time. <sigh>

So with the MAF and upstream sensor replaced, no apparent leaks around the manifold area, and normal resistance reads on the downstream sensor, any ideas as to where to look next? I know that acceleration in the car could feel smoother, as if its hesitating some, so perhaps something within the fuel system? I have read that issues with the fuel pump and fuel filter might be possible culprits.

As always, any guidance is appreciated.

- Mumford67
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post #24 of 43 Old 01-21-2018, 01:10 PM
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Oh man, that sucks. I'm sorry. I know you have limited time(s) you are able to work on this car, I was seriously rooting for it to be solved.
This pretty much covers what you have already done but maybe check it out and re-do/recheck the lines/hoses in car: https://parts.olathetoyota.com/p0171-code-toyota

Everything is indicating up to pre-cat. Air leaks, pcv, 02, intake. Only thing I can think of is to re-check. Even air-filter. Use strong lighting and 2-3 pairs of eyes go over every inch of hose, intake, etc look for leaks/cracks/hissing--take it all off and put back on..maybe a clamp? check airbox and its hose for any cracks/leaks--flex the rubber (press down) so cracks become more evident. According to that link, check fuel injectors as well. you could try running some lucas fuel injector cleaner (get 2 bottles) and/or seafoam to see if that cleans it. they say to put a bottle in full tank of gas--screw that: use 1/4 to half tank so it's stronger (lucas says it's safe). All I can think of.

* Also maybe while the air filter hose is disconnected from the throttle body, maybe shoot some carb cleaner/engine cleaner around and just past the butterfly valve. Just manually turn the throttle linkage to open and clean the gunk with a good spraying and a rag/paper towel(s). That will remove any potential air resistance.

** And if that fails, I would go &&%%[email protected]@!!!!. In deciding if fuel pump/filter or the intake I would first go ahead and do the fuel filter and/or pump--easy job just time consuming. If that didn't work, I would remove/unbolt the intake (pretty easy), and replace the gasket (which is cheap).

Last edited by l33ch; 01-21-2018 at 07:24 PM.
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post #25 of 43 Old 02-02-2018, 06:56 PM
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any update?
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post #26 of 43 Old 03-08-2018, 09:03 AM
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Hey, I don't know if you're still struggling with this engine code but I thought I'd jump in just in case anyone else is struggling with this same (particularly annoying) code. I got the car from my parents and they had put about 40k on the car without searching the source of the code on the advice of a mechanic because "this one isn't a big deal." Lol. Well anyway, I got the car and started to go through stuff. I replaced the PCV valve because A. It's cheap, and B. In some cases a clogged or bad valve can actually cause this code. I cleaned the MAF sensor, changed the air filters. None of that worked, so I went through the steps of checking vacuum lines, bought a Bluetooth scangauge and found my readings were similar to yours. I found some fairly inexpensive O2 sensors on Rock Auto that turned out to be densos and popped them in there and that turned out to be the problem for me even though in my case it seemed as though the voltage fluctuation was appropriate. Now I'm wondering how bad a shape the caddy is in because of all those miles of incorrect fuel mixture getting dumped in there....
P.s. - something else that could easily cause this code and isn't often mentioned is if there are any stuck open solenoids or broken vacuum hoses in your EVAP system. This would cause an unmetered air leak and you wouldn't see the broken hoses unless you're underneath the car by the gas tank. ***** either the Vent solenoid or the Purge solenoid being stuck open could theoretically cause this under the right condition. In my case I also had a rusted open vent hose on my fuel filler neck but because all the solenoids were working correctly there was no vacuum leak just an evap code. ANYWAY, That was long but hopefully helpful to someone.
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post #27 of 43 Old 03-15-2018, 09:36 PM Thread Starter
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Thanks for the info, Daniel and l33ch.

Here's the update. Last month, I decided to have the car checked by a local mechanic I trust. He ran diagnostics and did a smoke test. While the intake manifold was sealed well, there was smoke coming out near the gas cap. He said that it shouldn't be happening but that that problem would likely trigger an evap code and note the P0171 code. He also said that he doubted that a bad fuel filter would trigger the P0171 code. His best guess was to have the injectors and fuel system flushed and then to run the car on premium gasoline for about a half a month. To make a long story short, in about an hour, the P0171 code came back and despite the mechanic telling us to give it time, it never cleared away. So, $250 later, we are still dealing with the problem. I'm beyond frustrated and now I'm feeling less than enthusiastic about my trusted mechanic. Even worse, another mechanic I've used multiple times in the past on my 96 RAV4 tried to tell me we couldn't just replace the fuel filter but that we would have to replace the entire tank. Knowing better, I'm now wondering if I have a local mechanic I can even trust with my son's car. Ughhh.

