With that many miles I would look at the cat they do not last forever. As for the code setting when he put the regular in it for that code to set the PCM has to see low efficiency for a set number of miles so that would take some time.

 

So what u think id have to change the cats or just run octane 89?

 

I have a 2004 ram 4.7 with a miner eva code i changes gas cap how long will it take for engine light to go off

 

You can try the 89 octane but I do not think that is your problem. Just make sure there are no other codes and you do not have an exhaust leak. If you try a tank of 89 octane and it does not change any thing and you can not find any other problems I would replace the cats.

 

So can i run this regular tank of gas then fill octane 89 or is that going to be hard on it?

 

How do u tell if the cats need to be replaced? And u think its just a o2 sensor?

 

Well If you had a exhuast gas emissions tester that would tell you but when you see that P0420 code and you do not have any other issues then I would say your cats are bad. As for it being a bad 02 sensor If you have a 02 sensor code then you should make sure you take care of that first. If you do not have any other codes then I would replace the cats.

 

Ill try using 89 octane and see what happens. Is it doing any damage with the light on though?

 

It won't damage anything. I doubt that the 89 octane will make the light go way. I would replace the rear O2 sensor. It could be bad. If the light comes back on, then the cat is failing. A good exhaust shop should be able to check it.
I would always run 89 octane with the Hemi. There have been many cases, especially on higher mileage engines, that some detonation occurs with 87 octane. This may not be able to he heard but can beat a piston and valves to death. it's usually about 10 cents more a gallon, or about $2.60 a tank difference. Cheap insurance in my mind.
I would replace the o2 sensor, reset the computer and see what happens if it were me.

 

Oh ya octane 89 isnt much differece than ill try that thanks. And how do i reset the computer do i need a programmer?