5742637-YIM-D-0620
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Johnson Controls Ducted Systems
Gas Heat Troubleshooting Guide
Before beginning symptomatic troubleshooting activities read
the flash code LEDs on the unit control board (UCB) and the
ignition control board (ICB). Fault codes have a quick sequence
of flashes indicating the flash code number followed by a pause
with the LED off. The ICB flash codes are repeated until the
fault is cleared. The ICB monitors itself, the centrifugal switch,
lockout due to > 16 pilot flame losses, primary limit, roll-out and
a flame present when the ICB expects no flame.
With power applied to the unit, if the LED on the ICB is flashing
the heartbeat and the furnace will not operate, then proceed to
the symptomatic troubleshooting section. If the ICB LED is not
flashing, then perform the ICB troubleshooting procedures. If the
ICB has a flash code other than the heartbeat, then determine
the flash code and locate its troubleshooting procedures in the
flash code troubleshooting section. Refer to Table 32 for flash
code identification and component causing fault.
Flash Code Troubleshooting
Power to the unit should be interrupted during the
troubleshooting of individual components unless otherwise
indicated. All troubleshooting procedures assume the unit is
wired per the wiring diagram. If there is any indication the unit
has been previously repaired, then the first priority is to verify
that the furnace is wired per the wiring diagram. Miss-wired
units will give false flash codes.
Ignition Control Board
The ICB controls the ignition of the pilot, the opening and
closing of the gas valves and the operation of the draft (inducer)
motor. It also monitors all the furnace safety components.
If the ICB LED is on steady, then verify the wiring of the unit to
the wiring diagram and if OK, then replace the ICB. If the unit
has power and the ICB LED is not flashing, then remove the 3
pin connector (single stage gas heat) from the ICB. Measure
the control voltage between terminals 1 and 2 of the wiring
harness connector. The control voltage must be between 18
and 30 volts. If control voltage is not present, then check the
3.2A circuit breaker to verify that the circuit breaker has control
voltage on both input and output terminals. If control voltage is
present on the input and not the output of the circuit breaker,
then reset or replace the circuit breaker as necessary. If control
voltage is present on both sides of the circuit breaker, then
check the power and common wires between the unit control
box and the ICB and repair as necessary. If the control voltage
is present, then verify the cleanliness of the harness and the
ICB connector, clean if necessary and reconnect the wiring
harness to the ICB and observe ICB LED. If it now flashes the
heartbeat, then the fault was a bad connection between the
harness connector and the ICB connector or a broken wire
exists in the harness. With the harness connected to the ICB,
gently move the wires in the harness while observing the ICB
LED. If heartbeat is steady, then verify proper operation of the
furnace. If the ICB does not flash the heartbeat, then gently
move the wires in the harness connector while observing the
ICB LED. If you get any flashes of the ICB LED, then there is
still a bad connection or a broken wire. If no flashes are seen
while gently moving the wires with the harness connected to the
ICB, then replace the ICB and verify proper operation of the
furnace.
Centrifugal Switch
The centrifugal switch is an integral part of the draft motor. The
centrifugal switch closes when the motor speed increases to
~2500 rpm and opens when the motor speed descends to
~2000 rpm.
If a flash code indicates the centrifugal switch is causing a fault,
then
1.
If the flash code indicates the switch is open with the draft
(inducer) motor on (flash code 2), then
a. Disconnect power to the unit. Using a screw driver, spin
the draft motor blower wheel. If bound or dragging, then
visually inspect the draft motor blower wheel area for
debris. If debris is present, then clear debris and verify
proper furnace operation. If clear, then replace the draft
motor and verify proper furnace operation.
b. Restore power to unit and induce a call for heat.
Measure the voltage across the two draft motor leads
Troubleshooting of components may require opening
the electrical control box with the power connected to
the unit.
Use extreme care when working with live
circuits!
Check the unit nameplate for the correct line
voltage and set the voltmeter to the correct range before
making any connections with line terminals.
When not necessary, shut off all electric power to the
unit prior to any of the following maintenance
procedures so as to prevent personal injury.
Label all wires prior to disconnection when servicing
controls. Wiring errors can cause improper and
dangerous operation, which could cause injury to person
and/or damage unit components. Verify proper
operation after servicing.
The furnace may shut down on a high temperature
condition during the procedure. If this occurs, the UCB
energize the supply air blower motor until the high
temperature limit has reset. Caution should be used at
all times as the supply air blower may energize
regardless of the room thermostat fan switch position.