I'm dealing with an old GMV950905DXBB.

All of a sudden no heat.

Three flashes, low fire pressure switch stuck open.

So, I tried tondiagnose the issue and found when turning on the furnace, the ID fan doesn't start, three flashes for the switch stuck open, blower kicks on and then will turn off.

I checked the P Switch and it was fine.

Swapped it out for a new one anyway.

I took the ID fan out to check for any blockages or damaged tubing on the switches, etc nothing.

So I wanted to test the ID fan motor itself.

I jumped it with external power and it fires up and works great.

My assumption is a bad board.

Does that seem correct? No heat for going on two days.

Question from user 11teenthousand at hvacadvice at reddit.

Answer:

Based on the information you provided, it does seem possible that the control board in your GMV950905DXBB furnace could be the culprit for the no-heat issue. Here's why:

1. The three flashes indicating "low fire pressure switch stuck open" typically means that the pressure switch, which ensures proper airflow, is not closing when it should. This can be caused by various factors, including a faulty pressure switch, blocked or damaged tubing, or insufficient airflow due to a malfunctioning inducer (ID) fan.

2. Since you have already checked and replaced the pressure switch and confirmed that it is functioning properly, and there are no apparent blockages or damage to the tubing or switches, the next possible cause could be a malfunctioning control board.

3. The fact that you tested the ID fan motor separately and it works fine when jumped with external power indicates that the motor itself is not the issue. This further suggests that the control board may not be properly activating the ID fan.

4. A malfunctioning control board can fail to send the correct signals to various components, including the ID fan, resulting in a lack of heat.

Considering these factors, it is reasonable to suspect that the control board in your furnace may be faulty. It would be advisable to contact a professional HVAC technician to diagnose and repair the issue accurately. They will have the necessary tools and expertise to perform a thorough inspection, confirm the root cause of the problem, and replace the control board if necessary.

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