The PO138 is an O2 issue (shorted to Voltage)
the PO171 is the lean mixture.
The sensor is where we need to start. First, this is a heated O2 sensor (HO2S). I need to know if this is a grand cherokee or one of the original body types so I can identify wiring schematics. What you need is a scanner that can read real time information. You will start the engine, allow it to warm up and then check to see the voltage that is produced by the HO2 sensor at an idle. If the voltage is 4.97 volts or less, replace the sensor. If it is more, we need to check for a short. NOTE: The wiring harness or wiring to the HO2S can be damaged, melted, corroded. And causing the problem. This is the 1/2 HO2S. Let me know which model vehicle you have and I can tell you where the sensor will be located. Also, let me know if you have access to a live data scanner.
Saturday, March 5th, 2011 AT 5:56 AM