I had a Coleman gas furnace installed 2 years ago. I turned it on for the first time this year and it ran for 4-5 days then quit. It threw an error (3 flashing red lights). When it throws the error the inducer will kick on for 20 seconds or so then kick back off. 30 seconds will pass and the inducer will kick back on, then it just does this over and over. The manual says the error could be the inducer, vent pipe, or pressure switch issue. I call the guy who installed it, he comes to the house a day later and we turn the unit on and low and behold it works. No error. We even turn it off and on multiple times to try and get it to throw the error. It won't.

Fast forward. Weather warmed up a little and furnace was turned off. Turn it back on and it runs again for about 5 days perfectly fine, then same error. This time, the error was still present when the tech came. He checks it and the pressure switch was not closing. He puts a new pressure switch on it and it fired right up. We turn it off, then back on and it throws the error again. He checks the vent pipe and nothing is clogging it. He has some gadget that measures the air draw/vacuum and all that checks out (-1.75), so pressure switch should be closing. Then, he checks amperage at the computer board for the inducer. It's showing about 2.3-2.5 steady. He then checks the amperage at the connections over at the inducer and the amperage is all over the place, from .5-4.5 (he is not moving the amp meter). He then moves the meter closer to the inducer and the amperage gets up to about 12-14 range right at the inducer. He gets a new inducer and put it on. Furnace is still throwing the same error, but will randomly fire up and work. He left last night about 7 or so and furnace ran fine until about 10:30. Now, same error.

Inducer is getting 120 VAC at the connections from the computer.
Thermostat is calling for heat and audibly hear the "click" then the inducer will run and shut off.

Has anyone ever seen/heard anything like this?
Any suggestions of what may be the problem here?

 

Is it closing ?
Is it remaining closed ?

Sounds like his clamp on current meter had an issue.
The inducer sounds like it's running correctly. If it wasn't.... the pressure switch won't close.

 

At times it has to be and others no. Because the unit will run sometimes. It's so sporadic.

He's replaced the pressure switch twice. Once, because we thought initially that's what it was. Then he put another new on it after he replaced the inducer because the inducer never fixed the issue. My tech is perplexed. I figured i would reach out to see if anyone on the web had any other ideas.

 

The pressure switches rarely go bad. The draft inducer is usually good or bad. If it's running.... it's ok. For a service tech.... a manometer is critical. It eliminates the guesswork out of exhaust vacuum.

If the proper minimum vacuum is not being applied to the vacuum switch.... that's what needs to be checked.

 

If the proper vacuum is not being applied then what would be the culprit?

 

they can slow down but still run.