Common Operating Sequence...

On a call for heat, they first start the induced draft motor (if the furnace has one). Then, the inducer causes a pressure switch to close. The control then either ignites the pilot, starts the direct spark ignitor, or starts the warmup of the hot surface ignitor.

After the pilot is lit and verified or the hot surface ignitor is warmed up (glowing bright yellow), the gas valve is energized. At the same time a timer begins to ensure that the burners were lit. In most cases a separate flame sensor proves that the burners are lit.

After the burners are lit, a timer starts and energizes the furnace blower after the set delay.

When the thermostat is satisfied, the control shuts off the gas. The inducer usually runs for about 30 seconds to purge the heat exchanger. A timer shuts the furnace fan off after a delay. This time delay is usually adjustable on the control board.

Troubleshooting the furnace control...

If the control does not cause the individual components to perform as described above, the control may be bad. Observe the operation and when the normal thing does not happen, you should check the operation of the related components.

Most of the controls have built in diagnostic indicators. These are normally led lights that operate in flashes to indicate the problem. There should be a chart on the control or on the furnace access panel that explains the flashing codes. For many of the controls, there is a green light to let you know that the power is on and the control is functioning. If this light is not on, the control is usually reset by shutting the furnace power off for about 30 seconds and then turning it back on.


Control Board Replacement...

Once you have identified a defective control board, you continue your control board replacement by finding the proper replacement. This can be achieved by finding the manufacturer's part number or by using the furnace model and serial number. There are also universal controls that can be used for many furnaces. 

 To begin the control board replacement, you should ensure that the power to the furnace is off. Then, you should mark all of the wires and write down which connections they go to on the board.

Then, you can remove the wires. The board may be held in place by screws or plastic clips. Remove the screws or clips and remove the old board.

Now, you can attach the new board with the screws or snap it in with the plastic clips. Carefully reconnect the wires you removed. VERIFY THAT THEY ARE ON THE PROPER TERMINALS AND THAT ALL WIRES ARE CONNECTED.

You should complete your control board replacement, by restoring the power to the furnace. Run the system through a heating cycle and verify proper operation.

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