Lang GCCO-T Service Manual page 49

NO HEAT Computer Mode
PROBABLE CAUSE
No 24 VAC on Interface
board
DISPLAY LOCKS UP
PROBABLE CAUSE
"Help" in display
"88888" stuck in display
Display has shelf "A"
TROUBLESHOOTING GCCO-C CONT'D
CORRECTIVE ACTION
Check for 24 VAC at TP4 to ground.
If 24 VAC is not measured:
Check back-up switch for normal operation.
If 24 VAC is measured:
Check for 24 VAC at TP6.
Replace Interface board if defective.
CORRECTIVE ACTION
Check probe for proper resistance.
Check that probe connections are secure.
Push "TEMP" button on control board and check to see if temperature
rapidly descends. If temp does descend rapidly, replace ribbon cable.
Check to see that contactors/relays are not stuck in the closed
position.
Replace contactor if defective.
Check for foreign objects keeping contactor closed.
Check for stuck button by pressing any button.
If computer beeps or chirps:
Check control panel transformer (12 VAC) for proper operation.
Check TP1 for at least 10.5 VAC.
Check TP2 for at least 4.99 VDC.
Check TP3 for at least 4.97 VDC.
If computer does not beep or chirp:
Check each button for movement.
Check that panel label has not been damaged in any way.
Replace button if defective.
Replace panel label.
Read Programming Codes.
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