packman53 Posted September 8, 2016 Share Posted September 8, 2016 I apologize for the length of this message, but I am in a real pickle right now and wanted to give as much detail. Background: I recently bought a used Legacy with 117k. Carfax came out clean, but after driving a little while the CEL came on with the P0420. It had run rough in the morning hours a few times, but I chalked that up to poor air intake gaskets as were talked about on this forum, but none of the lean codes were thrown when I got it checked. After reading about the P0420 issue on this forum I found that it most likely related to the rear O2 sensor. I let it go for 1.4k miles and then I experienced the following problem. Main Problem: The car experienced a rough idle in the morning (nothing new) yet began to hesitate while driving at normal speeds, the At Oil Temp light came on with the air bag light and I could hear clicking coming from the passenger side of the car whenever the vehicle hesitated. I turned the car off/on at a stop light and this issue went away for about five minutes. By that time I made it to my destination. I come back to the car a few hours later and it literally died on me. The CEL, blinking CC, er HC code in the Odometer, the temp gauge railed high, and no response from the car when I try to start the engine. It had gone into a failsafe mode. Initial Troubleshoo/ Dealer Solution: I tooled around on a few Subaru forums looking for a solution. Yet no one seemed to find a definitive answer to these problems/symptoms. Therefore, I took it to my local subaru dealer (Napleton Autoworks). They diagnosed a failure in the ECU. I bit the bullet and payed dealer prices to change it out. Dealer got me the car back the other day and it ran like a dream. No CEL issues related to P0420. Symptoms Return: I went for a drive and set the cruise control...BAM CEL pops on with P0420 issue, but I am able to drive with no other problems. I figure it is a residual issue with the downstream O2 sensor. Yet, the next day I go for a drive then the car begins to idle high in park, and as I drive the car begins to hesitate while I simultaneously hear clicking coming from the passenger side of the car again. Thinking its the same problem I immediately turned the car around and got it home. Secondary Troubleshoot: I disconnected the battery, drained residual capacitor power, disconnected the main engine wiring harness, and disconnected the rear O2 sensor. I plugged everything back together. Turned the key to put power into the car and let it sit for about 20 seconds. Started the car and the CEL did not illuminate. I then followed the typical ECU relearning procedure and let it idle for ~30 minutes; turned it off and let it sit for 30+ seconds and took it for a spin. No problems, could set the CC without issue. Preemptive Fix: It made me think that it could be the bad downstream O2 Sensor. I purchased a Denso sensor and installed it. There were no issues while driving, able to set CC, accelerate without issue, no rough idle, etc. Problem Returns: Took the car to work today and it made it all of 5 miles before the CEL came on. I still need to get to code read out, but I anticipate P0420 again. Further known details: The dealer has record that the Cat was replace less than 8k miles ago for a factory recall by the previous owner. With that knowledge I feel like I can rule out the Cat and downstream sensor (since I just replaced it). When I find out the code I will let you know, but I am looking for any advice at this time. Dealer Next Steps: The next step with the dealer is a replacement BIU if this issue presists, but I feel like they are grasping at straws with that solution. Is it possibly a bad wiring harness? As an engineer I take pride in determining the root cause of a problem and I don't trust these mechanics to get it right. If anyone has a thoughts, I am open to suggestions. Link to comment Share on other sites More sharing options...
ehsnils Posted September 8, 2016 Share Posted September 8, 2016 Unformatted word forts aren't easy. It's still possible that the cat is bad even though it was recently replaced. Or that it was swapped with a bad after the recall replacement. A fresh cat is worth a lot. But check the battery voltage. If it's starting to go bad you can get a lot of strange things. Also check groundings. Link to comment Share on other sites More sharing options...
packman53 Posted September 8, 2016 Author Share Posted September 8, 2016 In my first round of issueI check the battery it read 12.59V. I have reviewed and checked the grounding as well. I cleaned a few grounding areas, but it seemd to be making a good connection. Is there a good way to determine if the Cat could be the root cause? Link to comment Share on other sites More sharing options...
packman53 Posted September 9, 2016 Author Share Posted September 9, 2016 I got the codes read for the CEL and it came back with P0420 cat bank 1. Has anyone experienced this failure, cascade into a failsafe mode? Link to comment Share on other sites More sharing options...
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