At my ends wit with this, and i’ve thrown to much on this blindly it feels. a month ago i had to replace my water pump for my car. 2009 Mustang V6 4.0 5spd. after doing this, i noticed the car became very sluggish and not wanting to accelerate beyond 3500rpm when driving, further more feels like i’m towing something. I was getting P0420, which was showing sensor 1 bank 1, which i promptly replaced. 3 days later, it came back, but the cars actions had not changed. In my venture i have replaced the fuel filter, air filter, 2 oil changes to ensure nothing looked off, spark plugs (coil pack and plug wires were done last summer). Still getting 0420 with the same reading, i decided it was time to take to an exhaust shop and just replace the other sensor on bank 1 and the catalytic itself. The shop then informed me that after pulling the single cat, that my cats had been gutted by the prior owner (some 8 years ago). After i picked it up at the shop, earlier today, with the fixed Cat and new o2 sensors on this side (passenger side) the car now, does not want to go beyond 2500 RPM, which made a drive on 75 quite frustrating. I also have cleaned the MAF. Car has little no mods (Throttlebody Spacer that has been on for the last 6 years), aftermarket coilpack - screamin demon from americanmuscle.com, and higher voltage plug wires from americanmuscle.
The catalytic converter is plugged up,
“The shop then informed me that after pulling the single cat, that my cats had been gutted by the prior owner (some 8 years ago). After i picked it up at the shop, earlier today, with the fixed Cat and new o2 sensors on this side”
The code points to a problem with the catalytic converter. Plain and simple.
The car should have been throwing the P0420 for the last 8 years.
It’s never been a thing till just recently.