| Slow green flashGreen LED on two seconds, off two seconds, repeating | Normal operation | Status |
| Slow amber flashAmber LED on two seconds, off two seconds, repeating | Normal operation with a call for heat | Status |
| LED steady offNo LED activity at all | The control board LED is not flashing at all, which means the board is not getting power or its on-board fuse has blown | Warning |
| Steady on (any color)LED lit solid, not flashing, in any color | Control failure | Hard lockout |
| Continuous green flashGreen LED flashing on and off continuously with no pauses | Twinning error | Warning |
| Continuous red flashRed LED flashing on and off continuously with no pauses | Twinning error on the TM8Y / TM9Y | Warning |
| Continuous amber flashAmber LED flashing on and off continuously with no pauses | The flame sense current has fallen below 1 | Warning |
| 1 red flashOne red flash, then a two-second pause, repeating | Flame was sensed when there was no call for heat | Safety fault |
| 2 red flashesTwo red flashes, then a two-second pause, repeating | The pressure switch is closed when it should be open | Soft lockout |
| 3 red flashesThree red flashes, then a two-second pause, repeating | The pressure switch contacts are open when they should be closed | Soft lockout |
| 4 red flashesFour red flashes, then a two-second pause, repeating | The main limit switch has opened its normally closed contacts - the furnace is overheating | Soft lockout |
| 5 red flashesFive red flashes, then a two-second pause, repeating | The normally closed flame rollout switch has opened | Safety fault |
| 6 red flashesSix red flashes, then a two-second pause, repeating | While the furnace was running, the pressure switch opened four times during one call for heat | Soft lockout |
| 7 red flashesSeven red flashes, then a two-second pause, repeating | The burners failed to light on three trials for ignition, so the control locks out for one hour and then tries again | Soft lockout |
| 8 red flashesEight red flashes, then a two-second pause, repeating | The flame was lost five times | Soft lockout |
| 9 red flashesNine red flashes, then a two-second pause, repeating | Reversed line voltage polarity, a grounding problem, or reversed low voltage transformer wires | Hard lockout |
| 10 red flashesTen red flashes, then a two-second pause, repeating | The gas valve is energized with no call for heat | Safety fault |
| 11 red flashesEleven red flashes, then a two-second pause, repeating | The main limit switch opened and stayed open for more than five minutes, which the control reads as a failed blower | Hard lockout |
| 12 red flashesTwelve red flashes, then a two-second pause, repeating | The model ID plug on the control board is loose or missing | Warning |
| 13 red flashesThirteen red flashes, then a two-second pause, repeating | The high-fire | Warning |
| 4 amber flashesFour amber flashes, then a two-second pause, repeating | The control is receiving a Y | Warning |
| 2 green flashesTwo green flashes after pressing the LAST ERROR button | There are no error codes stored in the control board's memory | Status |
| 3 green flashesThree green flashes after holding the LAST ERROR button more than five seconds | Confirmation that the stored error code memory has been cleared | Status |
| Steady off (YP9C modulating)No LED activity at all on the modulating control board | No 24V power at the modulating control board | Warning |
| 1 green flash (YP9C modulating)One green flash, two-second pause, repeating | Normal operation with no call for heat | Status |
| Rapid green flash (YP9C modulating)Green LED flashing rapidly | The control is in Factory Speed-up mode, used only during factory run-testing | Warning |
| 1 amber flash (YP9C modulating)One amber flash, two-second pause, repeating | Normal operation with a call for cooling | Status |
| 2 amber flashes (YP9C modulating)Two amber flashes, two-second pause, repeating | Normal operation with a call for heat | Status |
| 3 amber flashes (YP9C modulating)Three amber flashes, two-second pause, repeating | Normal operation - the burner is still on at the end of the heating cycle after the wall thermostat has been satisfied | Status |
| 4 amber flashes (YP9C modulating)Four amber flashes, two-second pause, repeating | The furnace has automatically cut back its heating output because airflow through the house side is restricted | Warning |
| 5 amber flashes (YP9C modulating)Five amber flashes, two-second pause, repeating | The furnace has cut back its heating output because the combustion air or vent system is restricted | Warning |
| 6 amber flashes (YP9C modulating)Six amber flashes, two-second pause, repeating | Heat pump applications only: normal operation with a call for heat pump heating | Status |
| Rapid amber flash (YP9C modulating)Amber LED flashing rapidly | Low flame sense current | Warning |
| 1 red flash (YP9C modulating)One red flash, two-second pause, repeating | Flame is present with no power being supplied to the gas valve - gas is reaching the burners when it should not be | Safety fault |
| 2 red flashes (YP9C modulating)Two red flashes, two-second pause, repeating | The pressure switch is closed while inducer pressure is below the switch setpoint - the switch is closed when it should be open, so the control will not start | Soft lockout |
| 3 red flashes (YP9C modulating)Three red flashes, two-second pause, repeating | The pressure switch is open while inducer pressure is above the switch setpoint - the switch is open when it should be closed, so the burners cannot light | Soft lockout |
| 4 red flashes (YP9C modulating)Four red flashes, two-second pause, repeating | High limit switch open, defective temperature sensor, or the 24 volt fuse is open | Soft lockout |
| 5 red flashes (YP9C modulating)Five red flashes, two-second pause, repeating | The rollout switch or the condensate pressure switch is open | Safety fault |
| 6 red flashes (YP9C modulating)Six red flashes, two-second pause, repeating | Gas valve communication error | Soft lockout |
| 7 red flashes (YP9C modulating)Seven red flashes, two-second pause, repeating | Lockout due to no ignition | Soft lockout |
| 8 red flashes (YP9C modulating)Eight red flashes, two-second pause, repeating | Lockout due to too many flame recycles - the flame was lost five times during a single heating cycle | Soft lockout |
| 9 red flashes (YP9C modulating)Nine red flashes, two-second pause, repeating | Reversed line polarity or improper grounding at the furnace | Hard lockout |
| 10 red flashes (YP9C modulating)Ten red flashes, two-second pause, repeating | Gas valve circuit shorted | Safety fault |
| 11 red flashes (YP9C modulating)Eleven red flashes, two-second pause, repeating | Main blower failure | Hard lockout |
| 12 red flashes (YP9C modulating)Twelve red flashes, two-second pause, repeating | The model ID plug is not present or not connected properly | Warning |
| Steady on red (YP9C modulating)Red LED lit solid, not flashing | A control fault has been detected, or 115 volts is present without 24 volts | Hard lockout |
| 2 green flashes (YP9C modulating)Two green flashes after pressing the LAST ERROR button | Indicator for 'No error codes in memory' | Status |
| 3 green flashes (YP9C modulating)Three green flashes after holding the LAST ERROR button more than five seconds | Indicator for 'Error codes cleared from memory' | Status |