A.O. Smith Boiler VB/VW- 750 User Manual

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TROUBLESHOOTING IGNITION SYSTEM
FaultMessagesandLockoutStatus(*=stagenumber:1,2,3,or4)
FAULT DISPLAYED DESCRIPTION RED LED LOCKOUT
“Display Fail” Communications with UIM interrupted Continuous Auto Reset
Check communication cable to UIM. Try moving to other Internal Communications connector on MCB. Also check the
connectors where the cable is plugged in for best connecting wires.
“Comm. Fail Stg.*” Communications with FCB interrupted Continuous Auto Reset
Same as above. Also try swapping FCB and UIM communications cables.
“Low AC Voltage” Line voltage less than 90 vac Continuous Auto Reset
Checkincomingpowerlineforlossofvoltage.Mayalsobecausedbyapowerlinebrown-out(momentarylossofvoltage)
“Low 24VAC” Voltage from transformer less than 18 vac Continuous Auto Reset
Check transformer output. Should be over 24vac. May be caused by excessive current drain or a faulty transformer
“Brown Out Reset” Indicates a brown out reset occurred - - - - None
CausedbyamomentarydipinvoltageontheMCB+5vdcpowerbus.Contactfactory.(Notconsideredaseriousproblem
ifitonlyoccursonveryrareoccasions.)
“Timeout Reset” Indicates a watchdog reset occurred - - - - None
Causedwhenthemicroexecutesthesoftwareinanimproperway.Contactfactory.(Notconsideredaseriousproblemifit
onlyoccursonveryrareoccasions.)
“FCB Comm Timeout” FCB did not receive command from MCB Continuous SOFT
Caused when communications between the MCB and the FCB are interrupted. May occur when a surge of power on a
nearbydevice(i.e.arcwelder)createsanEMIburst.Notaseriousproblemifitoccursrarely.
“No Flow” Water is not owing Continuous SOFT
Causedbytheowswitchbeingopenwhenitshouldbeclosed.Checkwaterlines,pump,owswitchcontacts,wiring.
“Blower Prov Stg* Blower pressure is too low it they should have been closed. Continuous SOFT
Check blower, switch contact, wiring, and for air leaks.
“Blocked Flue” Flue is Blocked Continuous SOFT
Causedbytheblockedueswitchcontactsbeingopenwhentheyshouldhavebeenclosed.Checkforueblockage,switch
contacts, wiring.
“Blocked Inlet” Air Inlet is blocked
Checkinletterminal,venturiscreenandburnertubefordebris.Ifinletisclearcheckblockedinletueswitchforcontinuity.
Inspect hose to switch to ensure it is connected and not cracked or spilt.
“High Limit” Outlet temp. exceeded high limit setpoint Continuous Auto Reset
Causedwhenthetemperatureoftheoutletprobeexceedsthehighlimitsetpoint.Thisisaninternal“software”limitswitchand
not an actual device. Check that the over temperature is not being caused by improper setup or operation of the boiler.
“MRHL” Outlet temp is too high Flashing HARD
CausedwhentheMRHL(athermostat)contactsintheOutletprobeareopenwhentheyshouldbeclosed.Faultmaybedue
to high outlet temperature, bad switch contacts in the probe, or disconnected wiring. Note: The MRHL can be checked by
disconnecting the outlet probe from the MCB and reading the resistance across the two active pins on the connector at the
end of the probe cable. The value should be approximately 0 ohms when the temperature of the probe is less than 220°F.
“Low Water” Water level is too low Flashing HARD
Caused by the Low Water Cut Out device not closing its feedback switch contacts when it should. Check water line, switch
contacts, wiring, LWCO.
“Low Gas” Gas pressure is too low Flashing HARD
Caused by the Low Gas switch being open when it should be closed. Check gas line, switch contacts, wiring.
“Powered Accessory” Power vent not running Flashing HARD
Caused by the Powered Vent device not closing its feedback switch contacts when it should. Could also be caused by a
problem with the relay output on the MCB. Check output to powered vent, switch contacts, wiring.
“High Gas Stg*” Gas pressure too high Flashing HARD
Caused by the High Gas switch being open when it should be closed. Check gas line, switch contacts, wiring.
“Inlet Probe” Inlet probe shorted or open Flashing HARD
Caused when the thermistor in the probe or the wiring to the probe is shorted or disconnected. Check the probe. Note: The
thermistor and wiring can be checked by disconnecting the probe from the MCB and reading the resistance across the two
activepinsontheconnectorattheendoftheprobecable.Thevalueshouldbeapproximately10Kohms(valuewillchange
slightlywithchangesintemperature).
“Outlet Probe:” Outlet probe shorted or open Flashing HARD
Caused when the thermistor in the probe or the wiring is shorted or disconnected. Check the probe. Note: the thermistor and
wiring can be checked by disconnecting the probe from the MCB and reading the resistance across the two active pins on
theconnectorattheendoftheprobecable.Thevalueshouldbeapproximately10Kohms(valuewillchangeslightlywith
changesintemperature).
“Tank Probe” Tank (Remote) probe shorted or open Flashing HARD
Caused when the thermistor in the probe or the wiring is shorted or disconnected. Check the probe. Note: the thermistor and
wiring can be checked by disconnecting the probe from the MCB and reading the resistance across the two active pins on
theconnectorattheendoftheprobecable.Thevalueshouldbeapproximately10Kohms|(valuewillchangeslightlywith
changesintemperature).
“Igniter Stg*” Igniter current is too low Flashing HARD
Caused by a low current draw problem with the igniter. This may occur if the igniter is old, damaged, or disconnected and
nolongerdrawstheproperlevelofcurrent.Thisconditionwillaffecttheabilityoftheignitertogethotenoughtorethegas
properly. Check the igniter and its associated wiring.
“Igniter Pwr Stg*” Improper power applied to the igniter circuit Flashing HARD
Caused by improper line power being applied to the igniter circuitry. Check line connections. Paying particular attention to
the earth ground connection. Also check that line voltage does not exceed 132 VAC RMS.
“Igniter Hdwr Stg*” Hardware problems with igniter circuit Flashing HARD

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