Outlet Thermistor Failure; Inlet Thermistor Failure - A.O. Smith ATI-140H-N Service Handbook

freely in the ceramic insulator.
See "Cleaning the Rod Assembly
(Flame Sensor and Air/ Fuel Ratio
Rod)" on page 28.
Unplug both ends of the wiring
harness from the igniter assembly
and control board, then reconnect
them to ensure good connection.
Also, verify that the green wire
that is part of the circuit is
attached to one of the screw
connections around the manifold.
6. Verify proper operation of the gas
solenoid valves. To do so, turn
the power off, then check the
resistance of the valves. A normal
resistance reading for SV1 and SV2
is 1.35-1.65 kΩ. Normal resistance
for SV3 is 2.07-2.53 kΩ. If the
resistance is out of range, replace
the gas valve. (Natural gas: part
320273-356, LP: part 320273-354.)
7. Check for blockages in venting,
such as bird nests, animals, or
trash.
Figure 5.

Outlet Thermistor Failure

Error Code: 311
See also "Wiring Diagram," page
22.
1. Check that the wire connections
are secure and free of burns or
cuts. The outlet thermistor is
embedded in the water outlet
connection. (See item 408 on page
70.)
2. Check the resistance value. See
page 15 for a list of normal
resistance values.
3. Clean the thermistor.
a. Drain the unit of water. (See
"Unit Draining and Filter
Cleaning" on page 24.)
b. Remove the computer
board to access the outlet
thermistor. The computer
is fixed in place by a single
screw at the top of the
board.
c. Remove the screw, then pull
out the sensor probe. Clean
the probe to a silver finish.
Do not lose the red rubber
High Limit Switch
Condensing Models: On-Demand Water Heater Service Handbook • 13
o-ring that wraps around
this sensor.
d. Replace and test.
4. If the error code persists, replace
the thermistor.
(Thermistor part no.: 319143-529).

Inlet Thermistor Failure

Error Code: 321
See also "Wiring Diagram," page
22.
1. Check that the wire connections
are secure and free of burns
or cuts. The inlet thermistor is
embedded in the water inlet
connection. (See item 407 on page
70.)
2. Check the resistance value. See
page 15 for a list of normal
resistance values.
3. Clean the thermistor.
3.1 Drain the unit of water. (See
"Unit Draining and Filter
Cleaning" on page 24.)
Clear Tube: Look behind
top, right-hand corner of
computer board assembly.
Figure 6.