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​​Code P0171 I have a 1998 Lexus ES300 and keep getting the code P0171 since before Oct 2021. Originally my car would drive fine until a couple hours after driving it I would shut it off and on trying to work and make food deliveries and it would die. All signs pointed to a bad battery or alternator and had a good friend replace them both including spark plugs. They all needed replaced. A week later it continued to have similar issues. I then replaced the Maf sensor, and took the old one it and they said it looked fine. (Note: I am not familiar with fixing cars) so I reinstalled the original without cleaning it and returned the new Maf sensor. But when I would try and start it all the lights would turn on etc. Basically all the symptoms you'd get for needing to replace your fuel filter, Starter or various Lean Bank 1 issues. The starter didn't seem to be doing that good either so my friend Devan replaced that as well. But the problem still continued. Just last week my friend changed the Oil Filter and last night I changed the Oxygen sensor (front) for Lean bank 1. And today the engine light came on again with the same code P0171. But I just remembered around 5 months ago the gas station attendant forgot to put my gas cap back on which I didn't realize until 2 days later getting gas when the guy refueling my car (I live in Oregon) told me it was missing. So I went back to the original gas station and the guy remembered me and said he had my gas cap and then put it back on (so I thought) Well I just checked and noticed he put a Honda gas cap on it. And I read that having the wrong Gas Cap wouldn't give me the code P0171, but I'm wondering what you all think? Could it be the Gas Cap? What do you all think might be causing this? I'm thinking the next on my troubleshooting list would of course be to get the correct Gas Cap for my car, and actually clean the Maf sensor. But I'm kinda lost at this point. I hope it's not the File Pump. I will mention aside from still having a hard time starting it, or stalling etc, after replacing all that I mentioned. It does run a lot better and feel a lot stronger. Now If I could only figure out the cause of this issue it would be great.

I forgot to mention I took it to the dealership to replace the alternator and then (supposedly) did a full diagnostic on my car and said everything looked great except I needed to break pads for the rear and could use an oil change. Never mentioned anything about all that I mentioned. Any replys are fully appreciated.

Devan if you see this, thank you for all your help bro!

Update: I cleaned the Maf sensor with Mass Air Flow sensor cleaner, and also purchased a proper gas cap and the Engine light came back on and got the same code 😑

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Thanks
Alex

 

Hello,

Did you try resetting the code by disconnecting the battery for a few minutes?
Which O2 sensor did you replace, the one on the front or the back of an engine?

There are a few spots that can result in a Lean Condition, the most common one is a vacuum leak. Check all the hoses around the engine bay, see if any are torn, loose, or otherwise damaged, if that's good, check and/or replace the PCV Valve, they are known to cause issues on those cars. Other thing you can try while the engine is cold is to spray some quick start or brake cleaner around all the intake joints, while also monitoring the O2 sensor readouts, if they will budge when you hit a specific spot, it would mean there is a vacuum leak in that joint. With that, do it on your own risk, there is a good chance to catch fire if you are not careful.

You can also check the VVT solenoid and its filter for Bank 1, which is the one on the back towards the Firewall, see if the solenoid is clean and responds well when you power it from the battery, the filter just needs to be free of any gunk, here is a good post about it.

If none of it will help, use an OBD2 scanner to check Live Data, check the readouts for Bank 1 Fuel Trims, as well as Temperature, O2 and MAF Sensors. O2 Sensor readouts should constantly fluctuate from about 0.1-0.2V to 0.8-0.9V, since your car is not yet equipped with A/F sensors, if it fluctuates between different voltages or is frozen at one specific voltage, the sensor or its wiring are bad.

MAF Sensor should read something close to 4g/s, if it is lower than that and a new sensor didn't make any difference, it could indicate a vacuum leak in the system. Temperature sensor, you can just disconnect it and see if makes a difference.

Hope this helps and best of luck!

 

Thanks for your reply! And yes I reset the code numerous times now, with.the device, and by disconnecting the battery. And it was the front Oxygen sensor. I cleaned the Maf sensor yesterday with Mass Air Flow cleaner, and purchased a new proper Gas Cap but just got the same code again. I will try some more troubleshooting today with your suggestions and keep you posted! Thanks again. I really appreciate!

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What brand replacement did you get?

Well, that is one of the more common mistakes people do with those cars.. On those engines it's a little counterintuitive, with the Bank 1 being located towards the Back of an engine, near the Firewall, the one on the Front is actually Bank 2.

Bank 1 and Bank 2 sensors have the same basic construction, the only difference being the length of the wire, so if you really want to, you can try swapping those sensors around to see if anything would change, though I am not sure if it is actually worth all the trouble instead of just replacing the other sensor. Denso is the OEM supplier, so those are the ones I would go with, you can find them here.

Hope this helps and best of luck!

 

I just took a screenshot of the codes again and this time it showed me these codes other codes as possible problems. Pic below.

 

Did you reset the codes after replacing the O2 Sensor?

That is actually quite interesting, a bad O2 sensor can very well cause your Lean condition. Slow response means that the ECU is not getting the expected reaction from the O2 sensor in time, meaning that if you were to floor the accelerator from idle, you should immediately see the O2 readings fly to the Rich side, if it happens with a delay or the readings are lower than expected, the ECU will not be able to adjust the Air-Fuel mixture, causing the engine to run poorly.

The way to check the sensor is to see its readouts with a scan tool - O2 sensor should fluctuate from about 0.1V to 0.9V, if the amplitude is noticeably less than that, like 0.4V to 0.6V, or it is shifted up or down, the sensor is bad and needs to be replaced. From idling, floor the accelerator and watch the readings immediately jump for a moment, before returning to normal, the same should happen when you release the throttle. If readings are sluggish or lower than expected, the sensor is likely bad.

Hope this helps and best of luck!

 

Thank you for all your knowledge and advice Arsenii, Im going to show my friend this info because he's offered to helpto
​​​​​​You think it's safe to drive for a few days until I can do more troubleshooting since all the codes say minor?

 

Depends on how is the car doing right now, if it doesn't misfire or stall at random, it is probably safe to drive for a day or two. With that, if the car is not doing so great, there are a lot of vibrations and the car stalls at every corner, there is a big chance to fry the catalysts or cause the local overheat, all sorts of fun stuff that you don't want to encounter if you plan to keep the car for a while longer. Since I haven't seen the car in person, I can't say anything specific, so it is your call..

Hope this helps and best of luck!

 

It actually seems to run better and better considering all the recent replacements. It fires right up. The only thing Is I've noticed my fuel burning quicker, and after I'm driving it for a couple hours making deliveries shutting the engine off and on, it will have a little hesitation on start up and mutter a bit (not as severe as before) like 1 out of 5 times, but will feel fine once I start moving. But I may just go talk to my dealership and have them diagnose it for me. Especially with all the new information you've provided. I think this car has a lot of life left in it. I've always felt like it drove amazing up until 6 months ago. And now with all the replacements, it seems to have a lot more power and acceleration.