Help Lexus Problem

[Sal Lubrano]

hello, my friend has a 1999 Rx300 and we are having a troble code of PO171. the obd2 is telling us that bank one is runnig to lean. meanwhile the tail pipe is giving out black carbon and getting horible gas mileage. we had changed the mass air flow, spark plugs, air filter and does not seem have any leaks in the hoses that we have checked. o2 sensores both 1 and 2 (before the cat )are reading .635 to .645 volts. o2 sensor bank 1 o2 (2) after the cat reads 0.00. any help would be greatly axcepted.

 

[garyk1970]

replace tthe o2 in question,or you have an intake leak.....engine thinks its lean because of the extra air and adds more fuel....

 

[gunslinger]

See these all the time. Usually fix them by cleaning the MAF.
Did you replace it with a new one from Toyota/Lexus, or was it a used piece?
Measuing voltage at the upstream O2s won't do you any good, these use AFR sensors and the voltage stays pretty much constant. The downstream has no effect on fuel trim, so you can ignore that for now.
Check the MAF again, and make sure it is sealed and clean, look at the PID for air intake temp and make sure it is in range.
I suppose it could be a bad B1 AFR sensor...but it's an expensive guess.

What are the long term and short term fuel trims?

 

[TurboDave]

If it's only one bank, I'd suspect an O2 gone bad.

 

[garyk1970]

If it's only one bank, I'd suspect an O2 gone bad.

x2,if the maf was bad,you would get lean codes for both sides....but Toyota/Lexus is not my specialty....

 

[klrv6]

Can you swap O2 sensors to see if the problem follows the sensor?

 

[TurboDave]

Ya, take the two sensors from bank 1 and put them in bank 2. Take the two sensors from bank 2 and put them in bank 1. Clear your codes and see what happens.

 

[gunslinger]

x2,if the maf was bad,you would get lean codes for both sides....but Toyota/Lexus is not my specialty....

You'd think so, but Toyota/Lexus monitors bank 1 first. Once it sees a lean condition on bank 1 it sets the fault and doesn't check bank 2 until it sees a good trip for bank 1...got to love OBD2 logic.

They also use stainless steel O2 bungs with stainless steel O2s....they pretty much weld themselves together...especially bank 1 where it's right against the firewall and keeps everything nice and hot. I've had to replace a few manifolds because of bad O2 heaters.

 

[GNeric]

I agree with gunslinger. Had a few of these myself and they tend to get real stuck in the manifold. Just did one 2 weeks ago. Had one run me in circles for a bit before a call to technical assistance set it straight. Replaced B1S1 AFR and all was normal again. Just have to make sure it is absolutely correct voltage readings before replacement.

Yes I do work at Toyota. Las Cruces Toyota.

 

[garyk1970]

ok,good info to know if i run across this in my shop .i learn something new everyday when it comes to cars....
also my tahoe has simular problem and it is my intake gasket leaking when its 50* or colder out....(good ole plastic intake )im not looking forward to changing it...

 

[klrv6]

ok,good info to know if i run across this in my shop .i learn something new everyday when it comes to cars....
also my tahoe has simular problem and it is my intake gasket leaking when its 50* or colder out....(good ole plastic intake )im not looking forward to changing it...

What year Tahoe? I did my Suburbans ('99) years ago and a ('99) for a good friend of mine a month or so ago. Not too bad of a job. If the dexcool ate the intake or the cooling system is dirty, flush it really good, replace the intake gaskets, water pump and quick disconnect fitting on the intake. If one thing has a problem, they all will.

Relating to the original post, the suburban had a bad O2 sensor heater that would throw a code when started below 40 degrees, readings were also off. It was a short that would only show up when cold. 0 volts would suggest that there is a broken connection in the sensor or wiring....or a dead sensor. I am sure a toyota tech here would know what to look at.

 

[garyk1970]

ive got lean codes on bank one and two.it skips when its cold out.checked out every thing three times,its the intake .
2003 with the 5.3.....