drhill26

I pulled error codes from my 2001 Rx (about 180k miles) yesterday of P0420 and P1150. For awhile I had the CEL light going on and off every few weeks (on for two weeks, then shuts off and stays off for two weeks, lather, rinse, repease). A months or so ago I put a new gas cap on and also put a cheap air filter in as I was cleaning my washable air filter. Since then the light has been on and I finally pulled the codes.

I found this thread : https://www.clublexus.com/forums/rx-...ault-code.html

The P1150 code seems pretty obvious, but 8 posts down someone mentions the P420 code is the rear sensor. Are these codes just the two separate sensors or could they mean something else? I read somewhere else that P0420 means the catalytic converter is bad. Is that the case?

code58

http://www.obd-codes.com/trouble_codes/lexus/
This site should give you the info you need. The page it will open on is Lexus specific. At the top of that page click on the link for generic codes (P0420 is a generic code) On P0420 in that list you can click on it because it's highlighted blue and it will take you to further info about the problem and what it may mean and what to do about it. This is a site I often use and feel if you wander the site there's a lot of good info there.

hypervish

I had a P0420 in my RX starting on October 24, 2010 (151k miles) ( i promise I did not remember the date/miles, I looked it up in my log book!! :P). The code would appear randomly it seemed. I was pretty scared and thought it was the CAT, and was worried about how expensive it would be. SO, I put it off for awhile as I did some research. It seemed to me that the sensor was a reasonable culprit. So on April 17, 2011 (158k miles) I replaced the sensor after the catalytic converter (Denso 234-4215), luckily it was cheaper than an oxygen sensor, I believe the sensor was around $90 at the time. The code has never returned to date, and I now have 193k miles. I haven't changed any oxygen sensors since that date either so i know for a fact that the sensor behind the CAT was the culprit.

drhill26

Thanks.

The P1150 is an upstream AF sensor for $100. From the obd-codes.com site, and hypervish, the downstream Oxygen sensor ($60) could be the culprit.

Is it possible that the upstream sensor would cause the downstream sensor to be reporting bad even if it isnt? Not that those both parts together are a big deal to replace, moneywise.

Baetke

drhill26,

The ECU determines catalytic converter efficiency by monitoring the exhaust gases at the input to the Cat (AF Sensor) and the output of the Cat (O2 Sensor) and comparing the two values. Imagine that you replaced the Cat with a straight pipe. Both sensors would indicate the same value ( if they were both O2 sensors instead of one A/F and one O2) but the idea is the same. If the Cat is working properly, lower emissions should be detected at the output of the Cat than at the input.

If either sensor malfunctions you could get a bad Cat trouble code so how do you figure out which one it is?

I sounds like you have a scan tool so hook it up and look at the STFT and LTFT numbers on Bank 2. If the sum of those two numbers is less than about 10% the A/F sensor is probably working just fine and the O2 sensor is probably the culprit. If the A/F sensor is bad, you will probably see STFT+LTFT numbers in excess of 20%.

Baetke

hypervish

You can test the upstream sensor : https://www.clublexus.com/forums/rx-...esistance.html

Remember, to use only a Denso sensor if you replace it. They can be found cheapest on Amazon.

drhill26

Last sunday when I pulled the codes I cleared them, cleaned a valve at the back of the air filter box, and drove about 400 miles (Friday night) when the CEL came back on. This time it was just the P0420 code (twice actually).

From the code reader I have (an Actron). Here are the various things that may be relevant.

02 Bank1 Sensor 2
Min (V)
Meas 1.271
Max (V)
Meas 0.00
===============
ST FTRM1 0.0
LT FTRM1 -5.5

ST FTRM2 0.0
LT FTRM2 - 4.7
===============

O2 S12 (V) 0.0
O2 S11 (V) 3.298
O2 S21 (V) 3.298

Baetke

Those fuel trim numbers and A/F sensor voltages indicate that your upstream sensors are working just fine. I would go after the downstream (O2) sensor. It could still be the CAT but it will be much cheaper to test/replace the O2 sensor. There is probably not a failure of the O2 heater because that would be detected and reported as a different code but the sensor could still be bad.

Baetke

drhill26

An update to this.

I replaced the downstream O2 sensor and that didn't do it. It's at the dealership right now. They are replacing the AF sensor and the catalytic converter. They said there is an exhaust leak. I'm at 190k miles so it doesn't seem unlikely, but $2800 doesn't make me happy either. :\

Baetke

Ouch, that's a chunk of change! Be sure and ask them WHY they replaced the cat and if it was bad, what caused the failure. Could the exhaust leak have lead to the failure?

drhill26

The cat is part of the center pipe assembly I was told. That was $2300. I wish I replaced the AF sensor on my own now as that was $500 (compared to $110 on amazon).

They did say the exhaust could have caused Cat codes to go off in the future. In retrospect I do remember occasionally smelling exhaust when exiting the car.