What was found
(CAN) AS THE CREW WAS ON TAKEOFF ROLL DOWN THE RUNWAY AND THE POWER LEVERS WERE ADVANCED TO FULL POWER, THEY RECEIVED A RT ENVIRONMENTAL FAIL INDICATION LIGHT. THEY REJECTED THE TAKEOFF AND RETURNED TO THE GATE. MAINTENANCE WAS ADVISED AND UPON FURTHER INVESTIGATION IT WAS DETERMINED THAT THE PRE-COOLER BYPASS VALVE HAD FAILED WHICH GENERATED THE LIGHT. THE VALVE WAS REPLACED AND THE AIRCRAFT GROUND RUN WITH THE SYSTEM TESTING SERVICEABLE.