Was driving home this evening, put truck into 2nd gear going downhill and shortly afterwards (IE I have no idea if gearing down in a hill impacted this) the Check Engine light came on.

Truck came with an edge unit on it, I checked the codes, the only ones that we're troublesome were p0401 and p0404.

My understanding is that those aren't worth worrying about, it's a Canadian truck so I don't think it will go into limp mode.

I cleared the codes, but they had reappeared 5 minutes later when I got home. I live in Vancouver, Canada. We have air care regulations here (for one more year) so I can't just mess with the exhaust system. All sorts of bad news... Failing air care is quite expensive so this time around it's probably worth just bucking up and getting the issue resolved. My understanding is that it's generally a gummed up valve or solenoid. The guy who I had inspect the vehicle before I bought it told me it was likely to be an issue in the future.

Truck is 2004.5 LLY 190k 2500HD

Anyone have any advice for where to start?