Troubleshoot: HW SV9541 (M2094) Smart Valve (Nat Gas) versus HW ST9160 (B1084) Furnace Control Board

Shotpoint

UNIT DESCRIPTION:
Comfortmaker Model #N9MP105B12B1 (Natural Gas), Sr #A050191193, Mfg Date: Jan 2005
- - - - - - - - - - - - - - - - - - -
ABBREVIATIONS:
"SV" = Honeywell Smart Valve: HW SV9541 (M2094)
"CB" = Control Board: HW ST9160 (B1084)
"PS" = Pressure Switch
"IM" = Inducer Motor
"LS" = Limit Switch Series (1 Main + 2 Rollouts)
"PL" = Pilot Light
"TS" = Thermostat
"CH" = Call for heat
- - - - - - - - - - - - - - - - - - -
PROBLEM DESCRIPTION:
Central Furnace not heating after thermostat call for heat
CH thru CB to SV check OK
SV diagnostic light indicates 2 flashes (pressure switch closed when s/b open)
IM does not start due to SV state above
PS tests contradict SV light, proving PS state is in fact "open" ... while SV thinks PS is "closed"
LS tests confirm that all three (3) are OK (in proper "closed" state)
PS replaced with new OEM to eliminate doubt, again confirmed "open" state ... yet SV (still) thinks PS is "closed"
- - - - - - - - - - - - - - - - - - -
TROUBLESHOOTING NOTES:
During initial CH (while SV light shows 2-flashes):
(A) If I momentarily break then quickly re-connect one terminal wire to LS,
>> IM starts and fan spin-up causes PS to "close" ... multimeter confirms PS "closed" continuity
>> SV light changes to 4-flashes (limit switch string "open")
>> PL sequence never starts
(B) If I momentarily break then quickly re-connect one terminal wire to PS,
>> SV light continues with 2-flashes (limit switch string "open")
>> PL sequence never starts
(C) Stand alone results in steps (1) and (2) are reasonable, given the momentary terminal breaks, but
>> Why would an "open" LS condition cause the SV to initiate the IM
(i) when normal (expected) LS state is "closed"?
-AND-
(ii) when the SV state is in 2-flash error mode (thinking the PS is "closed")?
(D) The wiring diagram shows that SV terminal
>> C1-1 & C1-5 serve the three LS function (wired in series)
>> C1-2 & C1-3 serve the single PS
>> Thus, PS is not wired directly within the LS series, so any interaction must come from the SV or CB
(E) Given facts in (4), and when the PS is truly "open" (even though the SV sees it as "closed"),
>> Why would a break in LS string continuity allow SV to move past its imagined PS (2-flash) error?

- - - - - - - - - - - - - - - - - - -
QUESTION:
Given the info here, is it safe to assume that the SV is the primary problem and must be replaced?
Or, is it possible that the primary problem lies in the CB? If so, how can I test to eliminate this possibility?
- - - - - - - - - - - - - - - - - - -
TROUBLESHOOTING DETAIL / STEPS (so far):
From clean power off reboot of furnace:
Initiate CH: SV shows 2 flashes (pressure switch closed when s/b open), inducer motor does not start
Action #1: Test continuity across PS terminals w/both wires removed = circuit tests "open", no continuity
Action #2: Test continuity across PS with small (manual) vacuum = audible click, "closed", continuity
Action #3: Confirm IM pressure port, hose and manifold are free of moisture and blockage = no improvement
Action #4: Confirm IM exhaust pipes are free of moisture and blockage = no improvement
Action #5: Replace PS with new OEM = SV continues with 2 flashes error condition, inducer motor does not start
Action #6: Conduct test (A) above = confirmed IM properly "closes" on IM spin-up, "open" otherwise
Action #7: Conduct test (B) above = no change in SV or IM state
- - - - - - - - - - - - - - - - - - -
Conclusions:
>> Since both the SV and CB are rather expensive to replace, would like to learn any troubleshooting tips or methods to determine which has failed?

Thanks in advance for any suggestions from this community!

Shotpoint

fredwilliams

so what part failed

Iceman1251

Yes. What part failed. Need answers. Have same problem.

EBEBRATT-Ed

check and see if your getting power to the hot surface ignitor. it could be a 2sv or 120v ignitor check your wiring diagram

pedmec

sounds like your smart valve is defective. if your getting a heat demand from the control board then the control board has done its job. the whole ignition sequence and safety circuit is part of the smart valve. smart valve has an integrated circuit board built-it. normally if you had an ifc (integrated furnace control) separate from your gas valve you would be able to see any shorts or burnt spots by flipping the board over after you removed it. but the smart valve is an all one unit and its not worth ripping apart on site (something i would do in my spare time) to see the failure. unfortunately one of the drawbacks to a smart valve.
when i have furnaces acting like this, and there are many, the integrated furnace control has failed. you get all kinds of crazy codes. don't spend to much time on it. change the smart valve. you have check all the positions of all the safeties and operating controls and they are correct. limits are not connected to the pressure switch but acts like it is. clear indication of an control going south