P0138 code in 99xj upstream or downstream?
 

So the other day I changed out the water pump, new thermostat and coolant flush and the temps are back to withing normal. So today on my way to work the CEL comes on. I was heading to work a little bit early so I had a few mins to spare and pulled into Advanced Auto. Code 0138 came up and stupid me forgot to ask the full details of the description. Can't remember if it was upstream or downstream. My mpg has dropped quite a bit and my driving habits are still the same. Is this the one after the cat or before? NTK is recommended but what is the part #? Thanks!

P0138 is downstream I believe. NAPA part number is 23099 for an NTK unit (OE Manufacturer)

Yup. P0138 is the downstream sensor.

alright cool, thanks guys I just ordered it from rockauto. thanks again!

I thought the upstream O2 is the one that affected mpg, no?

both do similar jobs and work in conjunction to make the engine run as efficiently as possible so yes they both affect gas mileage. the upstream does affect it more than the downstream though.

P0138 is definitely downstream.

On OBDII, the upstream is the primary input for determining fuel/air ratio, but I did read a technical paper from Chrysler where they stated that some (minimal) feedback input from the downstream is involved as well.

Regardless, start with replacing the downstream for a P0138. But why not replace the UPSTREAM too? If one oxygen sensor is failing, the other assuredly is not far behind.

So what does it mean when it (FSM) says the same thing for both P0137 and P0138......"Downstream and Pre-Catalyst O2S Shorted to Ground"? Downstream sounds like after cat and Pre-Catalyst sounds like upstream.:confused1:

Here's how to be sure:

Does the rig burn unbelievable amounts of fuel and idle like a cammed out camaro?
YES: Upstream
NO: Downstream

:thumbup:

Here's some diagnostic data pertaining to P0137 and P0138:

P0137-1/2 O2 SENSOR SHORTED TO GROUND

The oxygen sensor signal input voltage to PCM is below 0.156 volts for 28 seconds after starting engine.

Possible causes:

O2 SENSOR OPERATION
O2 SENSOR SIGNAL CIRCUIT SHORTED TO GROUND
O2 SENSOR SIGNAL CIRCUIT SHORTED TO SENSOR GROUND CIRCUIT

P0138-1/2 O2 SENSOR SHORTED TO VOLTAGE

The oxygen sensor input voltage to PCM is above 1.5 volts.

Possible causes:

O2 SENSOR OPERATION
O2 SENSOR SIGNAL SHORTED TO VOLTAGE
O2 SENSOR SIGNAL CIRCUIT SHORTED TO O2 HEATER CIRCUIT
O2 SENSOR SIGNAL OPEN
O2 SENSOR GROUND CIRCUIT OPEN
PCM

Here's some FYI data from DogeRam.org about O2S Goal Voltage:

Upstream Oxygen Sensor Goal Voltage
From Star Center News #5, February 2000 - tests for those who have access to OBD II or OBD III code readers.
Since the introduction of OBDII with both upstream and downstream O2 sensors, the downstream O2 sensor has been used to evaluate catalytic converter operation and to control converter efficiency.
When looking at the upstream goal voltage, it should fluctuate above and below the normal center (.4 to .6 volts) based upon what the downstream O2 sensor is reading. The vehicle should be driven through all speed ranges when checking goal voltage. If the goal stays high or low all the time, a thorough check of the downstream oxygen sensor should be made. Be sure you check the sensor, wiring harness, connectors, catalytic converter, and inspect for exhaust leaks.
Although a MIL lamp may not be illuminated, there are instances when the downstream O2 sensor goal voltage is out of range, causing driveability problems or low fuel economy.
If the downstream O2 sensor tends to stay below center all the time, the PCM will adjust the goal voltage higher in an effort to correct the low O2 readings of the downstream sensor. This can result in low fuel mileage.
If the downstream O2 sensor has a tendancy to stay above center, the PCM will set a low goal voltage which results in a lean fuel mix which can cause the engine to run hotter and aggravate a spark knock condition.
Sometimes a service technician will reset the PCM and take it for a test drive. The PCM does a recalibration after the reset, and the probelm may go away until the PCM has completed a relearn of the system and then begins to attempt to correct for the downstream sensor error. If the symptoms reappear after several days of driving after a service is performed, check the downstream O2 sensor, the exhaust system, the wiring, and the catalytic converter. Note: a worn out catalytic converter can produce symptoms similar to a bad downstream O2 sensor.