I am working on a 2000 Corvette with a tune and long tube street headers that was all done around 3 years ago by someone else. The car came in with 2 DTC's (P0134 &P0154) for insuficient switching on both front O2 sensors that set a check engine light. The car had what appeared to be Bosch rear sensors installed in front and no rear sensors installed. After talking to the header manufacturer and doing the wiring harness checks I installed 2 new AC Delco rear sensors in front bungs per the manufactures recommendation. Upon start up the front O2 sensors are still reading between 450-475mV and the car will not go into closed loop operation and stays open loop while monitoring with a Tech 2. During a test drive the O2 readings went to around 800mV with an occasional switch to 300mV and stayed operating open loop. The MAF, IAT, MAP, and ECT readings are all correct for the operating conditions. I read the tune with HP Tuners and all of the table values appear to be correct and the car should be going into closed loop operation. My next step is going to be trying a Bosch sensor to see if I can get the car to go into closed loop and the 2 DTC's to stay cleared. Has anyone ran into a problem similar to this. Thanks Rex