Alright, so I've been dealing with this P0420 error (Cat Efficiency Below Threshold Bank 1) for a while now.. here's what I've done so far: about 2 months ago, I replaced all 16 spark plugs (8 on intake and 8 on exhaust side iridium) and all the spark plug wiring.. once I did that, the error disappeared. Fast forward to 2 days ago, the error returned.. I just finished replacing all 4 02 sensors (2 upstream 2 downstream) and the problem still persists.. I would be inclined to believe that my cats are bad but I'm not suffering any noticeable performance issues.. tomorrow I plan to let the truck get nice and hot then to a before cat after cat temperature test to see if the cats (there are 2) are running efficiently.. I tapped both of the cans with a rubber mallet and it doesn't seem like my cats are broken up and rattling around or anything.. so, assuming that the cats test well on the temp test, any thoughts as to would be causing this issue? Thanks in advance

 

sure sounds same as mine did before i changed the cats. let us know what you find.

 

How many miles?


A good exhaust shop will have the tools to flow check them. By the time they restrict flow, you usually see symptoms anyway.


A lack of chemical reaction will not be seen in performance. While temperature is a good indicator, it doesn't tell the whole story. If you have a way to see your O2 data, you can get a more positive idea IF the cat is actually malfunctioning or if you have a flakey O2 sensor.


Maybe you're being to nice to your truck and need to get out there and floor it a while.

 

put a gallon of lacquer thinner in with 4 gallons of fuel. Run it on the highway for 100 miles (high speed).. and the cat will be clean. I did this aboat a year ago and zero problems.