TROUBLESHOOTING
TROUBLESHOOTING
16 • On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models
Error
Code
Indoor
or
Outdoor
Model?
Model
Type
Locate the
thermistor
by its
callout:
311
Indoor
or
Outdoor
540
See “Heat
Exchanger
Thermistor”
321
Indoor
or
Outdoor
240
340
540
See “Inlet
Thermistor,”
or 50.
331
Indoor
Or
Outdoor
540
See “Outlet
Thermistor,”
or 50.
341
Indoor
Only
240
340
540
See
“Exhaust
Thermistor,”
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.
Table 3:
Error
Code
Indoor
or
Outdoor
Model?
Model
Type
Locate the
thermistor
by its
callout:
311
Indoor
or
Outdoor
240
340
See “Outlet
Thermistor,”
or 50.
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 Table 4 for a list of
normal resistance values.
5. Clean the thermistor as follows:
a. Drain the unit of water. (See
“Unit Draining and Filter
Cleaning” on page 32.)
b. IF you are testing the
outlet thermistor, remove
the screw that secures the
computer board. Move the
computer board out of the
way without disconnecting
any of its wires. This will
Table 4: Thermistor Resistance Values
TEMPERATURE:
32 41 50 68 77 86 104 113
°C
0 5 10 15 20 25 30 35 40 45
RESISTANCE:
TEMPERATURE
THERMISTORS
23.76 15.43 12.56 10.28 8.47 7.02 5.85 4.12
RESISTANCE:
TEMPERATURE
EXHAUST
THERMISTOR
30.04 24.12 15.87 13.00 10.71 8.87 5.21
TEMPERATURE:
122 131 140 158 167 176 185 203 212
°C
50 55 60 65 70 75 80 85 100
RESISTANCE:
TEMPERATURE
THERMISTORS
2.53 2.16 1.86 1.60 1.21 1.05 0.81
RESISTANCE:
TEMPERATURE
EXHAUST
THERMISTOR
4.41 3.74 2.73 2.35 2.03 1.76 1.53 1.33 1.16 1.02
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