Hello guys, I'm new to this forum.
I was driving my car and the check engine light turned on. I reset it and after 15 miles it came on again. So I went to Autozone (they check it for free) and the OBD code was P0172. Car is a 98 GS400 60k Miles, with K&N Filter. I just bought the car a week ago.

Their computer readout said "The fuel system for Bank 1 was too Rich. (bank one identifies the location of Cylinder #1, While Bank 2 Identifies the cylinders on the opposite bank."

The guy at autozone said the O2 sensors might be bad. O2 sensor ran $180 each.

I've checked and Chilton does not make a manual. So I went to ebay and bought a Lexus CD service manual. (waiting for it to arrive)

Has anyone else had this problem? I would like to fix it myself because I've done it in all my cars. I had no drop in fuel mileage, and the car accelerates just fine and no hessitation. Idles at 500RPM> Is that normal?

I've done a google search for P0172

Code P0172 can be caused by several problems:
1) Fuel line pressure
2) Injector Blockage
3) O2 sensor malfunction
4) Manifold absolute pressure sensor
5) Engine coolant temp. sensor

The previous owner said he hasn't changed the o2 Sensor since he had it so I am assuming it is time to have it change. Should I at 60K? I've checked the gas gap, reset the ecu and the light is on again when I drive about 5-10 miles or so.

 

hey man i got the same issue! i have a 2000 gs 300 with 103,000m and im having the same issue, however i used to work at a lexus dealer so i got buddies their. they checked it up for me and said that if the oxygen sensor was the issue then the scanner would say so. however my friend told me that 9/10 the seal on the gas cap goes bad. so i recomend that you change that o-ring seal on the gas cap but their are other sources where it could be originated from. im still trying to resolve mine so as soon as i have an answer or a solution i will let you know. if you get it first please let me know. it happened to me 2 days after i got the car.

 

I thought it was the gas cap at first, so I exchanged the gas cap from my ES to my GS, reset the ECU and light is on again after 5 miles or so. Also, prior to filling my car gas, I opened the gas cap and it makes that suction sound. (Gas expands here in the hot texas) So, I don't think it's the pressure on the fuel system. I just ordered two O2 sensors (pre-cat) and it should be here in two days. Hopefully it's the O2 sensor, but I'll let you know.

 

I've changed both the precat O2 sensors and Man... they were dirty (filled with carbon deposits). I cleaned the MAF sensor, reset the ECU, but the Check engine light comes on again. I'll probably dump a bottle of Chevron Fi bottle and that's it for me. If that doesn't work, I'm taking it to the shop and have them run the diagnostics. I'm not gonna change the post cat O2 sensors unless the shop says their bad. Any news on yours Aristo? I'm crossing my fingers it's not the ECU, because I've read somewhere here that their's was defective and it costs quite a bit.

 

I'm in the same boat as you guys. My Lex dealer said they couldn't find anything but its probably one of the sensors. Let me know when you guys figure out the problem, I freakin hate driving with the Check engine light on

 

Thank goodness for CL I'm also having the same problem. I'm also noticing
slow pick-up from a dead start can this also be a result of bad sensors.

 

My check engine light just came on @ 80,000 miles. What remedy do you guys think I should try?

 

Good News, My check engine light is gone after driving it for 60 miles. (I guess time will tell)
It usually goes on after 5 miles or so.

I added a Lucas fuel treatment, recleaned and soaked my MAF sensor and then replaced the K & N Filter with a stock OEM filter. I started I thread here about K & N Filters. (It's worth reading)
https://www.clublexus.com/forums/sho...297#post938297

I don't know which did the trick or it's a combination of them. Hope this helps.