1996 Jeep Grand Cherokee Fuel System

P0138 - Oxygen Sensor Circuit High Voltage (Bank 1 Sensor 2)

How do you come up with a fuel pump problem? This code is usually caused by a wiring problem with the harness going to the rear oxygen sensor on the driver's side. Start by looking for corrosion on the connector terminals and if the harness fell down onto hot exhaust parts and two wires melted together. If you don't find anything obvious you'll need a scanner to view live data. Read the oxygen sensor's signal voltage which will typically be between 0.2 and 0.8 volts. If it is too high, unplug the sensor. If the voltage drops, the sensor is shorted. That is not very common. It's much more common to find the signal wire and 12 volt heater wire shorted together.

Yup. In this case a lean code and a high voltage code mean two different things. With a lean code the oxygen sensors are working and they're just reporting what they're seeing. That's where your fuel pressure could be an issue. The high voltage code means there's something wrong with the way the sensor is working which means a wiring problem about 90 percent of the time.

Also be aware only the front O2 sensors measure the way the engine is running and they should switch between rich and lean about two times per second. Your code is for the second sensor which is behind the catalytic converter. That one only monitors the efficiency of the converter meaning how well it stores oxygen during the lean pulses and mixes with the extra fuel during the rich times and burns it. If the converter is doing its job the rear sensor will switch between rich and lean perhaps once a minute. Low fuel pressure might trigger a lean code but that would just reference the side of the engine, not a particular sensor. A lean code won't be set by a rear sensor. He will set a converter efficiency code when it sees no change taking place in the composition of the exhaust gas. The way it knows that is by the switching rate between rich and lean. As the converter's efficiency decreases, less and less change takes place as the gases pass through it. The switching rate of the rear sensor will increase because it is matching the switching rate of the front sensor. The unburned gases going into the converter are the same as the gases going out so the sensors switch between rich and lean at the same rate. It's that rear sensor's switching rate that the computer looks at to determine when to set a converter efficiency code.

The high voltage code you got suggests the catalytic converter is most likely working fine but the sensor is unable to monitor it.

First we have to examine all the symptoms and clues. If the engine will not restart after stalling, check for loss of spark, especially if it will restart after cooling down for a half hour. That would be due to a failing crankshaft position sensor or camshaft position sensor. I have additional tests that will confirm that.

If a loss in fuel pressure is the cause it can manifest itself in different ways. I had a plugged pickup screen in my gas tank about ten years ago and the symptoms were it ran fine at highway speeds but stalled as soon as I started coasting. That's when the largest volume of fuel is pumped and it couldn't get through the screen. I've been driving around for over a year with a fuel pressure gauge clipped to my radio antenna so I could watch what was happening now. I just replaced that screen again two weeks ago. The symptom this time was it would spit and sputter at anything over 3/4 throttle or over 80 mph. The inability to exceed 80 mph equates with not being able to drag around my tandem axle enclosed trailer that's bigger than my '88 Grand Caravan I use to pull it. Both times the problems showed up consistently when it was hot out, as in over 80 degrees. I never lost spark and that was proven by the fact I could keep the engine running by playing with the gas pedal to get the fuel pressure back up.

Running rough and stalling can be a result of a spark problem that gets worse or a fuel problem that gets worse under certain conditions. You can use a fuel pressure gauge like I did but for everything else you need a scanner to view live data to see what's going on. Also check for diagnostic fault codes. The fuel supply system is not monitored so there won't be any codes if that is causing the stalling. Most scanners have a record function that lets you take a recording of the sensor data when the stalling occurs. Because that data passes through the scanner's memory, the recording actually starts a few seconds before you press the button.

One last thing we mustn't overlook is did you recently disconnect the battery or let it run dead? If so, "minimum throttle" will need to be relearned. Until then the idle speed will be too low and that could cause misfiring. The clue is the engine will be hard to start and will tend to stall at stop signs unless you hold the gas pedal down 1/4".

Dandy news. Happy to hear it's solved. I can share a little info related to fuel pressure. I was fighting a problem on my '88 Grand Caravan daily driver until a couple of weeks ago. It started over a year ago when it would cut out at anything over 3/4 throttle, but only on really hot days, (of which we get about two a year here), and while pulling a tandem axle enclosed trailer that's bigger than the van. I ran with a fuel pressure gauge clipped to the radio antenna for a year and noticed when pulling a load that the pressure would drop as low as 20 psi and the engine would still run fine. Normal is around 45 to 50 psi. At 15 psi it would finally start to sputter and cut out but the pressure would pop back up if I lifted the accelerator for a just a fraction of a second, and would be okay again for up to half a mile.

The cause, which I had guessed correctly based on a different but similar set of symptoms ten years ago, was a plugged strainer clipped to the fuel pickup. A new one cost 12 bucks. I was unable to exceed 80 mph which equated to not being able to drag my trailer around. Now I have no trouble with the trailer and I won't tell you how fast I can go with no sputtering.

Newer vehicles seem to be more sensitive to proper fuel pressure. I was surprised to see mine still running with no symptoms at 20 psi. I didn't have a scanner connected to see if it was running lean, but at this point I don't care. I know I can buzz off into the sunset now at will.