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Bosch 96% AFUE Gas Furnace Installation, Operation & Maintenance
04.2019 | Bosch Thermotechnology Corp.
Data subject to change
Symptoms of
Abnormal
Operation
Associated
LED
Fault Description(s) Possible Reason Corrective Action Cautions And Notes
Furnace fails
to operate
NONE
— No 115 volt
power to
furnace, or no
24 volt power to
integrated control
module
— Blown fuse or
circuit breaker
— Integrated control
module has an
internal fault
— Door switch open, or 24 volt
wires improperly connected or
loose.
— Blown fuse or circuit breaker
— Integrated control module has
an internal fault
1. Assure 115 and 24 volt power to
furnace's integrated control module.
2. Check integrated control module's
fuse (3A). Replace if necessary.
3. Check for possible shorts in 115 and
24 volt circuits. Repair as necessary
OR replace bad integrated control
module.
— Turn power OFF prior to repair.
— Replace integrated control
module's fuse with 3A
automotive fuse.
— Read precautions in
Electrostatic Discharge
section of manual.
RED
Furnace fails
to operate
1 FLASH
— Flame sensed
with gas valve off
— Short to ground in flame sense
circuit.
1. Correct short at fl ame sensor or in
fl ame sensor wiring
— Turn power OFF prior to repair.
Furnace fails
to operate
2 FLASH
— Pressure switch
closed with
inducer off
— Induced draft blower pressure
switch contacts "stuck".
— Shorts in pressure switch circuit.
1. Replace induced draft blower's
pressure switch.
2. Repair short.
— Turn power OFF prior to repair.
— Replace pressure switch with
proper replacement part.
Induced draft
blower runs
continuously
with no
further
furnace
operation.
3 FLASH
6 FLASH
13 FLASH
— Low fire pressure
switch open with
inducer on
— Pressure switch
cycle lockout
— High fire pressure
switch open with
high inducer on
— Pressure switch hose blocked,
pinched or connected
improperly, blocked flue, or
weak induced draft blower.
— Incorrect pressure switch
setpoint or malfunctioning
switch contacts.
— Loose or improperly connected
wiring.
1. Inspect pressure switch hose.
Repair if necessary. Inspect fl ue for
blockage, proper length, elbows, and
termination.
2. Replace pressure switch.
3. Tighten or correct wiring connection.
— Turn power OFF prior to repair.
— Replace pressure switch with
proper replacement part.
Furnace fails
to operate.
4 FLASH
5 FLASH
11 FLASH
— Limit/RoIIout
switch open less
than 5 mins
— Limit/RoIIout
switch open more
than 15 mins
— Limit/RoIIout
switch open from
5 to 15 mins
— Faulty chamber limit switch or
fan mounted limit switch.
—
Insufficient conditioned air over
the heat exchanger. Blocked
filters, restrictive ductwork,
improper circulator blower
speed or failed circulator
blower
— Misaligned burners, blocked
flue, or failed induced draft
blower.
— Faulty rollout switch - resettable.
— Faulty inducer.
— Loose or improperly connected
wiring.
1. Check chamber limit switch or fan
mounted limit switch. Replace if
necessary.
2. Check fi lters and ductwork for
blockage. Clean fi lters or remove
obstruction. Check circulator blower
speed and performance. Correct
speed or replace blower if necessary.
3. Check burners for proper alignment.
Check fl ue and air inlet piping for
blockage, proper length, elbows and
termination. Correct as necessary.
4. Check rollout switch - resettable.
Replace if necessary.
5. Check induced draft blower for proper
performance. Replace if necessary.
6. Tighten or correct wiring connection.
— Turn power OFF prior to repair.
— Replace pressure switch with
proper replacement part.
Table 25 Troubleshooting Chart
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