I have a 2003 Dodge Ram 1500 with the 5.9 and recently my check engine light came on with a P0138 code. This is the downstream o2 sensor correct? Also would this be why my gas milage has dropped down to 7ish mpg?

I recently replaced the intake and plenum gaskets because of the excessive oil consumption problem i was having, so I kinda thought that it would be the upstream o2 that is bad because of all the oil that would be going by it but i dont have a code for that sensor.

Thanks

 

p0138- Sensor Circuit High Voltage (Bank 1 Sensor 2)

 

Downstream sensor has no bearing on what the PCM does, it only reports that the cat is not working within spec. NOW if the sensor is going off because running that rich with all the oil in there clogged the cat, THEN it would be perfectly feasible that your gas mileage would take a sh*t...

 

sooo, i guess my first step of action would be to just replace the downstream sensor and see if that takes care of my problem? If not I would surmise i need a new cat.

 

Yep, but if it proves to be just the sensor, then you need to look elsewhere for you drop in MPG problem. Front sensor fail would have the PCM changing the A/F ratio in an atttempt to get the truck in spec, which would screw with MPGs, but not so with the back one...

 

sounds like a plan

 

sounds like a plugged CAT, have you checked your Plenum in the truck?

 

Yes, I replaced the plenum and intake gasket about 3 weeks ago.

Is it feasible that the upstream sensor could be faulty without the PCM throwing a code? Its recommended that when you redo the plenum that you change the upstream o2 with it, because more often than not they are faulty. Maybe ill just go all out and replace both of them while im at it. The truck has 112,000 and its the original sensors.

 

well then do like me and cut those bishes off.......... LOL

 

Lol that is an option. How did you get your downstream sensor not to throw a code?