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Not your ordinary P0420 DTC thread

4773 Views 8 Replies 3 Participants Last post by  rqjoe
P0420 is Catalyst System Efficiency Below Threshold (Bank 1.) I have looked at many past threads on this but wanted to share my experience with this DTC which recently caused my CEL to come on. My 2015 is almost exactly 2 years old with 36,500 miles and engine/exhaust/fuel system is all stock. I do not even have an AP. All of a sudden one day after I left from the gas station the CEL came on. The car was running great, the same as always. Next day I go to Autozone to have them tell me what code is set. There is only 1 and it is P0420.

Then I go to the dealer and they confirm the code and schedule me an appointment for a full diagnosis (appointment is next week.) Being the paranoid, un-trusting type, I go home and over the weekend check a few things in the troubleshooting sequence for this code from the service manual. I can't check everything because I don't have a datalogging device yet for the O2 sensor but I did check for visual damage and a couple resistance checks of the sensor harness wires. I found nothing abnormal. But I did, of course, disconnect the connector at the 02 sensor to do the resistance checks. I put things back together and now have driven several drive cycles and the CEL is NOT on any more. ?????

All I did that would affect anything is disconnect/reconnect the rear 02 sensor connector. One of the troubleshooting steps says to look for water in the connector. I found none. I am not sure what to make of this.

Oh, and one final interesting thing -- at first i could not get the connector apart so I thought I'd try to take off the small bracket which holds the body side of the connector in place so I could get a better grip on it. There is one small bolt going through the bracket into the engine valve cover (or head, I forget.) After I got the bolt out I realized it won't work because this bracket is also welded to the coolant line to the oil filter housing. But, anyway, as soon as I cracked the bolt loose I see a "foaming" ring of liquid stuff form just under the bolt head! First thought is this bolt was some sort of drain plug but then sanity took hold and I realized it couldn't be since it just held a bracket in place on the head/valve cover. I removed the bolt and smelled and felt the liquid. I could not tell what it was; it was not water and it felt ever so slightly oily.

The only conclusion I could come to was some sort of thread-locker or anti-seize. But it did not look like any thread-locker I had seen before and I am not even used to this being on such bolts and furthermore, why would it "foam up" when the bolt torque was released. Not sure how this could affect the 02 sensor connector but I just thought it interesting and it was somewhat just above the connector. So I am not sure what I am saying here, if I am saying anything at all, (Iron Man) except maybe it appears just cycling a connector can have a real effect on a sensor working. At least it appears so far, that is all it took to fix this issue. In the next couple days if the light does not come back on I will cancel my appointment with the dealer; it would have cost me $70 for the diagnosis. :dontknow:
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Edit that to break it up into paragraphs. Huge wall of txt that people will not take the time to read

So I had P0141 code on my GMC van. This is a 'rear H02S heater circuit malfunction' code. It comes on when the sensor heater bias voltage does not back off within a specified amount of time. I turned off the CEL and it just came back on. Then I disconnected the sensor harness and reconnected it. Wallah! No CEL. So twice now on two different vehicles, just disconnecting and reconnecting the O2 sensor connector has resulted in no CEL. :dontknow:

P0420 on the WRX has be coming on intermittently ever since my original post. Comes and goes at odd times and intervals. I had been waiting for warmer weather to crawl underneath and try a few things like coating the connector with grease then changing out the O2 sensor.

But, just recently the CEL has not been coming on!! There is only 1 thing I have consciously done............... drive the car differently!

99% of the time I drive like a little old man and never exceed 2500 RPMs. Don't know that I had EVER had it above 5000. So it hit me that maybe I should 'blow it out' or 'clean it out' by hitting some higher revs while driving.

I did this a couple times and the CEL seems to have disappeared. Could it really be that simply running those higher revs periodically, actually prevents a P0420 CEL?
Ha Ha. I know what you are saying, but driven like a family sedan by some old guy, the car does really get that excellent fuel mileage I get. But, alas, it may go down some now if I have to get on it harder just to keep the CEL off.
Well.......... driving the car harder did not actually help. I think it was just a placebo effect deal. The code kept coming back sporadically.

BUT, Seafoam did help! I ran a can in the gas tank, then did one of the intake spray bottles. Putting it in the gas
seemed to help most. It is as if either the cat or the O2 sensor was 'fouled' and Seafoam did indeed clean it up. No more light or code.

I am now a Seafoam believer and my car will forever drink the Kool-Aid.
Had this code driving home from Chicago AFTER my dyno tune. However, the fix was just get an updated tune to delete the code (high flow downpipe). It only happened once while on the freeway cruising in 'I' mode.
Yeah, yours was surely from a different cause than mine.

Seafoam really worked on mine. No CEL for 2 months now and no re-application of Seafoam yet either.
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