ECU code 6 - circuit testing

i recently had a 97 jdm type-r put into my 94 gs-r. everything seems to be okay except it's throwing a code 6 (ECT sensor), and therefore in limp mode. i changed the ECT sensor but the code's still there. can anyone tell me how to check the circuit to the sensor? does that mean just checking the voltage, because if so, they check out fine. any ideas on what might be causing this? could air in the coolant lines do it? please help!!!!