From TSB
P0136 - HO2S Sensor Circuit Malfunction (HO2S-12) The downstream HO2S sensor(s) are continuously checked for maximum and minimum voltages. The code will set when the voltages fail to meet the calibrated limits.
�· Disconnected sensor
�· Pinched, shorted, corroded wiring or pins
�· Crossed sensor wires
�· Exhaust leaks
�· Contaminated or damaged sensor
�· Chafed/damaged wiring
http://www.v8sho.com/SHO/TSB0197HO2SServiceTips.htm 
