P0138 code Dodge Ram 1500 P/U

I'm new here. After reading various forums about trouble codes, this group seems to be on top of this stuff. I'm no expert mechanic so bear with me a little. The vehicle is a 99 Dodge P/U, 3.9 V-6, 126,000 miles that I purchased new in 99. I do most of the maintenance myself. Recent work performed: @115,000 miles I replaced the Cat Conv with a Summit model after losing an argument with an open man-hole at about 55 mph. The vehicle ran fine afterwards. @ 103,000 I did my last tune-up. (plugs, wires, cap, rotor, A/F, PCV. Champ plugs, Mopar wires...etc. Trans service last year and used Castrol synthetic fluid and new Napa filter. This thing never shifted this smooth. Anyway, about a week ago it starts slipping a little and I find the lines are gone and there's a rust hole weeping in the pan - so I'm losing fluid. Add some and I'm okay. Will do the lines and pan soon. But, at about the same time it starts stalling at red lights. Sits there fine, starts getting rough, then dies. Starts right up and goes fine. Couple days later the MIL lamp comes on. I have an Actron scanner. It reads P0138 - high voltage at o2 sensor. High voltage I read is rich condition. It does this for a few days and the light goes off, but the truck runs just as bad. Gas mileage is about half of normal. This scanner will show a list of stuff happening while the engine is running. RPM, mph. temp...etc. Tons of other info too, but I'm not familiar with what any of it means. I basically have 2 questions. 0138 is supposed to be the second sensor (after cat). If the engine is running rich, why doesn't the first one pick this up also? It's the same exhaust. Or do I not completely understand o2 sensors? (very possible)
Second question: What can I do with this real time info to help me get started on the repair. I'm not crazy about buying parts until I luck out. LOL.
Thanks for taking the time to read this and sorry it's so long.
T-bolt
PS: I did search for this - just got more confused. Sorry.

For the age and mileage of the truck I'd suggest going to the dealer and purchasing both the upstream and downstream 02's and installing them. Reasonably easy to do on that truck and it most likely will restore your like new fuel mileage and drivability. They'll probably need your vin number to be sure you get the correct part numbers and just a suggestion from working on those trucks every day at the dealer is replace your igntion coil as well if it's original. They tend to swell and crack and lead to weak spark, intermittent misfires and sometimes low power output.poor mileage. The deale should have all these in stock. good luck and let me know how you make out.

Jeff, thank you for the reply. I have done o2 sensors, but not on this vehicle - they don't look too bad. Out of curiosity, do you put dielectric grease in the plug connections?
I agree on buying from the dealer. Been burned more than once with ill-fitting, may or may not work "lifetime warranty" junk. Gets old quick.
Thanks again,
Ron

No on the dielectric, it will wick down the wiring of the sensor into the internals of it and short the heater element and or contaminate it in short order.

Jeff, thanks for the heads up. And an FYI to anyone reading this. The dealer O2 sensors are only 12 bucks more than Bosch at a part store! Here's the kicker. The dealer has 2 different parts numbers - 1 upstream and 1 for downstream. The part store says they're the same part! The coil's 50.00. Only the coil is in stk. I'll order everything Monday.
And yes, they did need the VIN. That tells me something right there.
I'll get back with the results, thanks again.

Yeah it's no shock they have that messed up! I can't believe it but it doesn't suprise me. There should be two different numbers because the 02's are not the same. The connectors are likely different and on most of the trucks the 02's physically look different. They probabky list only 1 number because it's probably one of there POS cut and splice generic that will continue to trun the CEL on because the heater circuit inside the 02 is a different resistance and requires more dudty cycle from the pcm to run which still generates the fault with new parts. It's probably an NTK brand 02 factory and if you went to there website it probably lists 2 different numbers. Again more proof that Bosch is junk!

I finally got to this. With the home computer dying, 300.00 for my son's tires and 250.00 in license plates, money has been short! I had a little to work with this week so I bought the coil, new plugs, A/F and did a PM. The plugs looked okay, but they're cheap. The coil looked screwed up - the thin plates were spreading apart with rust creeping out. Afterwards, it stalled about 1000 ft from the house and then ran fine. (Maybe 30 miles) The coil was obviously a big part of this.
What I don't understand is: before I did the work, I took a chance and got it tested so I could get plates. It passed. Why would the light come on, store a code, go off by itself and 3 weeks later pass with no work done? It was still running lousy when I took it there. What am I missing here?
And, Jeff, I can't thank you enough for the free diagnosis. I would never have thought coil on my own.

