P0420 Any Options Left?

[004Gunner]

So I've had the P0420 code pop up 3 times now hoping that it was just the heated oxygen sensor everytime I cleared it. Now this time I finally got around to checking the sensors and both have resistances within specs. The A/F sensor was just replaced 15k miles ago so I knew it wasn't that but checked it anyway. The oxygen sensor is original so I was hoping it was that, but it checked out ok. I don't have any leaks on the system as far as I can tell. Any ideas or is it time to suck it up and replace the cats? I have Cali spec btw.

 

[psquared77]

I'm going through the same thing.....I get the CEL P0420 code about this time of year the past 3 years. Comes in the fall and leaves in the spring. I was hoping it was the O2 sensors, but same as you they checked fine and I just had the A/F sensor replaced a few months ago. Here is my brain basher though....I PASSED inspection on my emissions in NJ (reset my CEL before going). I am stumped. Gas mileage is good. Performance seems fine. The only other symptom is my fan and thermostat lights go out when the CEL comes on. The dealership of course wants to charge $100 to tell me what I already know, and then $3900 to replace both cats (quote was actually down from 2007 quote) :thumb3: Hopefully someone can offer us some insight......

 

[TheDurk]

I'm going through the same thing.....I get the CEL P0420 code about this time of year the past 3 years. Comes in the fall and leaves in the spring. I was hoping it was the O2 sensors, but same as you they checked fine and I just had the A/F sensor replaced a few months ago. Here is my brain basher though....I PASSED inspection on my emissions in NJ (reset my CEL before going). I am stumped. Gas mileage is good. Performance seems fine. The only other symptom is my fan and thermostat lights go out when the CEL comes on. The dealership of course wants to charge $100 to tell me what I already know, and then $3900 to replace both cats (quote was actually down from 2007 quote) :thumb3: Hopefully someone can offer us some insight......

Wow, I can't believe you passed NJ with a new reset. I thought they dinged you if the ECU wasn't showing all the tests complete. Anyway, a degraded rear O2 sensor can throw a P0420 even if its heater resistance is still within spec. Rear sensors have ZERO effect on performance or mileage; all they do is allow the ECU to throw the P0420 code. I would certainly swap it before thinking about doing cats, especially if it is possibly original. I changed mine at 100K and I have gone 20K with out a CEL (was throwing p0420). I had done front A/F at 70k for a P1135. I also have a Cali.

What's a fan light?

 

[psquared77]

Good to know. Thanks for the info. I reset my CEL by disconnecting a battery cable for a few minutes before leaving for the inspection station. I didn't touch the ECU (don't know what it is). I knew I wouldn't pass inspection with the CEL on. I threw a P1135 a while back too-changed out the A/F sensor. Is it possible I have a short somewhere that is leading to some of my dash lights shorting while throwing the P0420 code (I double checked it at Auto Zone last week to be sure.) I am definitely showing my ignorance in my desparation to solve this.



Oh, the "fan light" is the bulb behind my fan/blower control knob that illuminates.....

 

[psquared77]

Wow, I can't believe you passed NJ with a new reset. I thought they dinged you if the ECU wasn't showing all the tests complete. Anyway, a degraded rear O2 sensor can throw a P0420 even if its heater resistance is still within spec. Rear sensors have ZERO effect on performance or mileage; all they do is allow the ECU to throw the P0420 code. I would certainly swap it before thinking about doing cats, especially if it is possibly original. I changed mine at 100K and I have gone 20K with out a CEL (was throwing p0420). I had done front A/F at 70k for a P1135. I also have a Cali.

What's a fan light?

Oh-and I was being somewhat sarcastic about replacing the cats before replacing sensors. I posted on here 2 years ago that the dealership wouldn't acknowledge that I could replace sensors and not the cats...according to them I had to drop the $4k.......Did you replace your O2 sensors with Bosch or something else?

 

[004Gunner]

Wow, I can't believe you passed NJ with a new reset. I thought they dinged you if the ECU wasn't showing all the tests complete. Anyway, a degraded rear O2 sensor can throw a P0420 even if its heater resistance is still within spec. Rear sensors have ZERO effect on performance or mileage; all they do is allow the ECU to throw the P0420 code. I would certainly swap it before thinking about doing cats, especially if it is possibly original. I changed mine at 100K and I have gone 20K with out a CEL (was throwing p0420). I had done front A/F at 70k for a P1135. I also have a Cali.

That would most likely be my case then. I have absolutely no other symptoms. I've been monitoring my gas mileage everytime I fill up and there is no change. From what I understand the only way to check the actual operation of the sensor is by using a diagnostic quality OBDII reader. I can borrow one, but I think I would rather throw in a new O2 sensor. I did notice that when I reset the code it would come back on faster if I used the ECT setting.

Psquared: I was thinking that you do have some kind of electrical malfunction caused by the CEL coming on.

 

[TheDurk]

Oh-and I was being somewhat sarcastic about replacing the cats before replacing sensors. I posted on here 2 years ago that the dealership wouldn't acknowledge that I could replace sensors and not the cats...according to them I had to drop the $4k.......Did you replace your O2 sensors with Bosch or something else?

I replaced both with Denso OEM equivalent sensors from URDUSA.com. Linky:

Underdog Racing Development
I bought in to the bad press given to Bosch sensors on this and other boards. All I can say is the URD stuff works great.

I am glad you were sarcastic, I have read posts where guys may have thrown away perfectly good cats when sensors might have done the job, thanks to stealers. I did not want that to happen to you. No disrespect intended.

 

[TheDurk]

I have a reader set-up to my laptop that allows me to see the voltage tracks for the exhaust sensors. However, I found it hard to be sure whether they showed my problem was the cats or the rear sensor. Either I am not smart enough or my tracks were ambiguous. Either way, I am very glad it turned out my P0420 was due to the rear sensor.

 

[psquared77]

I replaced both with Denso OEM equivalent sensors from URDUSA.com. Linky:

Underdog Racing Development
I bought in to the bad press given to Bosch sensors on this and other boards. All I can say is the URD stuff works great.

I am glad you were sarcastic, I have read posts where guys may have thrown away perfectly good cats when sensors might have done the job, thanks to stealers. I did not want that to happen to you. No disrespect intended.

Thanks for the link. No disrespect taken at all. I received some good advice on this site regarding the cats 2 years ago. If I get the P0420 popping on again, I will order the sensors from URD.