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I've been recently encountering an annoying p0171 & p0174 on my 2001 Lexus LS430 V8 4.3L, I hooked up a scanner after removing the battery overnight to see how the ST and LT fuel trims look like. ST are constant at 19.47% (Both of them are having the same exact number) While LT's are at 35.24 (Also both of them are the same number).

O2 sensor 1 Bank 1 & Bank 2 both are reading 0 Volts in idle, If I rev it a bit it can increase to 0.2-0.6v, and once gas pedal is released, they will come back to 0 volts instantly.

So far, I have replaced the MAF sensor, Replaced Intake manifold gasket, Replaced Injectors(Mileage is far better now), and checked all the vacuum hoses for any holes or cracks. all good. I even sprayed WD40 all over the intake edges and hoses just to ensure if there is any vacuum leaks. and the RPM is steady at 720-740.

Now whenever I erase the codes they will never come back if I'm driving around(higher rpm), Once I come to a stop or a parking, they will come back again.

This is really annoying as I've paid too much trying to solve these annoying errors and nothing worked yet. Car drives OK, and accelerates just like it's supposed to do. But for some reason.. she doesn't like the air-fuel mixture :( can anyone help me please?

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  • I would guess that you have a vacuum leak that you are missing. These codes are caused by not enough fuel or to much air. If your idle speed is goofy I might wonder about IAC being stuck open allowing to much air at idle.
    – Jupiter
    Oct 22, 2021 at 13:36
  • Agree with @Jupiter here ... the ST/LT fuel trims are huge and dumping a lot of fuel, which indicates you've got extra air from somewhere (at least that's my thinking). Oct 22, 2021 at 13:43
  • Thanks guys for your help. I really appreciate it, Don't you think something is weird having both Banks readings exactly the same for both ST and LT? Also. I didn't see any drop in mileage so far. actually it even got better after replacing the injectors (getting around 23 MPGs on highway now), I do suspect it the O2 sensors.
    – Se7sNator
    Oct 22, 2021 at 14:31
  • Does the system go into closed loop operation while driving?
    – HandyHowie
    Oct 23, 2021 at 8:55
  • @HandyHowie thank you for that info, it makes sense. I was trying to pull a rabbit out of the hat trying to see why it was worse at idle. I should of left that part out.
    – Jupiter
    Oct 23, 2021 at 9:42

2 Answers 2

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The fact that the oxygen sensors are reading low at idle, but the reading increases when the revs are increased, would indicate a small air leak that becomes less significant as the throttle is opened. This is also backed up by the error only appearing at idle and not while driving.

Air leaks can be fairly well hidden, there could be an air leak for example in the brake booster. You could try plugging the pipe to the booster and see what happens. Obviously don’t drive without the booster disconnected.

Have a look to see if vacuum is used anywhere else on the car.

Does the system go into closed loop operation when driving?

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SOLVED!! the power steering air control valve connects to two vacuum hoses, both these hoses where completely de-attached and trust me. you can NEVER notice them as the valve is well hidden deep down behind the power steering lines. Can't imagine that the mechanic didn't notice that while changing my injectors and intake gasket. Now finally my STFT/LTFT are trending between -1 to 1

Thank you so much guys for your help! enter image description here

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