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.
What is it?
The oxygen sensor is part of your car’s emission system which keeps the engine running cleanly and efficiently in all sorts of operating conditions. If your car’s engine isn’t performing up to par or the “Check Engine” light goes on, have it inspected immediately. Failure to do so can reduce your mileage per gallon of fuel or cause your vehicle to pollute. Every new car, and most cars produced after 1980, have an oxygen sensor. The sensor is part of the emissions control system and feeds data to the engine management computer. The goal of the sensor is to help the engine run as efficiently as possible and also to produce as few emissions as possible.
Automotive oxygen sensors make modern electronic fuel injection and emission control possible. They help determine, in real time, if the air fuel ratio of a combustion engine is rich or lean. Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine. But when information from oxygen sensors is coupled with information from other sources, it can be used to indirectly determine the air-to-fuel ratio. Volvo was the first automobile manufacturer to employ this technology in the late 1970s, along with the 3-way catalyst used in the catalytic converter.
The sensor does not actually measure oxygen concentration, but rather the amount of oxygen needed to completely oxidize any remaining combustibles in the exhaust gas. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess.
The reason why the engine needs the oxygen sensor is because the amount of oxygen that the engine can pull in depends on all sorts of things, such as the altitude, the temperature of the air, the temperature of the engine, the barometric pressure, the load on the engine, etc.
When the oxygen sensor fails, the computer can no longer sense the air/fuel ratio, so it ends up guessing. Your car performs poorly and uses more fuel than it needs to.
Most automobile manufacture suggests replacing the sensor every 100,000 miles. If you have over 100,000 miles on your vehicle and have never had the oxygen sensor replace now is a great time. Certainly this will help improve your gas mileage. Call us today for an appointment.