14 • Condensing Models: On-Demand Water Heater Service Handbook
TR
OUBLE
SHOO
TING
3.2
Remove the computer
board to access the inlet
thermistor. The computer
is
fi
xed in place by a single
screw at the top of the
board.
3.3
Remove the screw, pull out
the sensor probe. Clean the
probe to a silver
fi
nish. Do
not lose the rubber o-ring
that wraps around this
sensor.
3.4
Replace and test.
4. If the error code persists, replace
the thermistor.
(Thermistor part no. 319143-214.)
Exhaust Thermistor Failure
Error Code: 341
This code will only appear on the
indoor models.
1. Check the wire connections to the
exhaust thermistor. They must
be secure and free of burns or
cuts. The exhaust thermistor is
located at the top, center of the
unit, above the secondary heat
exchanger.
2. Check the resistance of the
thermistor. See Table 3 and Table
4 for lists of normal resistance
values.
3. Clean the thermistor.
3.1
Remove the screw holding
the thermistor plate, then
pull out the sensor probe.
3.2
Clean the probe to a silver
fi
nish. Do not lose the white
gasket that seals this sensor.
3.3
Replace and test.
4. If the error code persists, replace
the thermistor. (Thermistor part
nos: 319143-131 and 319143-111.)
Air-Fuel Ra o Rod Failure
Error Code: 391
1. Ensure that the unit is plugged into
a socket that is properly grounded.
A floating ground can cause this
error.
1. Check that the wire connections
are secure and free of burns or
cuts.
2. Check that the clear plastic tube
connecting the proportional gas
valve to the combustion chamber
is attached. See Figure 6.
3. Clean the air-fuel ratio rod (flame
rod). See “Cleaning the Rod
Assembly (Flame Sensor and Air/
Fuel Ratio Rod)” on page 28.
Abnormal Main/Solenoid
Gas Valve
Error Code: 510
Driving circuit fault for the main or
solenoid gas valves is not normal.
1. If the hot water is shut off, but
flames are still visible through the
burner sight window, immediately
shut off gas and power to the unit
and contact the technical service
department.
2. Check the wire connections to the
main gas solenoid valve for burns
or cuts.
3. If the unit was recently rebuilt,
make sure that each gas valve wire
is plugged into the proper place.
(For each connector, the number
stamped on the metal must match
the number that is printed on the
wire’s label. See Figure 7.)
4. Verify that the supply gas pressure
is within the specified limits. Too
high of an inlet gas pressure may
cause the main gas valve to jam or
could be damaged.
5. Check the voltage across the main
gas valve wires (blue - light blue
#3) during operation. The voltage
reading should be 93-120 VDC. If
the voltage is not within normal
range, replace the computer
board, part number 320273-369.
6. With the power off, check the
resistance of the valve. Normal
resistance reading is 1.35-1.65 k
Ω
.
If the resistance is out of range,
replace the gas valve, NG: part
320273-356, LP: part 320273-354.
Abnormal Solenoid Gas Valve
Error Code: 551
Driving circuit fault for the solenoid
gas valves is not normal.
1. If the hot water is shut off, but
flames are still visible through
the sight glass, immediately shut
off gas and power to the unit
and contact the technical service
department.
2. Check the wire connections to the
gas solenoid valves for burns or
cuts.
Solenoid
Connector
Numbered
Wire
Stamped
Number
In each case, these numbers
must match.
Figure 7. Gas Valve Wiring
