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I-UBX-UBZ-UDX-UDZ (03-21) 1034344-0
MAINTENANCE—CONTINUED
Maintenance Procedures—Continued
Flame Rollout Switch Maintenance (Model UDZ Unit Sizes 030–125 Only)
• The cause of a flame rollout switch (see Figure 20) activating must be determined. See Figure 12 for flame rollout
switch location. Activation of the manually-reset flame rollout switch could be caused by one or more of the following:
• Restricted or plugged heat exchanger
• Too much building exhaust
• Manifold gas pressure too high
• Restricted combustion air inlet or exhaust outlet in combination with defective pressure switch
• Electrical power interruption during operation
• Unit being operated with line voltage disconnect (24V thermostat is required)
• If a flame rollout switch trips, inspect the burner/control compartment for signs of excessive heat and burned wiring.
If there is damage to the control compartment, repairs must be made before resetting the switch.
• If the compartment appears normal, reset by depressing the red button on the switch (15 to 20 minutes are required
for the switch to cool sufficiently before resetting). A distinct click will be felt when the switch resets. Operate the
furnace. If the flame rollout switch trips again, determine and correct the cause before resetting the switch.
• If it is determined that the flame rollout switch needs replacing, use only the factory-authorized replacement part
that is designed for that size of heater. The disconnect switch is located in the sealed electrical box inside the
control compartment with the toggle on the rear of the heater.
Interlock Door Switch Maintenance (Models UBZ and UDZ Only)
If it is determined that the interlock door switch (see Figure 20) needs replacing, use only a factory-authorized
replacement part that is designed for the heater. For the approximate door switch location, see Figure 12.
Transformer Maintenance
Use a voltmeter to verify that there are 24V output from the transformer (see Figure 20). If the transformer is not
functioning, it must be replaced. Use a replacement transformer identical to the factory-installed model. For the
transformer location, see Figure 12.
Disconnect Switch Replacement (Models UBZ and UDZ Only)
If it is determined that the disconnect switch (see Figure 20) needs replacing, use only the factory-authorized
replacement part that is designed for the heater. Always replace the electrical box cover.
Vent or Vent/Combustion Air System Maintenance
Check the complete system at least once a year. Inspection should include all joints, seams, concentric adapter box
(models UBZ and UDZ), inlet air guard or inlet air cap (models UBZ and UDZ), and the vent terminal cap. Clean all
openings and replace any defective parts.
TROUBLESHOOTING
Unit Troubleshooting Using DSI Control Module
The seven-segment display on the DSI control module (refer to Circuit Board (DSI Control Module) section) may
be used to troubleshoot the unit. The control module monitors the operation of the heater, and the display indicates
normal operation and various abnormal conditions. If the heater fails to operate properly, check this display to determine
the cause and/or to eliminate certain causes. Remove and reapply power to the control module to view the last five
fault codes stored in its memory—the most recent to least recent fault codes will be displayed. See Figure 21 for a
flowchart for troubleshooting the unit using the DSI control module.
NOTES:
• If troubleshooting indicates that repair of the DSI control module is required, note that its only
replaceable part is the fuse (see Figure 11), which is a type ATC or ATO 3A fuse, color code violet
(PN 201685).
• IMPORTANT: When using a multimeter to troubleshoot the 24V circuit, place the multimeter’s test
leads into the connectors located on the ignition control. Do not remove connectors or terminals
from the electrical components. Doing so can result in misinterpreted readings caused by the
control module’s fault mode monitoring circuits.
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