I was going down the highway just now when my Ram threw up another check engine code. This time it is a p0420: catalyst system efficiency below threshhold bank 1. Does anyone know what this means? About a month ago I replaced the front O2 sensor in the cat, and I replaced the plenum gasket. The O2 sensor is a genuine Mopar brand. About 2 years ago, I replaced the cat. conv. And a tune up wasnt more that 2 months ago. I dont drive it a lot, so the mileage wont be too long on the plenum and O2 sensor. Any ideas would greatly be appreciated before I go putting more money into my Dodge. Dont get me wrong, I love my truck, but I am getting tired of working on it. Thanks in advance. I think all the vehicle info is in my signature except that it has a little over 115,600 miles on it.

 

anyone have any ideas? or do i need to search prior posts?

 

sounds to me like your O2 sensor is detecting too much unburnt fuel or hydrocarbons in the exhaust. Might wanna find an air/fuel ratio tester and have it checked or maybe even run it by a local emissions place and have them check it.

 

your rear o2 sensor is detecting excessive un-burnt fuel meaning either the o2 is bad or the cat is not working correctly. You can measure the input and output temps on the cat with a temp sensing gun to determin if the cat is working correctly or not. output should be hotter than the input and the input temp shouldnt be over 550 deg or so. much hotter than that and it could be clogged.

you also might check into your fuel injectors... they may be dirty and leaking excessive fuel while your motor is running.