P0138 fault code

This pertains to my 01 Ram 1500 5.9L Regular Cab Short Box. A couple months ago after changing out the water pump I had my check engine light come on. I brought up the code P0138 on the odometer. Truck ran
fine, took it to Auto Zone and they cleared out the light. Just last week the truck is still perfect and the day of my inspection, the light comes on again. Truck runs fine. Due to the NYS emissions I had to wait another day to bring it back. One of my buddies reset the light at dealership. I ran it through a drive cycle and the next day it passed fine. Exactly a week later today it came on again. I took it to Auto Zone and asked them about my problem. I don't know about their computer, but it said I have 3 o2 sensors. I told the guy I have 2. One on both sides of the cat. The way the code reads is HO2S Circuit High Voltage Bank 1 Sensor 2. We come to the conclusion it's the sensor after the cat? I told him fine because I have never replaced a sensor on this truck. He sold me a Bosch that is OEM. I could have bought one that you splice the wires...I said no way. Now what my question is that by replacing the sensor, does this solve my problem? The truck never misses a beat. Replaced plugs at 30,000 miles. Everything is stock. The guy at Auto Zone tells me that o2 sensors should last 70,000 miles, not the 50,000 I have. I would appreciate the help. Thanks.
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Well, if the code doesn't come back, then yes, your problem is solved :) Seriously, a code that refers to a post-cat sensor is almost always a fault with the sensor itself (unless the catalyst has broken down and blown out of the converter), so replacing it probably cured the problem.
Some last 70K, some last 200K... other last 5 minutes. Electronic parts are funny that way...
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Tom...does this truck have two or three sensors? I have only found two. I plan on replacing it after work today. We'll see how it goes.
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Four, actually... straight from the FSM:
3.9L/5.2L/Light Duty 5.9L Engine: Four sensors are used: 2 upstream (referred to as 1/1 and 2/1) and 2 downstream (referred to as 1/2 and 2/2). With this emission package, the right upstream sensor (2/1) is located in the right exhaust downpipe just before the mini-catalytic convertor. The left upstream sensor (1/1) is located in the left exhaust downpipe just before the mini-catalytic convertor. The right downstream sensor (2/2) is located in the right exhaust downpipe just after the mini-catalytic convertor, and before the main catalytic convertor. The left downstream sensor (1/2) is located in the left exhaust downpipe just after the mini-catalytic convertor, and before the main catalytic convertor.
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