I been using the Helix HF cat for more than 8K miles and had no issues with P420 code until now. So why does the code pop up now and not before? After doing some research, I came accross this.

The most likely cause:
1. Catalytic converter deteriorated (I doubt it's this)
2. O2 sensor failed (I doubt it's this)
3. Exhaust leak (most likely the cause)
4. ECM failed (ECU most likely not)

Out of all these, I think mine is caused by exhaust leak around the exhaust gaskets since I removed and installed my HF cat multiple times using the same gaskets which are old and added a cat-back afterwards. I think once I get some brand new gaskets, P420 code won't pop up anymore.

So if you have a HF cat then installed a new exhaust or a DP, it could be the gaskets causing exhaust leak especially if you have a 3in HF cat and the cat-back tapers to 2.5in like stock exhaust.

Otherwise I'm to why this code suddenly pops up.

 

Hi,

Hmmmm, I have the cell with my RT HF cat, didn't start right away

I may have a leak. When my Helix cat back comes this week, I'm going to try the anti-fouler trick, but I should also look for leaks.

Take care,

FB