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SUF SERVICE HANDBOOK
TROUBLESHOOTING
State Water Heaters Technical Training Department
Ashland City, Tennessee © 2004
23 STC-077
TEMPERATURE PROBE
STEP 6: CHECK RESISTANCE OF THERMISTOR IN UPPER AND LOWER TEMPERATURE
PROBES.
Condition:
• Disconnect CN3 and CN8 plugs from control board.
• On/Off switch is off.
• Multimeter is set to lowest ohms scale above expected resistance.
Check resistance between black wire connects on CN3 and CN8 plugs as shown in illustration
above.
STEP 6 TEMPERATURE PROBE RESISTANCE
IF… THEN
ohm reading does not approximately correspond to table at given
temperatures
replace temperature probe if thermistor is open.
shorted, or resistance value is off significantly.
ohm reading does approximately correspond to table at given
temperatures
continue to Step 7.
WATER TEMP
OHMS RESISTANCE
°C °F
3
21
40
70
26,435
11,974
37.5
49
100
120
5,862
3,780
54.5
60
130
140
3,066
2,503
71
82
160
180
1,698
1,177
PROBE PLUG
PROBE PLUG
TEMPERATURE
UPPER
LOWER
TEMPERATURE
NOTE: The upper and lower temperature probes contain
thermistors which are heat sensitive resistors. The control
board interprets changes in thermistor resistance as
changes in water temperature.
These thermistors are very reliable and should only be
replaced when:
The resistance test indicates an “open” (infinite resistance)
or a “direct short” (no resistance) circuit.
The nature of the service problem is temperature control
and the resistance readings are significantly off when com-
pared to the values in the table here at the given tempera-
ture.