I have a Ducane 92% high eff 70,000 BTU furnace that is less than a year old. The contractor and I have been trying to solve an intermittent issue for the last 6 weeks. Hopefully one of you pros might see what we are missing.

The issue.
LED code 03 [Pressure switch]
Intermittent, sometimes runs for 12 minutes before cycling, sometimes longer than 45 minutes without cycling.

This same LED code seems to be used for:
Pressure switch [ condensate drain or combustion air]
Inducer or Inducer Blower Switch

So far, we have tried the following:
- The pressure switch has been changed and tested.
- We have run the air inlet off, to allow the furnace access to combustion air both from the air inlet and room, to ensure lack of combustion air was not an issue.
- The condensate drain has been cleaned, and the drain line is clean.
- The IBS has been changed, and temp at the outlet vent is below 120F, and switch is rated at 140. [Oh, by the way, the differential is 54 F from intake to outlet, with a 40-70 rating.] It is not the temp limit that is cycling, so this shouldn't be involved, but just while temp is being brought up.
- I checked that the furnace was level and not leaning back, to ensure condensate was not pooling in the inducer somewhere, and that this was causing the cycling.

I had a voltmeter on the IBS once, and when the furnace was running normally it was 120V AC either side. [expected] It did cycle flame off while I was watching this. I could swear that I got 120V AC on the Smartvalve side of the IBS, and 0V on the inducer blower side. The LED stayed bright/dim for 30 secs, then started O3 error code. When the code started flashing, the Smartvalve side of the IBS went to 6.4 V AC. BUT..... the IBS switch was changed, and it still cycles!!!!!! I have not been able to measure voltages right when it cycles to catch new data, but have run downstairs when the furnace cycles, and seen the 03 LED error code.

So far, my contractor has been great, and he has not charged me for the switches, condensate trap clean, etc. But this keeps going on and on, and both he and I are getting frustrated with the furnace and each other. Any help would be greatly appreciated!

Thanks, JP