Ducane CMPB075U3A furnace turning off after 2 minutes

[cocooner]

Model Number

CMPB075U3A

Age

More than 10 years

Attempted to start my furnace, and I noticed it's short-cycling this season. It starts up as usual, I can hear the burners fire up, and both the inducer motor and blower motor are spinning. Furnace is an old Ducane CMPB075U3A (20 years old) high-efficiency model.

Problem
Around 2 minutes after pumping out hot air, it shuts down, and the error light flashes 3 times (pressure switch open, should be closed).

What I've tried

• Replaced air filter
• Opened some additional vents
• Confirmed I'm able to manually spin the inducer motor without any resistance
• Confirmed the (single) pressure switch checks out with a DMM, and I can hear clicking when gently blowing into the tube
• Confirmed these tubes aren't clogged (there's one branch going to the burner area which I haven't taken apart yet)
• Confirmed condensation pump is in working order, and there are no clogs in the internal reservoir

While reviewing a photo of the board for this post, I noticed there is a brown area around one of the components (looks like burn marks). However, checking photos of this board (ST9160B1076) on ebay, many of these listed boards seem to have this same issue. Is this the culprit?

For what it's worth, cooling still works just fine, although I feel like the blower output has been too weak in the last several years (but HVAC tech said it was fine when he replaced the AC 2 years ago).

 

[cocooner]

Looks like I lost the ability to edit the post, so here are some updates:

• Confirmed inducer motor input voltage is hovering around 110-120VAC under load, and sounds normal/smooth
• Inducer motor warms up from high 67F (cold state) to 74F when in operation (using IR thermometer)
• Confirmed it drains condensation properly after shut off
• Scoped the first few feet of the vent going outside, no debris/clogs
• I can see the duct work flexing when the blower motor starts up

So I'm at a loss here, unless the error light is sending me on a wild goose chase, and it really is the flame sensor (but there's supposedly a seperate error code for that if this was the issue).

 

[Jake]

I don't have any experience with HVAC other than replacing the condenser motor.

Hopefully another tech with HVAC experience can shed some light on this issue.

Other than that, its best to get a HVAC tech to come out and see what's going on.

Jake

 

[cocooner]

Thanks for the response Jake! I really wanted to figure this one out (and fix) myself, since I feel like I'm getting close, but eventually will have to place a service call.

 

[Jake]

Yes, I agree.

Jake

 

[cocooner]

Started taking everything apart, and after checking the drain again, I felt some resistance that would explain the pressure sensor alarm. So I disconnected all hoses for inspection, and I may have found the root cause

 

[Jake]

Yes, looks like you did. Good find.

Jake