2003 ST 4 Stroke 9.9/15HP Johnson outboards Service Manual, Page 144Get this manual
POWER UNIT CYLINDERCRANKSHAFTPISTON
Oil SeallO-Ring
Check: Oil seal O-ring Cut, nicked or damagedReplace
THERMOSTAT Inspect: Thermostat
Deposited saltClean Corrosion, wearReplace
Check: Operation
Thermostat valve operating temperature 5862C (136144F)
Thermometer
If out of specificationReplace
Check step:
1Suspend the thermostat in container filled with water by putting thread in its valve2Heat the water and read the temperature with the thermometer when the thermostat valve opens