What was found
LEFT ENGINE ANTI-ICE INTAKE ILLUMINATED INFLIGHT AFTER TAKEOFF. THE QRH WAS COMPLETED. THE LIGHT CYCLED ON AND OFF APPROXIMATELY EVERY 5 SECONDS OR SO. THE FLIGHT CREW RETURNED TO DTW WITHOUT FURTHER INCIDENT. RAN LT ENGINE WITH INTAKE HEAT ON FOR 30 MINUTES, COULD NOT DUPLICATE THE PROBLEM. AS PRECAUTION, REPLACED LT DE-ICE MODULE CONTROL WITH SERVICEABLE UNIT, S/N OFF A00171, S/N ON A00191, P/N ON 44730-501 PN OFF 44730-1.