I recently had a check engine light pop on and I retrieved the codes, the first one is P0138 which is O2 sensor circuit high voltage (bank 1 sensor 2). What could be the possible cause for this code? The second code is P0740 which is torque converter clutch circuit malfunction. Again what could be the reason this code is showing? This is a 99 Ram 2500 4x4 with about 63,000 miles on it and I take really good care of it and basically only go offroad to hunt which I have been lately but the truck is in awesome shape. If anyone could help me out here I would really appricate it...............

 

P0138 - Bank 1 Sensor 2 is your left hand downsteam sensor. (FYI, bank 1 is left, bank 2 is right; sensor 1 is upstream, sensor 2 is downstream). Anyways, it could be the wiring has a problem with it, or, most likely, the 02 sensor is bad.

P0740 - Basically, the 2001 Ram service manual puts it nicely, "Relationship between engine and vehicle speeds indicated failure of torque convertor clutch lock-up system." After sending the signal to engage the torque convertor, the PCM likes to see a certain engine RPM drop. Maybe caused by a bad torque convertor solenoid.