d124716

Hello All, I am in a bit of trouble with my 2002 GS300 with 300K miles on it. I've been taking good care of this car and the time has come to pass the emissions test but P0420 (check engine light) is keeping me from doing so. Looking over the diagram of the exhaust system I got a little confused. Trying to understand exactly which catalytic I need to replace to make sure I'm not tossing money out the window. If someone could give me a quick tour on the exhaust system I would really appreciate it as P0420 doesn't tell me which catalytic failed. Looking at the exhaust system as I understand it (please correct me if I'm wrong) there are 3 catalytic converters on this vehicle – one on each bank (attached to headers) and one right before exhaust resonators. How do I know which one to replace? Please help. Any additional troubleshooting steps helping to nail down the failed components would be super! (there are no other codes on the car besides P0420)

PS
I've replaced the O2 sensors on bank 1 (pre and post "headers" catalytic) as I had a code for those sensors some time ago. The car has been a champ for 300K miles and I want to keep on "trucking" but state will not renew my tags unless P0420 goes away!

PSS
I've build an O2 "simulator" circuit out of 555 timer on my bench but am very reluctant about plugging it into the vehicle as to really afraid to screw up the ECU AND not sure which sensor this circuit should be plugged into since I cannot tell which catalytic is out of order.

DMPesso

AHH the infamous 420 code. 420 is my favorite number ;-)
Replace whatever O2 sensors you havent replaced yet. the 420 code is either an exhaust leak or an O2 sensor. Very rare that it is an actual CAT. Dont assume its the CAT and dont listen to any shops, start off by checking for an exhaust leak and if you are SURE there are none, change the O2 sensors if they havent been changed in less then 80-90k miles. Most likely one or the other. The code 420 just says CAT converter because the O2 sensor or leak of air is not allowing the CAT converter to run at optimal efficiency so it is telling you something is wrong. Hope I explained it good enough for you

brainsurge

You can check the function of the o2 sensors via obd. i had 3 empty catalytic converters in my '01 gs 430 when i bought it.
check for exhaust leaks and while you're under the car knock the cats. if they sound hollow, replace the 2 front ones with magnaflow cats. i removed the third one, since it serves no real purpose and i pass emissions with only 2.

d124716

Guys thanks for rapid replies! Quick clarification:

How many o2 sensors are on GS300 – I was able to locate 4 (2 on the bank 1 catalytic, and 1 (??) on bank 2 catalytic, and one past the rear catalytic). Should I replace the remaining 2 that I haven't replaced?

Also, how to conduct the o2 test with obd mentioned above?

d124716

one more question, would anyone have a "DI-44" section of service manual for GS300? I have a pdf version but ASUSUAL the one section that is being referenced to checking procedure of o2 sensors is missing

PS
quick correction to my post above in regards to o2 sensors - 2 sensors on Bank 1 (pre and post cat), 1 sensor on bank 2 (pre cat) and one more sensor underneath the car past cat. on bank 2 (found it). So I know where the sensors are, question is how do I tell if they are good or not. Tossing 200$ for the sensors is putting me 200$ away from possible purchase of the $400 new cats ((((. Thus I would want to be sure

aneidiaz

Listen up! I have a GS430 and there's more complications there then a GS300. This is a simple fix. DO NOT change your cats UNTIL you replace ALL gaskets BEFORE and AFTER the cats and anywhere down the exhaust line that has leaks/deteriorated. Plus ALL o2 sensors. Get this done all at once and the code should go away. If it doesn't then it's the cats all together. But the formula I just gave you is the most CERTAIN AND INEXPENSIVE way to get this done with AUTHENTIC Lexus parts for cheap. Go on www.sewellparts.com and sign up with your Club Lexus account. You'll get KILLER DISCOUNT prices. Hope this helps in any way

brainsurge

Get a Bluetooth obd adapter like this
http://www.amazon.com/iSaddle-Bluetooth-CAN-BUS-Diagnostic-Windows/dp/B008U1MOM8/ref=sr_1_2?ie=UTF8&qid=1410856595&sr=8-2&keywords=obd+bluetooth
and the app torque pro, to read your o2 sensor voltages with most android phones. check for leaks and knock your cats. if you find leaks have a look into your cats if the converter is intact. if not replace. if cat intact, replace o2 sensors.

gudbramble

i removed the third one, since it serves no real purpose and i pass emissions with only 2.