The one that goes into the base of the ECU in the passenger footwell. Teh contacts are famous for getting mucky and giving a poor contact. Remove teh chip and clean the connections on both it and the ECU witha non abrasive cleaner.

On my FRST I made a couple of cardboard shims to go between the chip and the ECU to hold it all nicely and used elephant tape to hold the chip in place. Worked a treat for quite some time.