Takagi 100 Series, Service Handbook

Page: 17 / 80

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On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models • 17
allow you to access the
outlet thermistor.
c. Remove the screw or clip
that secures the thermistor,
then pull the sensor probe
out.
d. Clean the probe to a silver
finish. Do not lose the red
rubber o-ring that wraps
around this sensor.
e. Reinstall the thermistor.
IF you cleaned the outlet
thermistor, reinstall the
computer board.
6. Return the water heater to service.
7. If the error code persists, replace
the thermistor.
Code 391: Air-Fuel Ratio Rod
Failure
The system displays this code if no
flame is detected by the Air-Fuel Ratio
(AFR) rod.
1. Ensure that the unit is plugged into
a socket that is properly grounded.
A floating ground can cause this
error.
2. Check that the wire connections
are secure and free of burns or
cuts. Verify that the green ground
wire is installed properly and is
making an electrical connection.
This ground wire is part of the AFR
and Flame Rod circuit. See “Wiring
Diagrams” on page 30.
3. Check that the clear plastic tube
connecting the proportional gas
valve to the combustion chamber is
attached. (See page 49 or 50.)
4. Clean the air-fuel ratio rod (flame
rod). See “Cleaning the Rod
Assembly (Flame Sensor and Air/
Fuel Ratio Rod)” on page 36.
Code 441: Flow Sensor Failure
(Easy-Link or Multi-Link System
Only)
The system displays this code when it
detects a flow sensor failure.
1. Disconnect the water heater
from the linked system before
you advance to step 2. The water
heater must be tested as an
individual unit.
2. Ensure that the water supply is
turned on.
3. Check the flow sensor/control
valve wires to see if they are
broken or disconnected. Refer to
Figure 41 (p. 49) or Figure 42 (p.
50).
4. Verify the flow rate using the
water heater’s controller. To do
so, select Flow Rate by pushing
the INFO button until the arrow
on the left side of the display
points toward “Flow.” Ensure that
the controller displays a flow rate
during operation. If a flow rate is
not indicated, proceed to the next
step.
5. Check the inline water filter for
debris.
6. Turn off the gas supply and
electrical power to the heater, then
remove the flow sensor/control
valve (p. 51). Check the flow
sensor as described in
“Checking the Flow Sensor,”
page 36. Clean or replace as
necessary.
Code 510: Abnormal Main Gas
Solenoid Valve (MV)
This code indicates a fault in the main
solenoid gas valve (MV) circuit.
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 serviced recently
and the gas valve had to be
removed, 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 6.)
4. Verify that the gas supply pressure
is within the specified limits. An
inlet gas pressure that is too high
can jam or damage the main gas
valve.
5. With the power off, check the
resistance of the valve. Normal
resistance reading is 1.35-1.65 kΩ.
If the resistance is out of range,
Figure 6
.
Solenoid
(locations vary)
Connector
Numbered
Wire
Stamped
Number
In each case, these numbers
must match.
Gas Valve Wiring