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Gas furnace pressure switch problems
When troubleshooting, understand you are working with potentially lethal voltages and a highly flammable gas. If you do not have the ability to do these operations safely, do not attempt them.
To understand how to diagnose pressure switch problems, you first need to understand how they work. The video below explains how they work.
First, when troubleshooting problems with pressure switches it is important to know that when there is a break in the sequence, the IFC will stop the attempt to start after a short time and go into lockout. With some of these actions, you will have only about 1 minute to troubleshoot the problem.
Pressure switches are used to stop the furnace from operating in an unsafe condition. On newer furnaces, on a call for heat, The IFC checks the pressure switch. If is in the closed position, the furnace stops the sequence and goes into lockout mode until the switch is open.
If you get a pressure switch code, and the inducer will not come on, remove one of the wires from the pressure switch. If the inducer comes on, the pressure switch is stuck closed. Tapping lightly on the pressure switch body may open the switch and the unit may start. This is not a repair. The pressure switch must be replaced. Only OEM pressure switches should be used.
If the switch is open, the inducer starts and the resulting pressure change closes the switch. If the switch does not close, after approx 1 minute, the IFC goes into lockout mode.
The pressure switch should be considered the indicator of the problem, not the problem.
If you get a pressure switch code, and the inducer will not come on, remove one of the wires from the pressure switch. If the inducer comes on, the pressure switch is stuck closed. Tapping lightly on the pressure switch body may open the switch and the unit may start. This is not a repair. The pressure switch must be replaced. Only OEM pressure switches should be used.
If the switch is open, the inducer starts and the resulting pressure change closes the switch. If the switch does not close, after approx 1 minute, the IFC goes into lockout mode.
The pressure switch should be considered the indicator of the problem, not the problem.
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On the left are some pressure switches.
When there is a call for heat, and the inducer starts, if , after 1-2 minutes, the inducer shuts off and pressure switch code comes on, The pressure switch is not closing. To test this, recycle the furnace. Remove the wires from the pressure switch. On a call for heat, the inducer should start. While the inducer is running, short the wires going to the pressure switch with a jumper. Do not short the pressure switch itself. The furnace should start. Caution: the burners may not burn properly. be ready to pull the power. If the unit starts, there is a pressure problem. It is not necessarily the pressure switch! The pressure switch is an indicator of pressure problems in the furnace. Check the tubing from the inducer. It should be clear. Also the fittings that the tubing fit into should be clear. Note: the pressure switch fitting may have a restrictor inside, do not remove. Other problems that could cause the pressure switch to not open are a dirty blower wheel on the inducer. The inducer motor may not be coming up to speed due to failing bearings. Check the inducer wheel to see if it is damaged. A blockage in the vent will also cause a pressure switch to not close. Water in the vent due to a sag in the piping could cause a pressure switch not to close. Ice on the venting or a plugged screen on the vent termination (only on 90% efficient furnaces) could also be the problem. A crack in the heat exchanger could also be at fault. |
The video below explains diagnosing pressure switch problems.
If the furnace is a 90% efficient furnace, the condensate drain may be blocked. This can cause a backup of condensate into the inducer housing which can cause the pressure switch to not make. The video below explains how to clean the drain.