I have a 2005 Honda Civic hybrid with a manual transmission with 162,000 miles, located in Detroit Michigan. When we bought the car around 55,000 miles ago it had a check engine light with an upstream a/f sensor gone bad, it was replaced with a denso sensor. Check engine light went off, few days later back on again with code p0420 inefficient cats, so I replaced the two downstream sensors and the light went away. Now with colder weather the light is back on again, the only thing that changed was the heat shielding around the third cat was removed because it was rusted and rattling. Could the lack of shielding be causing the light or was it something else? Thanks!!
If the cat doesn't reach optimal temperature during driving it will cause these codes. The o2 sensors only have to be off by like 5% for them to throw a check engine light. More than likely you will need to replace the catalytic converter.
I agree with @Cjbman.
You can try higher octane gas and cleaning the CAT. Scotty has a video on this topic.
