Got a P0138 "O2 Sensor High Voltage". 03' 1500 4X4 5.9L.

Bad sensor? I think it's the downstream from the cat/conv.

I'm running into a Magnaflow then duals out. Got the truck that way with the code.

Any ideas?

 

bad sensor. might as well do all 3 (mine has 3 sensors) they last about 70,000 miles. the other one or two if you have three wont be to far behind

 

Ok thx buddy. Didnt wanna start replacing good parts. I'll get on em' today.

 

I know the feeling, mine have been bad for a year, been procrastinating on them LOL

 

The way i understand it, with a bad o2 sensor you can run way to lean and eat some plugs or burn valves, holes in pistons. Or run way too rich. Not sure on the Rams. Hope the o2 sensor doesnt hurt my wallet.

 

I thought there were 2 sensors, 1 upstream and 1 downstream from the cat, unless you are in Calif then you have 4. Maybe not...

A code reader should be able to read out the voltages seen from the O2 sensors and tell you which one is bad. I have 145k miles on mine and the last time I looked at the sensors they were reading fine. That is, once I fixed what was making the reading from the sensor look too rich, which is what threw a code.

 

Not too much wallet pain...

Pre-cat, $27 @ amazon.com
NGK-23139-Oxygen-Sensor-Packaging

Post-cat, $41 @ amazon.com
NGK-23140-Oxygen-Sensor-Packaging

 

On a Hemi, the POST cat sensor is a "nuisance CEL", it's telling you that the cat (or the O2 itself) is bad, the PCM makes NO changes for this code other than posting it on the dash. Now the PRE-cat sensor going off will have the PCM changing things (A/F ratio, etc.) in an attempt to rectify the situation that caused the code to generate. When this is a bad sensor, it can result in either a too lean or too rich mix. It's MUCH MORE imperative to change a bad front sensor right away than it is to change a failed rearward sensor...

Oh yeah and DON'T buy Bosch, Hemi's HATE them...

 

Ditto what Hammer said. I just went through this on one of my 4runners. Rear sensor was intermittently reporting a "CAT deficiency". It didn't and doesn't do anything as far as adjusting A/F ratios, etc. It just likes to play emissions cop and turn the CEL on. Since changing out the sensors is much cheaper than installing new CATs (my 4runner has 2 CATs $$$$$), I just swapped out the sensor first. Had over 100k on it, so it was time anyway. So far, the CEL has been dormant. When I went underneath to change the rear sensor, I could see that the forward sensor was newer, so maybe it will last a while longer.

When I went back through the service history of this yota, I saw where it had been taken in for CEL codes 3 times. Each time it was the sensor that was the problem, and not a real mechanical issue (if you consider a sensor non-mechanical; I do, depending on which sensor it is). First time, cleaning the MAF fixed the problem. Second time, replacing the AF (forward) sensor fixed the problem. Third one was related to some accellerator positioning sensor. So, my fingers are crossed that this time was just a sensor too. $62 vs. $1500+

Ditto on Bosch as well. Yota's like OEM parts and hemi's seem to be particular as well. Toyota uses Denso sensors. The guy at Advance tried to sell me a Bosch for $170. Found the Denso I needed for $62 shipped on ebay. Higher price does NOT always mean higher quality.

 

Well, didn't get around to changing it today. BUT.......

Earlier i got back in the Ram after going to the store and no CEL. Hoping it might be some bad wiring. Gonna crawl under it tomorrow.