Fault Code 42
INDUCER OUTSIDE VALID SPEED RANGE—This fault indicates that inducer motor is not operating at
calculated RPM or has not started within 10 sec after a call for heat.
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 and W/W1 thermostat terminals on furnace control board. — — 4
4. Observe operation of furnace for next 8 minutes or until fault occurs. — — 5
5. Does fault occur again? 96 —
6. Does a different fault occur? 760 —
7. Go to page number indicated in Index for fault code flashed. — — INDEX
8. Go to page number indicated in Index for CLEANUP AND STARTUP INSTRUCTIONS. — — INDEX
9. Does fault occur within 11 sec after a call for heat? 52 35 —
10. Do you have 120v between I1 and I2 on furnace control board? 11 14 —
11. Do you have 120v between terminals PL7-6 and PL7-12? 13 12 —
12.
You have an open wire or bad terminal on either the BLACK or WHITE wire between furnace con-
trol board and inducer motor MTR1.
—— 14
13. Are all pins and wire leads intact on connectors between furnace control board and inducer motor? 15 14 —
14. Fix problem. —— 8
15.
Turn power off and disconnect jumper wire across R and W/W1 thermostat terminals. Turn power
on and wait a few sec for self test before proceeding to next step.
—— 16
16. Put setup switch SW-4 for EMER. HEAT in ON position. — — 17
17. Jumper R and W/W1 thermostat terminals on furnace control board. — — 18
18. Does inducer motor turn on? 30 19 —
19. Does inducer wheel rub against inducer housing? 14 20 —
20. Turn power off. —— 21
21. Does inducer wheel turn freely? 24 22 —
22. Replace inducer motor or inducer motor assembly. — — 8
23. Replace furnace control board. —— 8
24.
Disconnect jumper wire across R and W/W1 thermostat terminals and put setup switch SW-4 for
EMER. HEAT in OFF position.
—— 25
25. Turn power on and wait a few sec for self test before proceeding to next step. — — 26
26.
Check 12-vdc line. To do this, connect a DC voltmeter across terminals PL1-6 BROWN (+) and
PL1-7 GREEN (-). Is voltage across BROWN and GREEN wires 11.5 — 12.5 vdc?
27 23 —
27. Turn power off. —— 28
28. Do you have continuity between PL7-2 and PL1-6? 22 29 —
29. You have an open wire or bad terminal on BROWN wire between connectors PL7 and PL1. — — 14
30.
Disconnect jumper wire across R and W/W1 thermostat terminals and put setup switch SW-4 for
EMER. HEAT in OFF position.
—— 31
31.
Check RPM feedback line. To do this, connect a DC voltmeter across terminals PL1-6 BROWN (+)
and PL1-5 ORANGE (-), then put setup switch SW-6 for COMPONENT TEST in ON position. Does
voltage across BROWN and ORANGE wires change between states as shown below?
• State 1—OFF (11.5 — 12.5 vdc)
• State 2—LOW (10.2 — 11.2 vdc)
• State 3—HIGH (8.4 — 9.4 vdc)
23 32 —
32. Turn power off. —— 33
33. Do you have continuity between PL7-1 and PL1-5? 22 34 —
34. You have an open wire or bad terminal on ORANGE wire between connectors PL7 and PL1. — — 14
35.
Does fault occur a few sec after low-pressure switch LPS makes?
NOTE: Check for 24v between the N.O. (Normally Open) contact on low-pressure switch LPS
and Com on furnace control board to know when low-pressure switch LPS makes.
36 48 —
36. Is inducer motor rotating in direction shown on inducer housing? 37 22 —
37. Turn power off and disconnect jumper wire across R and W/W1 thermostat terminals. — — 38
38.
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.
—— 39
39.
Turn power on and wait several sec, then jumper R and W/W1 thermostat leads on furnace control
board.
—— 40
40.
Do you have more than 1.0 in. wc pressure drop (0.80 in. wc on trophy unit) across heat exchang-
ers when low-pressure switch LPS makes?
NOTE: Check for 24v between the N.O. (Normally Open) contact on low-pressure switch LPS
and Com on furnace control board to know when low-pressure switch LPS makes.
47 41 —
41. Turn power off. —— 42
42. Is inducer blower wheel okay? 44 43 —
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