It comes on when engine hits 1500RPMs and turns off at idle. I went to Autozone but they couldnt read it.

Recently rebuilt heads and replaces front two o2 sensors. Any ideas on why this would occur? How to fix it?

 

Would be able to help you more if you told us the year, model, and engine of the car you are dealing with.

 

95 Z28 stock 5.7

 

Most likely Autozone does not have the proper equipment to scan your car or does not how to do it. To be scanned, your car needs an OBDI scanner with an OBDII cable.

 

some can read it, some cant. its been hit and miss for me with my 95 blazer (obd2 compliant, obd1 running system). some places can read it, some cant. try different places to see if they can work. if not, stop at a dealership and ask to have the code run. we do it for free at toyota