43. If possible, replace inducer wheel. Otherwise, replace inducer motor assembly. — — 8
44. Is there any frost build-up on combustion-air inlet? 45 46 —
45.
The problem is caused by reversed vent and combustion-air pipes or some other moist air entering
combustion-air pipe.
—— 14
→46.
You have inadequate combustion-air supply. This may be caused by:
- Excessive restriction in combustion-air or vent pipe.
- Internal restriction in coupling box or secondary heat exchangers
—— 14
→47. Replace pressure switch assembly. — — 70
→48. Is unit a trophy unit? 50 49 —
→49. Does fault occur before blower motor turns ON? 47 51 —
→50.
Replace pressure switch assembly with pressure switch assembly used in NON-trophy models or
replace furnace control board. If you replace furnace control board make sure replacement control
has a date code of 9837 or later. Only replace the pressure switch assembly or the furnace control
board NOT both.
—— 70
→51. Replace furnace control board. —— 70
52. Turn power off and disconnect jumper wire across R and W/W1 thermostat terminals. — — 53
53. Turn power on and wait several sec before proceeding to next step. — — 54
54. Disconnect collector box tube to pressure switch assembly. — — 55
55. Jumper R and W/W1 thermostat terminals on furnace control board. — — 56
56. Does Fault Code 42 occur within 11 sec after a call for heat? 57 58 —
57. Reconnect collector box tube to pressure switch assembly. — — 10
58. Reconnect collector box tube to pressure switch assembly. — — 36
59.
Go to page number indicated in Index for CLEANUP AND STARTUP INSTRUCTIONS. If problem
persists on an intermittent basis, contact your distributor.
— — INDEX
60. Turn power off and disconnect jumper wire across R and W/W1 thermostat terminals. — — 61
61.
Turn power on and wait several sec, then jumper R, W/W1, and W2 thermostat terminals on fur-
nace control board.
—— 62
62.
Does fault occur a few sec after high-pressure switch HPS makes?
NOTE: Check for 24v between the N.O. (Normally Open) contact on high-pressure switch
HPS and Com on furnace control board to know when high-pressure switch HPS makes.
63 67 —
63. Turn power off and disconnect jumper wire across R, W/W1, and W2 thermostat terminals. — — 64
64.
Connect 1 side of a slope manometer with a tee to collector box pressure tap. Connect other side
with a tee to gas valve air pressure fitting. Refer to pressure check diagram in Appendix C.
—— 65
65.
Turn power on and wait several sec, then jumper R, W/W1, and W2 thermostat terminals on fur-
nace control board.
—— 66
66.
Do you have more than 1.9 in. wc pressure drop across heat exchangers when high-pressure
switch HPS makes?
NOTE: Check for 24v between the N.O. (Normally Open) contact on high-pressure switch
HPS and Com on furnace control board to know when high-pressure switch HPS makes.
47 44 —
→67. Does fault occur a few sec after main burners ignite? 63 68 —
→68. Observe operation of furnace in high heat for next 8 minutes or until fault occurs. — — 69
→69. Does the fault occur? 22 59 —
→70. If problem still persists check for internal restriction in coupling box or secondary heat exchangers. — — 14
Fault Code 43
PRESSURE SWITCH CALIBRATION FAULT—Indicates low-pressure switch LPS and high-pressure switch
HPS "make" points during high-heat purge are not within calibration range. This is determined by reading
inducer motor RPM when low-pressure switch LPS and high-pressure switch HPS make. Then the following
comparison is made:
0.55
‹
RPM (low)
RPM (high)
‹ 0.90
If this relationship is not within range specified, furnace control board starts flashing fault colde 43, shuts unit
down (inducer runs for 15 sec), waits 2 minutes, stops flashing fault code, and restarts heating cycle.
It can also indicate that the high-pressure switch HPS made before the low-pressure switch LPS during either
low-heat or high-heat purge.
STEP ACTION YES NO GO TO
1.
Turn power off, remove blower access panel, and disconnect all thermostat leads from furnace con-
trol board.
—— 2
2. Turn power on and depress door switch. Use a piece of tape to hold switch closed. — — 3
3. Jumper R, W/W1, and W2 thermostat terminals on furnace control board. — — 4
30
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