First off this forum has helped me out so much, so thanks for all the help. I have a 99 RX300 with 130,000 miles on it. It seems like i get a check engine light about every 6 month on this SUV. I have tried to research on search forum and came up with most probable cause is the O2 sensor that is after the main cat. Is this correct?? Also where is the best place to buy an O2 sensor? I am really thinking of leaving the Lexus family after i trade this one in, just way to many problems it seems to be having. Thanks for the help once again.

John

 

You'll have to ask others on where to get the best price.

If you're getting this every so often, I'd just ignore it. The P0420 code can come on due to heat of the incoming air, particularly in summer. I saw this code with friend in Mexico City since their vehicles didn't get enough oxygen (thinner air) during the summer and this code would come up as the vehicle got older. It's not necessarily a symbol anything is wrong with the sensor or the cat.

 

Three possibilities come to mind.

Catalytic converter failing.
O2 sensor misreading signals.
Exhaust leak before 02 sensor.

You need a diagnostic tester to check signals coming from the 02.
Check for exhaust leaks.--Most common leak is the donut gasket in between the Y-pipe and the Cat converter.

 

Thanks for the responses and the visual. I can check for leaks. How hard is it to change that donut seal? As far as the diagnostic test i would have to bring it back to Lexus and pay them $145 to do that. I deleted the code and it came back about 35 miles later. If I cant find a leak what would be the best option? Thanks for the help again.

 

Not hard. There are two 14mm bolts holding that gasket in there. That gasket can get rotted out as I found my Mexican experience. Pemex gas just loved eating them, but eventually I took them all out and went straight pipe from the engine to the cats because I got tired of the popping noises on rotted gaskets.

Mechanics my have an advanced OBD2 scanner that could reading the readings on that sensor, which is is Bank 1, Sensor 2, so don't fork over that $145.

I always would change the O2 sensor before I'd change the cat. The only time I changed out the cat and not the O2 sensor was when I performed a gas analysis test (same thing as an emissions test) at a shop in Mexico City and was able to see with my prior catalytic converter that I had a few issues (high sulfur gas will kill it), had a bit of a high hydrocarbon count.

 

Size up the situation .. the nut and bolt may be rusted that it might be easier to just saw the old ones off. Some nuts may be spot welded. Also make sure you can move at least one end of the plumbing, when working on the middle section [about 2 times the width of the gasket, else you will crush it before mating the two parts].


Salim

 

Salim

I wouldn't saw it off.
Those 14mm nuts are spot welded in the flange.

Mine were pretty rusted but they still came out.
Although my bottom nut was half way deteriorated.
I still threaded the bolt in and secured it with another nut.