As for why it could be the 02 element was sometimes passing the monitor and sometimes not! When it's not it would throw a code and likley cause the drivability issue you were noticing including the mileage and rough idle. i've seen lots of vehicle pass with no lights on that ran poorly but to the sniffer were good enough. I think you'll notice a big improvement with that truck, probably hasn't run that well since new. i appreciate the kind words and thanks. Alot of posters on here could learn from your example.

Well maybe I spoke too soon LOL. Drove it tonight and it stalled twice. But I'm getting in line with your train of thought, I think. There are differnet things going on here. The only difference between today and yesterday is probably more loss of tranny fluid. Somehow just the way it "feels" when it stalls, it seems like the trans is involved. It acts like a a car that stalls when you come to a stop with a stick, but forget to put in the clutch. Just one fast shudder and it's off. When it's going, it's much better - coil issue solved that.
So, help me understand something. O2 sensors with age get; weak? act funny and send inaccurate signals? This creates a "less than optimal" report to the computer and it has to adjust for this, therefore everything else follows the incorrect lead? Am I comprehending this right? Sorry, but my only schooling in this is in my garage!!! Not the best I admit.

Oxygen sensors are in a nutshell a sensor that generates a voltage based on the 02 content in the exhaust gases and the engine's computer uses this signal to determine the fuel control of the engine through injector duty cycle etc. When the 02 ages it becomes contaminated from chemicals in the exhaust which reduces it's ability to sample the gases and the heater element inside them tends to wear out of course as they age. Essentially what happens when the 02 gets lazy is stays too close the the middle of it's voltage gnerator average of about .45 for too long and doesn't react quick enough to maintain optimal fuel control. This can cause sudden decel stalls, surging at idle, higher emissions, etc. You changed the 02's in your truck yet?

Jeff, I appreciate the explanation. I've only done the plugs and coil so far. Money was short this week. I can get voltage readings of both O2's with the scanner, engine running, if that helps. Any particular rpm I should target? Maybe I'll hook it up and note the voltage when it stalls out.
There is a lot of real time, engine running info with this scanner - I just don't know what most of it means. If there's data in there that you could make use of, I'll write it down.

A voltage reading can only confirm that the 02 isn't at a stuck bias or measurment, ie won't swith off .9. A dvom isn't quick enough to measure the amount of times the 02 crosses centre. This is called cross counts and is more indicative of an 02's function then just min and max settings. I'd still be changing the 02's out before going much deeper into this without a top level scan tool and maybe a scope, when your 02 heater is right on the edge of pass or fail you can get the exact symptons you r experiencing with no codes. An 02 that isn't hot enough can stall at decel and idle and feel like an igntion cut out but still start right back up. As well the way chryslers wire the circuit when it shorts it can throw the other sensor readings off ie the tps, crank sensor etc.

Thanks for the interesting explanation. It's easier for me to mess with this stuff when there's some type of basis of what's going on. Even "scratching the surface" knowledge beats nothing at all. The voltage numbers showing in the scan tool last month were going high to low and if I remember right, even went above .9 up to 1.??. Either way, the next step is replacing both and see what happens. This may take a week or so to get the money - I really gotta stop the tranny leaks. I'm sure it's not helping anything when it starts slipping every other day.
Thanks again very much - I'll post the results.

Well, I got more of it done. I had enough money to make up and install the trans lines. It took an entire day to put them where they were. The pan itself was okay, just weeping a little around the bolts and a little snugging dried everything up. I had enough $ left over for the downstream sensor so it's in. It's running better - the stalling is about half. What I have noticed, now that the weathers changing, is it runs much better when the outside temp is down. We had some really hot days early in September. When I would leave work it ran poorly. The nights were 30 deg cooler and the truck ran better in the morning. Could I have a temp sensor that's getting like the owner, old and tired? LOL.

I suppose it's possible but temp sensors are not a common failure on any mopars
I still think you're feeling the sluggish 02's and as you know they can be sensitive to temperature fluctuations. A good idea as well on your truck would be to remove the throttle body, you'll nned a new gasket that goes under neath, remove it and the IAC motor from the side of it and clean it out really gopod with some carb cleaner and reinstall it, It shoudl help the idle a bit as well. When you get money ahead complete the tune up with a new dist cap and rotor and good wires and a pcv valve. With that completed and the new 02's you'll have covered qbout 95% of all things that cause stalls on those trucks