So, I've been thinking about going back to my original plan and replacing the fuel filter by myself (and the fuel pump while I'm in there). I've watched some videos and it definitely looks like a doable job.

But before I do...I hadn't run into mention of the PCV valve causing this problem before. It looks like checking/replacing it is a relatively simple procedure so I will tackle that this weekend. Like you said, it's a cheap solution and after all of the money I've sunk into fixing this problem so far (we're now near $500), it would be nice to have an uncomplicated, cheap solution vs another $60 part like the downstream O2 sensor or the fuel pump/filter assembly, both requiring removal of seats and a lot more effort than the PCV valve. Fingers crossed this $10 solution is the one.

So one more question - I know you said that the downstream O2 sensor shouldn't be causing the problem. But if the PCV valve is not the solution, do I attack that or the fuel pump/filter next? In other words, what is the more likely culprit?
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post #28 of 43 Old 03-15-2018, 10:19 PM
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Hey, sorry to hear you still have problems with that silly code. It is an annoying one. I can't remember if you already did this, but vacuum hoses are very cheap so it may be worth replacing them all just for the heck of it. If you already have (again, I don't remember), I would say replace the downstream 02 sensor. I'm not an expert and I'm biased because those are what fixed it in my case but it is cheaper. If you're going to replace both the fuel filter and fuel pump it'll be a bit more expensive than an 02 sensor. Also, if the problem is the pump or the filter not allowing enough fuel flow I'd be surprised if you didn't have other problems that are related to a lack of adequate fuel supply. I just actually changed my fuel pump (and filter while I was in there) because the pump went bad after sitting for 2 weeks and I couldn't start the car. Let me know if you decide to tackle the pump and filter and I can send you links to the parts I used from Rock Auto. And for whatever it's worth, the 02 sensor I got from Rock Auto was the cheapest one but when it arrived it turned out to be a rebranded denso and has worked great. Don't feel like you need to shell out money for the best sensor because let's be honest, its an old car :p hope that was helpful. Again, I'm not an expert, just my two cents from dealing with my own dear echo
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post #29 of 43 Old 03-16-2018, 02:19 PM
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Well PCV should be changed every so often (I can't remember when exactly--maybe every 30-50K?, but if it's black and nasty--change it), but if it's been awhile that is a cheap replacement (like 3-5 bucks). It can trigger rough engine operation and if clogged, could cause a back-leak to where the hose connects if that hose isn't in the best condition (sometimes mine will shift off a couple mm every so often..i need to change the clamp), and other evap/emission issues--not to mention impediment of how the engine is supposed to direct extra/lingering vapors in the engine for combustion,etc..so yeah it can affect operation of the car.

A new fuel pump and filter will cost you about 90+10 (pump and filter). The smoke coming out of gas cap..hmm.....Just out of curiosity, I would steal (er...borrow) a gas cap from another toyota..and see if it seals better. Gas Caps should not leak, and if leak-detector smoke came out = bad. That will cause evap/emission/fuel delivery issues. A new gas cap will cost 10-15 bucks--maybe? Like the HVAC system, the fuel system is a (should be) a sealed--PRESSURIZED--system. So if the cap doesn't seal all the way, pressure is less/lost w/ respect to fuel delivery.I'd like to say, that's the problem (after ALL you have done so far),(Why I suggesting stealing/borrow one from a Toyota/Scion/Lexus first so confirm so not out $15 if that isn't it).

Last edited by l33ch; 03-16-2018 at 03:20 PM.
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post #30 of 43 Old 03-16-2018, 10:41 PM Thread Starter
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Originally Posted by l33ch View Post
The smoke coming out of gas cap..hmm.....Just out of curiosity, I would steal (er...borrow) a gas cap from another toyota..and see if it seals better. Gas Caps should not leak, and if leak-detector smoke came out = bad. That will cause evap/emission/fuel delivery issues. A new gas cap will cost 10-15 bucks--maybe? Like the HVAC system, the fuel system is a (should be) a sealed--PRESSURIZED--system. So if the cap doesn't seal all the way, pressure is less/lost w/ respect to fuel delivery.I'd like to say, that's the problem (after ALL you have done so far),(Why I suggesting stealing/borrow one from a Toyota/Scion/Lexus first so confirm so not out $15 if that isn't it).
Some clarification here, as I was afraid I might not have been fully clear. The smoke didn't come from the gas cap itself. We viewed the smoke leaking out from underneath the car in the area around the top of the filler neck, near the gas cap area. Mechanic didn't seem immediately concerned as the computer wasn't picking up an evap system error code, though he did mention that there might be a potential issue with the charcoal cannister that would need to be checked later.
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