PO171 & PO420 Codes
Thread Starter
Driver School Candidate
Joined: May 2016
Posts: 5
Likes: 0
From: Wisconsin
PO171 & PO420 Codes
Hey,
I have a 2000 GS400 with 227k mi. Recently, my CEL has been going on and off. I've checked the codes before and after to see if they are the same. They are: PO420 Efficiency Below Threshold and PO171 System too Lean (Bank 1).
I cleaned the MAF sensor, changed both air filters, and the throttle looked fine.
Here's the interesting part; since I've owned it (2nd gen), the fuel level indicator will not drop past 1/8 of a tank without her running out of gas.
I'm considering trying to drop a sparkplug and see if it burns off leftover fuel that's been sitting and check for leaks but, I wanted to ask if anyone thinks differently. Eg replace the O2 sensor first or have experience in knowing it's definitely the cat.
Anything would be helpful. Thanks!
I have a 2000 GS400 with 227k mi. Recently, my CEL has been going on and off. I've checked the codes before and after to see if they are the same. They are: PO420 Efficiency Below Threshold and PO171 System too Lean (Bank 1).
I cleaned the MAF sensor, changed both air filters, and the throttle looked fine.
Here's the interesting part; since I've owned it (2nd gen), the fuel level indicator will not drop past 1/8 of a tank without her running out of gas.
I'm considering trying to drop a sparkplug and see if it burns off leftover fuel that's been sitting and check for leaks but, I wanted to ask if anyone thinks differently. Eg replace the O2 sensor first or have experience in knowing it's definitely the cat.
Anything would be helpful. Thanks!
Lexus Test Driver
Joined: May 2007
Posts: 803
Likes: 3
From: NC
For P0420 look at:
Gas leakage on exhaust system
Heated oxygen sensor
Three-way catalytic converter
for P0171:
Air induction system
Injector blockage
Mass air flow meter
Engine coolant temp. sensor
Fuel pressure
Gas leakage on exhaust system
Open short in heated oxygen sensor (bank 1, 2 sensor 1)
circuit
Heated oxygen sensor (bank 1, 2 sensor 1)
ECM
Gas leakage on exhaust system
Heated oxygen sensor
Three-way catalytic converter
for P0171:
Air induction system
Injector blockage
Mass air flow meter
Engine coolant temp. sensor
Fuel pressure
Gas leakage on exhaust system
Open short in heated oxygen sensor (bank 1, 2 sensor 1)
circuit
Heated oxygen sensor (bank 1, 2 sensor 1)
ECM
Racer
Joined: Jul 2015
Posts: 1,304
Likes: 184
From: North Carolina
When mine ran out of gas with 1/4 tank it was the fuel pressure. I would have that checked and see if it helps.
https://www.clublexus.com/forums/gs-...ml#post9484003
Btw, I have a '99 GS4 with 218k.
https://www.clublexus.com/forums/gs-...ml#post9484003
Btw, I have a '99 GS4 with 218k.
Thread Starter
Driver School Candidate
Joined: May 2016
Posts: 5
Likes: 0
From: Wisconsin
Thank you very much both of you! Keeping this amazing car up can be frustrating because it's solid but, becoming expensive. Upper and lower ball joints "failed" 
the other day -front driver- and, now, my mechanic (who's the best I've had yet) said the diff was leaking, front sway bar wear and tear, NAPA gave me the wrong caliper after another mechanic did a shotty job on a refurbished pin but did correct the problem (very thankful they stepped up), and ABS sensor is out for the count after the ball joints broke. I'm nearing my wits end. Trust me, I wish I could just take it in and have it taken care of with the best parts because it's the best vehicle I've ever drove.
the other day -front driver- and, now, my mechanic (who's the best I've had yet) said the diff was leaking, front sway bar wear and tear, NAPA gave me the wrong caliper after another mechanic did a shotty job on a refurbished pin but did correct the problem (very thankful they stepped up), and ABS sensor is out for the count after the ball joints broke. I'm nearing my wits end. Trust me, I wish I could just take it in and have it taken care of with the best parts because it's the best vehicle I've ever drove.
Last edited by Benny701; Jun 28, 2016 at 08:42 AM.
Thread Starter
Driver School Candidate
Joined: May 2016
Posts: 5
Likes: 0
From: Wisconsin
I did try that...a couple times, to say the least. CEL always, eventually, came back on with same codes. Still working on it.
Racer
Joined: Jul 2015
Posts: 1,304
Likes: 184
From: North Carolina
Hang in there. It can be frustrating when you have a lot of repairs at once, but these cars are worth it! Definitely get the fuel pressure test when you get the chance. The dealer can do it (expensive) or if you have a good indy. Most of the time when it stalls with 1/8-1/4 tank that points to the fuel pump. Let us know what you find out.
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