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Old 10-15-2020, 10:37 PM   #1
Chip Whitley
Drives: 2007 Auto Hatchback
Join Date: Jan 2017
Location: Locust Grove, VA
Posts: 7
Getting a P0171 code even after taking it to the shop

So about two weeks ago my 07 Yaris Hatchback's check engine light came on throwing code P0171 (System Too Lean - Bank 1). I made an appointment for a diagnosis with a local shop that has a good reputation. My MAF sensor is three months old so they said that wasn't the problem and then recommended a smoke test to find any leaks. They couldn't find a single leak so then recommended I replace both O2 sensors. I also had a new muffler installed which is unrelated to the code, but it was time for a replacement as my old one was getting crazy loud. Anyway, I left my car with them Monday and picked it up yesterday.

To be honest, it runs better now then it did when I bought it a few years ago. Much quieter with a bit more pep although it's still slow as hell since it's a Yaris. I thought everything was fine, but the check engine light came back on with the same code on my way home from work today. I called the shop back and made an appointment for Monday morning. I'll leave it there for the day as I have to work. Also, the mechanic mentioned something to me about using a bottle of sea foam fuel injector cleaner before I ended up agreeing to the new O2 sensors. Should I try that before I take it back in Monday? Any idea what else it could be? I'm $750 poorer now because of this issue so any advice would be greatly appreciated.
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Old 10-16-2020, 12:34 AM   #2
Drives: Yaris 2007 1.3 2SZFE 64kW
Join Date: May 2012
Location: CZ
Posts: 214
Personally I do not think O2 sensor will help. There are other codes indicating O2 sensor issues. On the other hand - a friend of mine had P0171 as well and it stopped popping in after he changed battery. Do not ask me why... So everything is possible.

Are you sure the new MAF is seated properly? Some rogue air can go around and compromise the measurement.

I would also clean the throttle body and disconnect the battery for a few minutes so that the ECU can re-learn from scratch the air intake values, position the TB flap correctly etc.
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Old 10-27-2020, 03:13 AM   #3
zoidberg444's Avatar
Drives: Yaris 06 - 5dr hatchback
Join Date: Aug 2015
Location: Birmingham, UK
Posts: 385
Is your MAF sensor genuine? I have heard so many stories of shitty Chinese sensors that are just faulty from out of the box. It would be best to go with a used working original from a scrapyard or get a genuine replacement part from Toyota.

Given the smoke test revealed no obvious vacuum leaks either your MAF is providing bogus information or you have some sort of fuel delivery issue - weak fuel pressure caused by a bad pump or bad injectors. You can also rule out a vacuum leak by plugging your OBD2 scanner in, going to live data and looking at long and short term fuel trims at idle and see if they start correcting down if you hold the throttle at say 3000-4000 rpm for a few minutes as the "pirate air" getting in the system will have less of an effect the more air the engine is consuming. Normally with vacuum leaks it probably would set the code at idle instead of when its driving down the road.
- Yaris 2006, 1.0, 5 door hatchback "Betty" - 200,000 miles
UK to Nordkapp in a Yaris
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Old 01-30-2021, 09:24 AM   #4
Drives: Yaris T1
Join Date: Aug 2020
Location: South Africa
Posts: 29
Lean? Isnt that due to not enough fuel? That could be anything. Funky or clogged fuel filter (Americans are too cheap to buy new Fuel filters from what I see). Maybe fuel pressure regulator or fuel pump? Ask the grease jockeys to test fuel pressure. Even 3rd world countries have fuel pressure kits :)

Yaris 1Kr-Fe engines are supposed to get a new fuel filter every 75 000 km from the TIS maintenance schedule that I see at the stealerships.

AS for fuel pumps? Well those eventually die at some point. 1 old fart that we saw in the media took his VW polo to 1 million Km. He went through 2 or 3 fuel pumps.

I checked my LTFT. It is +0.78% at 190 000km. Yours must be something like 25% to fire off a code... :(
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