I recently had a problem with my Cat. I had a loss of power. We replaced the Cat about 1 month/4000 miles ago with an aftermarket unit recommended by Midas. It runs smooth and everything. However, this week, the check engine light came on. It shows P0420 which is Cat converter efficiency. Should I look at the O2 sensors? Should I take it back to Midas?

 

try cleaning the maf real quick then reset the cel light and see if that helps if not time for a new o2 sensor order it from http://www.urdusa.com/

 

Same thing happened to me. First was the P0420. One week later she threw off a P0171 (Too Lean)Turned out to be the O2. Got it replaced and now she runs beautifully. I could have sworn it was the cat.

Ryan