Thermo Products MDA1-50N, MHA1-75N Diagnostic Features, LED will also flash once at power-up

All installations and services must be performed by qualified service personnel.

IV. Troubleshooting

NOTICE: BEFORE TROUBLESHOOTING, FAMILIARIZE YOURSELF WITH THE START UP AND CHECKOUT PROCEDURE.

ALL INSTALLATION AND SERVICES MUST BE PERFORMED BY QUALIFIED HEATING CONTRACTORS.

To assist you in troubleshooting this furnace, it is equipped with an integrated ignition control with diagnostics. These diagnostics evaluate what control system has experienced a failure and activate a flashing light on the control in different sequences to help pinpoint the failure which has occurred. Also provided in appendix C is a troubleshooting flow chart located in the back of this manual.

IMPORTANT: For your convenience, Thermo Products has installed a sight glass in the blower door of this furnace. The number of flashes in groups should be observed through this sight glass and recorded before turning off power to the furnace. Otherwise power to the furnace will be interrupted upon or removing the blower turning off power or removing the necessary to restart the furnace and have the failure occur again.

DIAGNOSTIC FEATURES

The 50A65 control used on this furnace continuously monitors its operation and the operation of the system. If a failure occurs, the LED light on the control will failure code. If the failure is internal to the control, the light will stay on continuously. In this case, the entire control should be replaced, as the control is not field-repairable.

If the sensed failure is in the system (external to the control), the LED will flash in the following flash-pause sequences to indicate failure status (each flash will last approximately 0.25 seconds and each pause will last approximately 2 seconds).

1 flash, then pause	System lockout
2 flashes, then pause	Pressure switch stuck closed
3 flashes, then pause	Pressure switch stuck open
4 flashes, then pause	Open high limit, aux. limit, or roll-out switch
6 flashes, then pause	115 Volt AC power reversed
7 flashes, then pause	Low flame sense signal
8 flashes, then pause	Check ignitor or improper grounding
Continuous flashing	Flame has been sensed when no flame should
(no pause)	be present (no call for heat)

The LED will also flash once at power-up.

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