I changed my plugs and 3 of the o2 sensors about 6 weeks ago. The only sensor that wasn't changed was the front passenger side. I noticed a great improvement in gas mileage. I got a CEL and my mileage took a crap about 10 days later. I disconnected the battery to get rid of the CEL. It came back about a week ago so I took it in to get the code pulled.

Doe anybody have ideas what it could be?

 

The 02 sensor that you didn't replace [bank 1 sensor 1]has an old slower range than the new ones and they all work together. After disconnecting the battery you have reset the PCM and it has new parameters based on the new sensors. If you couldn't get the 02 out, take the inner fender out for more room.

 

I should have added that the front passenger sensor was replaced in the summer of 04. I am thinking about replacing it again since me and a few others also think that is the problem. The truck also has about 128k on it and the cats have never been touched.

 

P0420 is the catt. converter efficiency code for bank 1 (passenger side) The ECM uses the o2s to determine if the catt. is working, but a bad o2 can't set this code. You have a failing catt.

 

Get rid of the 4 stock cat's and put on 2 Hi-flow cats

 

I replaced the other O2 sensor. The CE light hasn't come back yet. I'll report back if it does though.

 

This code has come back. It has been running real rich and gas mileage has gone down a lot. I'm getting 200-250 miles per tank.

 

Oh yea , loss of power and sh^ty milage with that code. I mean really bad milage and it's NOT and O2 sensor fault .

Your catalist monitor needs to fail THREE times before that code is set.

Check for a leaking cat (they split under the heat sheild), leaking exhaust clamp.

I don't think it's plugged , It's possible tho. You'll here a hell of allot tinging and tanging after shutdown and it may even glow in the dark when there plugged..

If you replaced the rear O2's - They leak @ the port allot of times.. - They all can .. Permatex RTV High Heat w/Copper is awesom stuff to use when installing the new ones..

 

I found a leak at both welds where the y-pipe was cut. I heard a ticking noise while driving so I checked out the welds. You can see the welds in this picture.



I went a local exhaust shop and they used some type of high temp rtv stuff to fill the leaks. The leak is at the top of the weld so it is hard to get to. I am going to see what happens.

I don't hear a lot of noise from the exhaust cooling down and I don't think the cats glow.

I am going to get Magnaflow parts # 23344 and 23345 if it comes down to replacing the cats. I might get headers also since I've been wanting some. The studs look like *$$ though.

 

What motor do you have?

 

4.6