There are a lot of scenarios that can cause a pressure switch to report an error. As an example, one that I have seen that is actually "self-healing", I have seen really cold days where there are ice crystals in the air and as the air is pulled into the air intake it can create an 'ice dam' and thus restrict the air flow enough that the pressure switch does not close. This can melt if the outside temp gets warm enough and it can also be dislodged simply by shaking the intake pipe. I have also seen birds nests and various debris block the air flow. One time I am pretty sure that the wind was strong enough and blowing in just the right direction that it was preventing the switch form staying closed. (I made this assumption because of the homeowners description of his trying to fix it and their location at the mouth of a canyon and the strong winds we had had.
You mentioned power cycling the unit. That would reset the error and allow for it to try to come on again. If it is running again it should be good the switch is open and will not run unless all is well.
As far as preventative maintenance, you have done most of what can be. I have seen a pressure switch that had water in it because one of the hoses was kinked. But there is not much else that can be done.
The normal sequence of operation is:
- Thermostat calls for heat.
- Control module runs self check, and verifies 1st stage pressure switch is OPEN, and limit switches are CLOSED.
- Draft blower is energized.
- 1st stage pressure switch closes.
- Igniter warm up begins (17 seconds).
- Gas valve opens.
- Flame sensor confirms ignition within 4 seconds.
- Delay to FAN ON begins (45 seconds).
- FAN ON delay elapses, and blower motor is energized.
It sounds like you're having a problem at step 3. Where either the control module is not energizing the draft motor, or the draft motor is dead.
I'm not familiar with your specific unit, so I'm not exactly sure which speed the control module runs the draft motor at. I would guess that 1st stage uses low speed, but I could be wrong.
To verify the motor is being energized, follow this procedure.
WARNING: You'll be working on live equipment. If you're not comfortable with this, please contact a local licensed professional.
- Turn power off to the unit, using either the breaker, or the serviceman's switch.
- Remove the wire harness from the draft motor.
- Restore power to the unit, and trigger a call for heat.
- After a delay, measure the voltage between the white IND-N pin (pin 3), and the red IND-LO pin (pin 2) on the inducer motor wire harness.
- Measure the voltage between the white IND-N pin (pin 3), and the black IND-HI pin (pin 1) on the inducer motor wire harness.
You should measure line voltage (~120 volts) between the white wire, and one; or both, of the colored wires. If you don't, the problem is with one of the other limits or the control module. If you do, the problem is likely with the motor.
You mentioned that the blower motor is starting, as soon as the thermostat is calling for heat. If this is the case, that could point to an open limit switch. As the blower should not come on until after ignition, during normal operation.
Best Answer
replace pressure switch it is a possibility that it is bad