P0420 - Catalytic converter and o2 sensor

[md1m]

I have a 2003 Pathfinder with 257k. Had a P0420 and installed new o2 sensors (upstream and downstream on bank 2). Also ended up installing new coil packs for all cylinders because I had two prior misfires, and all coil packs were original, so decided to just put in all new ones.

 

The new o2 sensors worked for a while, but the P0420 came back so I put a spacer on the o2 and cleared the code and it has remained cleared. Assuming it stays off, does this seem to indicate it's probably the cat? I know it can be other things, but just wondering if it's most likely the catalytic converter.

 

I'm putting on maybe 250 miles a month now, and even pre covid was only driving 500/month, so not wanting to do more than I have to. 

[Mr_Reverse]

Most of the times in my experience, a P0420 and or P0430 has been catalyst failure. I have had a bad sensor set the code once. I do a simple test that works well to determine if the catalyst is working. Using a noncontact (IR) thermometer, with the engine warmed up to operating temperature, hold the RPMs at about 2000 for a minute. Then measure the temperature at both ends of the catalyst. With a healthy cat, the outlet temperature will be 150°f+ higher than the inlet. If the temperature at the outlet is less than 100°f higher than the inlet, the catalyst is failing. If it is the nearly the same on both ends, the cat has died. If it is hotter on the inlet than the outlet, that indicates a restriction in the catalyst. 

 

Years ago, I would have a couple of failed cats in a year. A few years after the E10 mandate, started seeing more failures. In the last few years, I have been seeing an average of 2-3 cars and light trucks with failed cats per month.

To me, the common factor is the fuel, though Honda's and Subarus seem to have been the worst for it. I believe the Ethanol is why I have seen an increase in catalyst failure. My mom was a Chemical engineer and said ethanol made no sense in our gasoline. With my 1993 Pathfinder, I have observed that my fuel economy runs about 18 mph highway with gasoline and about 14 mph highway with E10. Since I do my emissions testing, I have noticed that when I am running E10, my HC and CO numbers are a bit higher than when I am running straight gasoline. That is telling me that I am not getting as complete burn with the ethanol. Makes sense since my O2 sensor and ECU  maps were made for gasoline rather than E10. My truck is seeing too much O2 in the exhaust and thinks it is running too lean, so it is always running a bit rich and that is causing the cat to work harder. Plus the lower energy density requires more fuel to get the same work. 

No, I am not a fan of ethanol for fuel in our gasoline engines. But what do I know, I am just the monkey that works on them and I don't get subsidies from the federal government like the ethanol industry does.