Hello everyone,

I have a major problem with my 2005 model LS 430. My car began experiencing sporadic misfires a few months ago, in April 2023. I could sense a vibration inside the car when it was at idle, It was after a rainy day and my first assumption was that it might be a damaged spark plug or coil. I checked all 8 of the spark plugs and the coils, but everything appeared to be in good condition. I checked with TechStream which showed 0 misfire counts with cylinders, further after noticing I had a CEL with TRAC OFF and discovered I had two problem codes:

- P0171 - System Too Lean - Bank 1
- P0133 - O2 Sensor 1 Bank 1 - Slow Response

I also checked the data with Techstream the Fuel trims, and I noticed Long FT were over 35+ which told me there is a vacuum leak possibly. I did have an exhaust leak but when I went to the workshop the mechanic advised me not to fix the exhaust leak because there is a misfire in the engine so he told me there will be leak again if fixed. I didn't bother to take his word, so I paid him and had him fix the leak which was a gasket leak in Bank 1 Catalyst Converter. Replaced the gasket and put fresh silicon and installed. I cleared the codes yet Moving on, I noticed the misfire problem wasn't fixed but this time only one code kept popping up which was P0171 - System Too Lean - Bank 1. Week later I had exhaust leaking in the mid filter suddenly just like how the mechanic told me.

Considering I had a terrible misfire experience due to bad O2 sensor on my Lexus LS400 where once the vehicle even shut down itself. I thought O2 Sensor 1 Bank 1 might be culprit causing misfires even though I didn't see the code P0133 ever again since April 2023. So I took to mechanic shop and had inspected the vehicle from underneath. We noticed that Idle Air Control Valve was broken. So I replaced it with a new OEM one as seen in picture below.




Moving on, I also noticed a hose under PCV Cover left side of engine was not connected. I am not sure how much dust the engine sucked over the past months due to this hose being unconnected. Anyhow I installed new hose as well as connected the Air Idle control hose to top engine (near TB). While the mechanic was working on these, I made a very dumb decision to remove the O2 Sensor 1 Bank 1 and spray it with WD-40 in the hopes that it would clean it while the mechanic worked on these. Then, I let the sensor dry for like 20 minutes and put the sensor back in.

After everything was done, I decided to erase the codes using TechStream and start up the vehicle. I was very happy that the fuel trims returned to normal range which meant no more vacuum leaks and the P0171 - System Too Lean - Bank 1 no longer appeared (Until this moment - been month now) yet I have created another problem. A new code P0031 - O2 Sensor 1 Bank 1 - Low Circuit began to appear just 5 minutes after I started the vehicle and until this day the code won't go away. Even if I delete the code, every 5 minutes later It would pop the CEL with TRAC OFF and the code would stay on. I realized it could be due to me spraying WD-40 on O2 B1S1 I might have accidently ruined the O2 A/F sensor. I tested for weeks, and the code won't go away, Keeps coming back 5 minutes later after I delete the code, and this is solely the only code that's appearing ever since. I still have the random engine misfire - TechStream shows 0 misfire count/reports from any of 8 cylinders.

I had no other choice but to buy a new O2 Sensor and replace the existing one which is what I did. I went to the dealer and brought a brand new O2 Sensor (Part #89465-50120) and had Bank 1 Sensor 1 replaced but to my surprise just 5 minutes later I am again greeted with CEL/TRAC OFF and code P0031 - O2 Sensor 1 Bank 1 - Low Circuit and now this code won't leave me alone. Even after I replaced the O2 Sensor 1 Bank 1 (B1S1).

I decided to do a comparison between:
B1S1 = Bank 1 Sensor 1 (One that I replaced / Keep giving fault code - P0031)
B2S1 - Bank 2 Sensor 1

I decided to check the data with TechStream again and I noticed B1S1 is having average 0.68v compared to B2S1 is having 0.72v which I think is normal during idle yet upon giving a rev I noticed B1S1 go upto 0.98v and drop to somewhere around 0.23v whereas B2S1 drop to 0.02v - I think 0.02v is too low? But no-fault code shown for Bank 2 Sensor 1? Now I am seriously confused what's going on! I checked the wiring harness of B1S1, and everything seems fine and connected. I am completely lost. Ever since I removed O2 B1S1 and sprayed WD-40 I am now being haunted by the code "P0031 - O2 Sensor 1 Bank 1 - Low Circuit" and even after replacing the O2 B1S1 itself with OEM It won't go away. It's just made me waste my money! I am not sure, even the old O2 Sensor might be good. Live data during revs for B1S1 looks like this:




Upon live data - The sensor does work fine? Please advise what are the possible options. I am sure that WD-40 can't have done much damage! Considering I replaced the sensor already! Should I replace B2S1 also? Even though I don't have any fault code for that. How do I get rid of P0031 and this random misfire that's going on especially when engine is warm and in idle. I don't even know if any component is being wear and tear due to this misfire. My RPM's normal idling range is around 200-400 - Mostly drops to 200ish when I put gear to park and turn off A/C/headlights (Seems like common issue with LS430 - I even cleaned TB/MAF - Won't fix).

Please help guys! I am soo lost with this issue. It may look like such a small issue, but I am personally not fan of CEL/TRAC OFF light on the dash and the random misfire. Plus, the fact P0031 appeared out of no where!

 

Unplug battery. See if codes come back.

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