1999 "EE" Evinrude 200, 225 V6 FFI Outboards Service Manual, P/N 787025, Page 59Get this manual
External ECU Sensors [}] Air Temperature SensorThe air temperature sensor monitors the temperature of air entering the air silencerThe AT sensor is Positive Temperature Coefficient (PTC) thermistor, resistor whose resistance changes with temperature and alters voltage values accordinglyWhen temperature increases, both resistance and voltage also increaseWhen temperature decreases, resistance and voltage likewise decreaseThe sensor receives voltage signal from the ECU, and another wire provides ground circuit back to the ECUThe ECU uses this signal to adjust the airfuel ratio in accordance with changes in incoming air temperaturesIf sensor values are out of limits, or the sensor or its circuit fails, the ECU will store service code and turn on the "CHECK ENGINE" lightDRC44570
Water Temperature SwitchFFI engines have water temperature switch located in the starboard cylinder headIt threads into seat in the water passage of the head, but does not actually contact waterIt monitors water temperature to protect against an engine overheatIts operation differs somewhat from the water temperature sensor in that it's an onoff switch, not thermistorIt receives voltage signal from the ECU, and if powerhead temperature exceeds switch limits, the circuit groundsThe ECU will initiate S.L.OW store service code, and turn on the "WATER TEMP" light