2011 Fusion 2.5l P0420

Hi all,

My parents’ Fusion has thrown a PO420. There’s a lot of background on this, I’ll try to be as concise as possible.

They bought the car used about three years ago with 90k miles on the clock. It’s at 125k now. Carfax reports one previous owner, and a pretty good service record at the same dealership that sold it. No accidents reported. I went over it when they bought it, it is a solid used car. The only funny thing was that four days after they bought it, it threw an upstream O2 code. We took it back to the used dealer. They said someone had left that sensor unplugged. ? They said they fixed it. I kept a close eye on the OBDII and it never threw another code until now.

Car threw P0420 about a week ago. I went in and looked at the only O2 sensor voltage reading I could, which is the downstream. I have a bidirectional Actron scan tool that I can graph the sensor with in real time. There is no PID listed for the upstream sensor voltage, which is odd. I can only view the amperage graph, which tells me nothing. I cleared the code to see if it would come back. It has not as of now.

Fuel trims look normal. LTFT is -1.3 and short term hangs out around that value too.

The data from the downstream sensor suggests a bad cat. At temperature and holding around 2500 RPM, it switches rapidly between .2 and .8 volts, sometimes hanging out at about .7 for a few seconds. Sometimes hunting all around that range, and then it goes back to switching

Fuel economy has dropped about 5 or 6 mpgs on average since the code first appeared. I think it is plugged or has melted down. There is no rattle of broken substrate that I can detect.

I noticed the coolant in the puke tank was a little low. I topped it off and I’m monitoring that. I don’t think it’s burning coolant, at least, not enough to visibly contaminate the oil or smoke up the exhaust. I get a faint whiff of coolant by the outlet of the radiator. Probably a little leak there.

Before they spend $400.00 on an OEM converter for me to install, how should I go about ensuring the new one doesn’t also fail? I can’t see any obvious signs that the car is running on a bad AFR. There are no idle or driveability issues, no obvious vacuum or exhaust leaks, and no other pending codes. I’m stumped. I assume the upstream sensor is OK, because it would be throwing a code if it wasn’t.

The best I can figure, the previous owner drove a long time on a bad upstream sensor and didn’t fix it. The used dealer fixed it and forgot to plug the new one in, but the cat was already compromised. It is now below threshold.

Is this a safe assumption? Should I run any more tests? I will be doing the infrared thermometer test tomorrow. I’ll update what I found when that is done.

Thanks! I’m trying to save my folks some money and headache.