Bryant 355CAV A Series Troubleshooting Manual page 23

pressure switch MPS remakes the variable speed furnace control continues heating cycle. If medium heat pressure switch MPS does not
remake within 45 sec after opening, the variable speed furnace control starts flashing status code 31, shuts unit down, waits 2 minutes, stops
flashing status code 31, and restarts heating cycle.
5. OTHER TRANSITIONS - If the variable speed furnace control is transitioning from low heat to high heat the variable speed furnace control
will first transition from low heat to medium heat and then transition from medium heat to high heat. If the variable speed furnace control is
transitioning from high heat to low heat the variable speed furnace control will first transition from high heat to medium heat and then
transition from medium heat to low heat.
STEP
1. Turn power off, remove blower access panel, and disconnect User Interface ABCD connector
(if used) or the R thermostat lead (if used) from the furnace control board.
2. Turn power on and depress door switch. Use a piece of tape to hold switch closed.
3. Put setup switch SW4-2 into the ON position, and jumper R and W/W1 thermostat terminals.
4. Wait 90 seconds before proceeding to the next step.
5. Is status code 31 flashing?
6. Is a different status code flashing?
7. Go to page number indicated in the Index for the section covering the status code.
8. Go to page number indicated in Index for CLEANUP AND START-UP INSTRUCTIONS.
9. Is 24-vac across GRAY wire on the medium heat pressure switch MPS and C
variable speed furnace control?
10. Is 24-vac across connector terminal PL1-12 and C
11. You have an open wire or bad terminal on the GRAY wire from the variable speed furnace
control to the medium heat pressure switch MPS.
12. Repair or replace wire harness.
13. Replace the variable speed furnace control.
14. The problem is caused by reversed vent and combustion-air pipes or some other moist air
entering combustion-air pipe.
15. Does the medium heat pressure switch MPS make, then break several times at start of cycle
before faulting out?
16. The problem is either a sag in vent pipe or a clogged drain trap or line. Also check for
improperly plumbed drainage tubing within furnace.
17. Is 24-vac across VIOLET wire on the medium heat pressure switch MPS and C
variable speed furnace control?
18. Is 24-vac across connector terminal PL1-3 and C
19. You have an open wire or bad terminal on the VIOLET wire from the medium heat pressure
switch MPS to the variable speed furnace control.
20. Turn power off and disconnect jumper wire across R and W/W1 thermostat terminals.
21. Connect 1 side of slope manometer with a tee to collector box pressure tap. Connect other side
with a tee to gas valve pressure fitting. Refer to pressure check diagram in Appendix C.
22. Turn power on and jumper R and W/W1 thermostat terminals.
23. Do you have at least 1.06 in. wc pressure drop across heat exchangers when status code 31
flashes?
24. Replace the pressure switch assembly.
25. Is inducer motor rotating in direction indicated on inducer housing?
26. Replace inducer motor or inducer motor assembly.
27. Turn power off.
28. Is inducer wheel okay?
29. If possible, replace inducer wheel. Otherwise, replace inducer motor assembly.
30. Is the inducer wheel properly mounted to the inducer motor shaft?
31. Is there any frost build-up on the combustion-air inlet?
32. Fix problem.
33. You have excessive restriction in combustion-air or vent pipe. Also check for proper vent
sizing for installation.
34. Continue to observe furnace operation for 20 minutes or until status code starts flashing.
35. Does status code 31 flash?
36. Does a different status code flash?
37. Turn power off and disconnect jumper wire across R and W/W1 thermostat terminals.
38. Connect 1 side of slope manometer with a tee to collector box pressure tap. Connect other side
with a tee to gas valve pressure fitting. Refer to pressure check diagram in Appendix C.
39. Turn power on and jumper R and W/W1 thermostat terminals.
40. Monitor slope manometer. Does the pressure reading appear to be stable when fault occurs?
ACTION
-24V on variable speed furnace control?
OM
-24V on variable speed furnace control?
OM
- 23 -
YES
9
7
-24V on 15
OM
11
16
-24V on 18
OM
13
24
27
30
31
14
37
7
41
NO GO TO
2
3
4
5
6
34
INDEX
INDEX
10
13
12
8
8
32
17
32
20
19
12
21
22
23
25
8
26
8
28
29
8
32
33
8
32
35
36
43
38
39
40
42