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Code 311, 321, 331, 341 or 351:
Thermistor Failure
The system displays this code when it
detects a thermistor failure.
1. Turn off power to the water heater.
2. Identify the row in Table 3 that
applies to you, then follow its
instructions.
3. After you have located the correct
thermistor, verify that the wire
connections are secure and free of
burns or cuts.
4. Check the thermistor's resistance
value. See Figure 4 for a list of
normal resistance values. Run
water through the heater for
several minutes to get all the
thermistors in approximately the
same temperature range.
5. Clean the thermistor as follows:
35.00
30.04
30.00
24.12
25.00
23.76
19.50
20.00
19.08
15.00
15.43
12.56
10.00
5.00
0.00
32°F 41°F 50°F 59°F 68°F 77°F 86°F 95°F 104°F 113°F 122°F 131°F 140°F 149°F 158°F 167°F 176°F 185°F 194°F 203°F 212°F
0°C
5°C
10°C 15°C 20°C 25°C 30°C 35°C 40°C 45°C 50°C 55°C 60°C 65°C 70°C 75°C 80°C 85°C 90°C 95°C 100°C
Figure 4: Temperature vs Resistance Graph
18
•
On-Demand Water Heater Service Handbook for X3P Models
a.
Drain the unit of water. (See
"Unit Draining and Filter
Cleaning" on p. 29.)
b.
Remove the screw or clip
that secures the thermistor,
then pull the sensor probe
out by grabbing the metal
base. Do not pull the wire
as you may cause the wire to
separate from the probe.
c.
Clean the probe to a silver
finish. Do not lose the red
rubber o-ring that wraps
around this sensor.
d.
Reinstall the thermistor.
6. Return the water heater to service.
7. If the error code persists, replace
the thermistor.
Temperature vs Resistance
15.87
13.00
10.71
8.87
10.28
7.39
8.47
6.19
7.02
5.85
4.90
Exhaust Thermistor
5.21
4.41 3.74
3.19 2.73
2.35 2.03 1.76
4.12
3.49
2.96 2.53 2.16
1.86 1.60 1.39 1.21 1.05 0.92
Temperature Thermistor
1.53
1.33
1.16 1.02
0.81
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