LSUMatt1514

As a lot of you know, I am having problems with my ECU throwing the P0420 code (catalyst efficiency below threshold). One question...if the behind the cat O2 sensor was bad wouldn't it throw it's own code to the computer, or could it be the root cause of the P0420 code? My wife had two O2 sensors go out on her Lexus, and the codes it threw to the computer told exactly which two sensors it was. I'm not getting that. I'm getting the P0420. I just don't want to go spend $150 to replace this O2 sensor if there's no guarantees that it will solve the problem. Seems to me that if it was bad, it would throw it's own code saying it was bad.

I just don't see how my cats have gone bad. I thought they were supposed to last the life of the truck. I do know that I won't be dropping $1,500 to replace them if they indeed are bad. As mentioned in previous threads, I continue to see no performance or gas mileage degradation but every couple of weeks the CEL light comes on and it is the same code time after time. Thanks guys.

Matt

ebelen1

Your cat can go bad if your 02 sensor was bad and you didn't change it.

JamesD

This may be a little technical for you, I don't know. How you tell if you have a bad convertor or a bad rear O2 is you need a lab scope or a power graphing meter (PGM). On your truck you have three O2's, two upstream and one downstream. The upstreams are one per side the down stream is after the cat or mounted on the rear of the cat. The code 420 can only be one rear O2 since you have a single exhaust system.

You hook up upstream O2 either left for right side to the meter and the downstream as well on. If the downstream O2 mimics the upstream O2 you have a bad cat. If it does not, most likely you have a bad downstream O2. This is where a qualified tech comes in. If you have a place to take it and they know this procedure then your good, if not then you need to find another garage since that is the only way you can tell without a 4/5 gas analyzer(tailpipe sniffer).

James

Mikestang

I think cats are supposed to last about 50k miles. You could just go to a muffler shop and have the old ones cut out and new ones welded in. It should cost like $100, not $1500.

Elton

i got 136k on stock o2 sensors and the cat and it still works fine and i well replace all that when i start throwing codes

LSUMatt1514

Well, I bought my truck here in Alabama however I am one of the lucky ones to end up with a California emissions equipped truck with dual cats. I can't just cut it out and replace it, because I don't know which one (or both) are bad. The front one cannot just be cut out and a new one welded in because of the way it is configured. I would have to purchase a new flange to flange cat assembly. I'm just curious if this O2 sensor would send a code to the ECU to trigger the CEL if it was indeed bad (like my wife's Lexus did). I have no idea, but if it is bad, it could be causing the P0420 code if it doesn't send one if it's gone bad itself.

MTL_4runner

Matt, to answer your question, there is a specific set of codes for the rear sensor being bad (the OBD code number depends on the mode of failure). Now in some cases the sensor can be marginally bad (meaning it has not reached the spec according to the ECU to cause a CEL, but it may be bad enough to have some affect on the system). So it is possible that the rear sensor could be incorrectly reporting that the cats are bad and in that case the sensor should be replaced. When doing parts replacement you are trying to do is eliminate the possibility of it being something else (this is exactly the same way a dealer will go about fixing your problem). Now I know no one wants to spend money unnecessarily, it is often necessary to do so to narrow down the possibilities (usually starting with the cheapest items first). You can also turn around and recoup some of the money by selling the second good sensor to someone else (if you find it does not solve your issue).

Here's a list of codes for the rear O2 sensor:
P0136 O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
P0140 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
P0141 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)

X2and4X

A lot of people including new mechanics will replace ECU and sensors to fix an issue. I will not even look into the problem. my truck was throwing codes about my O2 sensors. The shop told me i needed to replace the sensor. I thought why did the sensor go bad or think something was wrong in the first place. I cleaned my air flow meter and whole intake. then reset the ECU and everything has been just fine. That was about 15k ago. I didnt have to replace the sensor

MTL_4runner

I think your situation may have been slightly different than Matt's, what OBD code was it specifically?

ebelen1

I agree w/ Jaime and BTW, he is one of the guys I listen to on this board as he's been pretty spot on.

I ended up replacing the cheapest parts that I could to figure out my CEL. I had the same code as you Matt, and after reset but before I could check it again, I bought a new front O2 sensor (A/F for you and me w/ Cali spec trucks). I cleared the code before the O2 install and came up w/ a different CEL code, which was PO171. As I researched, the front 02 sensor can be the culprit for this and since I bought one, I installed it.

Reset the computer and bam, another CEL. This time PO171 again. Checked up on the board and explained my situation, people say clean the MAF. Do this reset the ECU, bam another PO171. Come back to the board and they say replace MAF. Replace MAF and the rest is history till another CEL pops its head.

I know you don't want to spend unnecessarily but if your O2 sensors are bad, your cat may suffer. Since you've had the code awhile and haven't changed anything, you could have (only a theoretical risk) made your cat situation worse. Just my two cents...