I need help with diagnosing some codes I'm running on my 01 4.7 Dodge Dakota quad cab. I have P138, P139, P141 (all downstream O2 censor codes) along with P302 (Cylinder 2 misfire). At first I thought I might have leaking injectors because my exhaust smells very rich, the truck is bucking under driffent throttle loads, and my gas mileage is around 13 mpg (way below what it should be). But now with all the O2 censor codes It leads me to believe that I might have a bad catalytic converter or the censor might just be bad or even something else i havent listed here. I finally gave up trying to figure it out and took it to me mechanic today. Unfortunately to diagnose it it'll cost me $75 that I don't wanna spend if i don't have to. Does anyone have some insight or has dealt with this problem before? thanks.

 

I'd start with changing the Bank 1 downstream O2. All three codes are for that one sensor so odds are it has a problem.

Don't use Bosch. Use NTK or Denso.

Pull the plug on cylinder 2. What does it look like? Post a pic here if you want. If the plug looks OK then swap the coil pack with another cylinder, and see if the code follows.

A somewhat common problem if the engine has ever overheated is that you'll drop a valve seat on #2.

 

I found a friend who will do a free diagnostics on the O2 censor codes. as far as the cylinder misfire I have checked the plug before and its fine. Ill have my friend run a diagnostic on that also.

 

I got a new O2 sensor and installed it last night. So far Ive gone about 50 miles and its seems to be actually taking MORE gas than it did before. its gone through a quarter of a tank in the 50 miles. The only difference i can notice is that it idles at about 500 less rpm's. Is it possible that my Catalytic converter is shot and that could be affecting my gas mileage so much?

 

Are there any more codes showing?

Normally you'd generate a "catalyst efficiency" code if there was a problem with the converter.

The downstream O2 does not adjust the mixture, it monitors the performance of the catalyst.

What does the tail pipe look like? Is it very sooty?

 

No more codes but the inside of the tailpipe is black as night.

 

Check the fuel pressure. You can rent/borrow a fuel pressure gauge. It should be near 50 psi.

You might have some clogged/dirty injectors.