I just picked up a 2007 Accord with 177k. On the drive home from the purchase, it threw a CEL. I plugged in an ELM327 and determined that it was a P0420. If I have to replace the cat, so be it- but I figured I'd try the anti-fouler extension trick on the downstream O2 sensor. I did this, and then attempted to clear the code. No luck. No scanner I try gets it to clear. I've used Torque app with 2 different ODB modules. I also tried an older dedicated scanner. The scanners do not indicate any sort of failure- but the CEL never goes out, and the code is still there if you scan again.

I've been using these ODB scanners for years and never had a car that wouldn't let me clear codes.

I was able to clear the CEL by disconnecting the battery.

Any idea why this is happening?

 

I have the same car and I've gotten the P0420 code. I changed the O2 sensor and never had the problem sense. Your sensor maybe bad. You can test it to see

 

The O2 sensor is putting out a signal for sure - I added a display in Torque Pro for bank 1 sensor 2 and I can see it going up and down. I think this is apparently the sign of a bad cat.

 

The P0420 clears after you drive it a few times or disconnecting the battery. It should be a permanent code that's why you can't reset it. That code does indicate a bad cat and didn't know you troubleshooted it before adding the fouler.

How long is the fouler. If it's one of the short ones I always use 2 and it works.

 

I initially installed 2 anti-foulers (one has the small 1/8" hole in the end). I'd say each extends by about 5/8 - 3/4". I disconnected the battery to clear the CEL, and after driving around for 5-10 mins I got a CEL again. BUT- this time it was a P0139. I'm interpreting this to say that I extended the sensor too far out, so I removed one and now have a single anti fouler in place. I cleared by disconnecting the battery, and tomorrow I'll see if the fix stays. If not, I'll just bite the bullet to do the cat.

 

I replaced the cat on my 99 Accord and that took care of my P0420 code. I even reused the original O2 sensor. I went with a MagnaFlow cat that I got off of e-bay for less than 150 shipped.