I hear this alot, " I'm working on furnace model:#######, and the board was giving me a pressure switch fault code, I just replaced the pressure switch and I'm still having the same problem".  My response to this is usually "why did you replace the pressure switch?".  Followed by a moment of silence, the usual reply is "because the board said I have a pressure switch issue". There could be a lot more going on here than we think.

 

                            The furnace pressure switch is a very simple part, relatively inexpensive, and easy to replace, so I get it. However, furnace pressure switches have ALOT of responsibility. They monitor the following: HX (heat exchanger), inducer, venting system, and drain system. 

 

                     

                           I have personally witnessed erroneous pressure switch faults, starting with the thermostat. If we have a slight amperage or voltage drop through the thermostat, this can cause the control board to "not understand" if you will, and it may fault out. Those faults may vary. So it may be necessary to bypass the thermostat during troubleshooting.

 

                          Another thing we need to do is verify voltage going to and leaving the pressure switch. Remember its a switch, power-in power-out. The control board sends 24vac up to the pressure switch and looks for it to return, so it knows it is safe to continue with the ignition sequence. 

 

                        Now, there are two possible scenarios at this point. The first being, we DO have power in and out of the switch like we should. Now what? Replace the switch? ABSOLUTELY NOT!!!!!    We now must verify the signal is making its way back to the control board. Trace the wire and look for breaks or pinches. If there is a molex connector at the blower deck, make sure 24vac is on both sides of that connector. Finally test the return wire from the pressure switch right at the control board connector. If voltage is present, we most likely have a bad control board. If no voltage present, we most likely have a wiring or connection problem.

 

                         Scenario number 2, we DO NOT have power leaving the switch. Now we replace the pressure switch right????? Ummmmmm, NO!!!!!! Now what we need to do is a visual inspection of the drain piping and vent system. Once that is complete, then we need to test the draw of the combustion blower. This is simple, all we need to do here is to "tee" in a digital manometer or magnahelic gauge (the tool needs to be able to read negative pressure in w.c.). A general rule here is if the inducer is drawing 30-40% more than the rating of the switch, we can consider that acceptable and now it would be ok to replace the switch. If the reading is very close to or lower than the switch rating, then we need to more investigating. At this point it may be necessary to remove the intake and exhaust one at a time during testing to see if the results change. If they do then we may need to investigate the vent system more thoroughly and refer to the installation instructions to see if we are over our allowable vent lengths. 

 

                        If our readings do not change with the vent system removed, then we will need to investigate a bit more at the furnace this time. The HX is pulled into a negative pressure by the draft inducer/combustion blower. If there is a breach (crack/hole ect.) in the HX, the pressure switch may not close. If the secondary HX (90%+ models) is plugged, the pressure switch may not close. After this investigation is complete and we know there is not an issue with the HX, then we are safe to assume that the combustion blower may be the problem here and probably needs to be replaced. 

 

                        WOW!!!!!! all of that for a pressure switch fault!?!?!?? Yep. Or you can just replace the switch because the board said so. Keep your fingers crossed tho, your customer just might call you back "my furnace is out AGAIN!"  

 

 

So, in summary:

 

Pressure Switch Fault Troubleshooting Summary

When faced with a furnace pressure switch fault, replacing the switch without proper diagnosis may not resolve the issue.

Key Steps for Troubleshooting:

1. Understand the Pressure Switch’s Role

   • The pressure switch monitors multiple systems: heat exchanger (HX), inducer, venting, and drainage.

2. Check the Thermostat First

   • A slight voltage drop through the thermostat can trigger false pressure switch faults. Bypass the thermostat to rule this out.

3. Verify Voltage at the Pressure Switch

   • Confirm 24VAC is reaching the switch and returning to the control board.

   • If power is present at both points, test the return signal directly at the control board.

     • Voltage Present: Likely a bad control board.

     • No Voltage Present: Likely a wiring or connection issue.

4. If No Power Leaves the Switch:

   • Inspect the drain piping and vent system for blockages.

   • Test the combustion blower’s draw using a digital manometer or magnahelic gauge.

     • 30-40% higher draw than the switch rating: Likely safe to replace the switch.

     • Draw near or below the switch rating: Investigate vent system issues or HX problems.

5. Check for HX or Combustion Blower Issues

   • A cracked or blocked HX may prevent the pressure switch from closing.

   • If no HX issues are found, the combustion blower may need replacement.

Conclusion

Pressure switch faults can have multiple causes — don’t just replace the switch based on the fault code. Thorough diagnostics can save time, money, and repeat service calls.

 

 

            Carlos