The problem child, a dirty 1998 Jeep Grand Cherokee 5.9 Limited with 166k miles:
The victim, me. I had started the Jeep about a week ago and noticed that the MIL (Malfunction Indicator Lamp), commonly referred to as a check engine light, was on. It wasn't running any differently that I could tell (key words there, "That I could tell), but it seemed to be burning through fuel more quickly, so I needed to find out what was going on. So, I drove it to work and checked the code.
P0137: o2 Sensor Circuit Low Voltage Bank 1 Sensor 2.
Taken from fixya.com:
"Essentially the same as P0136, P0137 refers to the second oxygen sensor on Bank 1. P0137 means the O2 oxygen sensor's voltage remained low for longer than 2 minutes. This, is interpreted by the ECM as a low voltage condition and sets the MIL. Bank 1 Sensor 2 is located to the rear of the catalytic converter and should produce an output signal relative to oxygen storage capacity of the catalytic converter. This rear (sensor 2) sensor is less active than the signal produced by the front sensor. However, if the ECM senses the sensor is inactive, this code will set.
Symptoms: There may be no visible symptoms to the driver, other than the MIL (Check Engine / Service Engine Soon) illumination.
Causes: A code P0137 may mean that one or more of the following has happened:
* Faulty o2 sensor Exhaust leak near the rear sensor
* Plugged catalyst
* Short to voltage on O2 signal circuit
* High resistance or open on O2 signal circuit
* Replace faulty sensor
* Repair exhaust leak near the rear sensor
* Check for restriction in catalyst and replace as necessary
* Repair short, open, or high resistance on o2 signal circuit"
Now, there's a specific flowchart of diagnosis that's supposed to be done to determine why this code was thrown, but I decided to get under the truck and just have a look-see.
^^ This picture was taken AFTER I found the problem, but see that wiring harness for the oxygen sensor behind the driveshaft? The old one was laying on the tailshaft coming out of the transfer case. A plastic clip that holds it up and out of the way had broken, so the harness fell onto the tailshaft and upon further inspection I found this:
The good news for me, was that I could skip all the pinpoint diagnosis needed to find a fault in the oxygen sensor circuit, and that the problem wires were part of the oxygen sensor and not the vehicle harness. I ordered up a new downstream oxygen sensor, installed it, cleared the code, and went on my merry way.
This just proves a point that a visual inspection can yield results, so if you know what to look for, or at least know WHERE to look, it can save you some headaches.