Samsung ML-651x series Service Manual page 226

6. Measure the thermostat resistance. Check if the thermostat is opened.
Although the thermostat is opened, check the following steps. And then replace the fuser unit.
7. Remove the FDB cover. Check if the FDB Triac is short.
Check the resistance among 3 legs of both transistors. If it is short, replace the FDB.
8. If there is no problem for above steps, repalce the FDB, engine board step by step. If the fuser unit is
defective, replace it.
Service Manual
ML-551x / 651x series
4. Alignment and Troubleshooting
4-131
Samsung Electronics