25.
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.
—— 26
26.
Turn power on and wait several sec, then jumper R and W/W1 thermostat terminals on furnace con-
trol board.
—— 27
27. Monitor slope manometer. Does pressure reading appear to be stable when fault occurs? 14 28 —
28. Check for a sag in vent pipe. If problem persists, replace inducer motor. — — 8
29. Disconnect jumper wire across R and W/W1 thermostat terminals and wait until blower stops. — — 30
30. Jumper R, W/W1, and W2 thermostat terminals on furnace control board. — — 31
31. Does high-pressure switch HPS make, then break several times at start of cycle before faulting out? 32 33 —
32. The problem is either a sag in vent pipe or a clogged drain trap or line. — — 17
33. Does fault occur before main burners ignite? 34 51 —
34.
Do you have 24v between C (Common) on high-pressure switch HPS and Com on furnace control
board?
36 35 —
35.
The YELLOW wire to high-pressure switch HPS is not making good connection. Repair or replace
it.
—— 8
36.
Do you have at least 1.90 in. wc pressure drop across heat exchangers when fault occurs? Deter-
mine this by connecting 1 side of slope manometer with a tee to collector box pressure tap. Con-
nect other side with a tee to gas valve air pressure fitting. Refer to pressure check diagram in Ap-
pendix C.
45 37 —
37. Is inducer motor MTR1 rotating in direction indicated on inducer housing? 39 38 —
38. Replace inducer motor or inducer motor assembly. — — 8
39. Turn power off. —— 40
40. Is inducer wheel okay? 42 41 —
41. If possible, replace inducer wheel. Otherwise, replace inducer motor assembly. — — 8
42. Is there any frost build-up on combustion-air inlet? 43 44 —
43.
The problem is caused by reversed vent and combustion-air pipes or some other moist air entering
combustion-air pipe.
—— 17
44. You have excessive restriction in combustion-air or vent pipe. — — 8
45. Turn power off and disconnect jumper wire across R, W/W1, and W2 thermostat terminals. — — 46
46.
Turn power on and wait several sec, then jumper R, W/W1, and W2 thermostat terminals on fur-
nace control board.
—— 47
47.
Do you have 24v between connector terminal PL2-4 and Com on furnace control board when fault
occurs?
11 48 —
48. Turn power off. —— 49
49.
Do you have continuity across terminal PL2-4 and N.O. (Normally Open) contact on high-pressure
switch HPS?
14 50 —
50.
The BROWN wire from high-pressure switch HPS to furnace control board is not making good con-
nection. Repair wire or replace harness.
—— 8
51. Turn power off and disconnect jumper wire across R, W/W1, and W2 thermostat terminals. — — 52
52.
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.
—— 53
53.
Turn power on and wait several sec, then jumper R, W/W1, and W2 thermostat terminals on fur-
nace control board.
—— 54
54. Monitor slope manometer. Does pressure reading appear to be stable when fault occurs? 55 56 —
55.
Replace pressure switch assembly. If problem persists, check wiring for an intermittent connection
in high-pressure switch HPS circuit.
—— 8
56. Check for a sag in vent pipe. If problem persists, replace inducer motor. — — 8
22
