Condensing Models: On-Demand Water Heater Service Handbook • 17
TR
OUBLE
SHOO
TING
on, this error code will result. Turn
off power before disconnecting the
remote from the heater.
3. Check for any signs of power
surges.
4. Check the voltage across the wires
coming to the remote. Normal
voltage should be 10-25 VDC. If
within normal range, replace the
remote, part 9009069005. If the
voltage is out of range, replace the
computer board, part 320273-369.
Temperature Controller
Problem
Error Code: 751
This code is only for the built in
controller on the indoor models.
(TM-RE41 is printed on the front,
lower portion of the controller.) This
error code only occurs when there
is a miscommunication between the
temperature controller (320273-
684) and water heater. This code is
not for the remote controller (TM-
RE40/9008172005).
1. Check that all wiring is properly
connected to the temperature
controller (320273-684).
2. Check for any signs of power
surges.
3. Check the voltage across the wires
that are connected to the remote.
Normal voltage should be 11-
25 VDC. If within normal range,
replace the temperature controller,
part 320273-684. If the voltage is
out of range, replace the computer
board, part 320273-369.
Abnormal Exhaust Temperature
Error Code: 941
This code indicates that the exhaust
temperature exceeded limits and that
the heater was not able to reduce
the exhaust temperature, forcing the
exhaust high limit switch to trip and
shutdown the burners.
1. Check for excessive vent length and
for any blockage in the intake air
and/or exhaust.
2. Check for dust or lint in the
burner and heat exchanger in
contaminated installation areas.
3. Check the manifold pressure. See
page 25.
4. Check the incoming water
temperature. If it is above 140°F,
the exhaust temperature may be
too high. Reduce the supply water
temperature.
5. Check the resistance of the
thermistor. Refer to “Exhaust
Thermistor Failure,” page 14.
Abnormal Combus on
Error Code: 991
See also “Error Code: 101.”
IMPORTANT:
Familiarize yourself with
the components before attempting these
procedures. Use the Installation Manual
and reference the component schematic
on the last few pages of the manual.
Safety first! These procedures must be
performed by licensed, qualified service
professionals only. A.O. Smith is not
liable for the actions of the technician if
he/she fails to adhere to any or all safety
procedures, specifically in regards to
electrical and gas safety.
1. Verify the gas type of the heater
and DIP switch position. The
manifold plate will have a stamp on
it to indicate the gas type.
NG: NG 88A
LP: LPG 88B
2. Verify the DIP switch settings.
2.1
Are the DIP switches set for
the correct vent size and
length? Refer to page 19
for correct se ngs. Each
90-degree elbow is equal to
5 feet of straight pipe.
2.1
Is the heater set to the
correct eleva on? Refer to
page 20 for the correct
eleva on DIP switch se ngs.
Compare this to the information on
the water heater’s rating plate.
Is the gas supply pressure within
speci
fi
ed limits when the heater is
in standby and when it is running?
If the pressure di
ff
erence between
standby and maximum
fi
ring is greater
than 1 inch W.C., the supply line and/
or regulator may be undersized.
NOTE: In Canada, do not exceed the
maximum allowable pressure drop
permi ed by B149.1.
Verify that the gas lines are sized
for the correct volume of gas.
Refer to the installa on manual or
the current edi on of the Na onal
Fuel Gas Code, NFPA 54 (U.S.) or
Natural Gas and Propane Code,
B149.1 (Canada).
3. Check for obstructions in the vent
or intake air lines. Clear them if any
are found.
4. Verify that the terminations meet
the minimum required clearances
that are outlined in the installation
manual and local codes.
5. Check for blockages or
contaminants inside the heater
cabinet, fan, burner, and around
heat exchanger. If the fan is unable
to push sufficient air, then the
