There's couple of reason why you got that code but it should mean that something wrong with the upstream 02 sensor.
You can try replacing it yourself, or have a mechanic put a scanner on your PT and pinpoint the exact problem. This is from PT Do-It-Yourself.net ....
P0138 -1/2 O2 Sensor Shorted To Voltage - Oxygen sensor input voltage maintained above normal operating range.
Possible causes:
Intermittent Condition
O2 Sensor Operation
O2 Sensor Signal Shorted To Voltage
O2 Sensor Signal Open
O2 Sensor Ground Circuit Open
PCM
1/2= O2 sensor before the cat.
2/2= O2 sensor after the cat.