Inducer motor or control circuit? Please help me troubleshoot.

averagerob

I have a Carrier Weathermaker infinity from 1992. I believe either the inducer motor is bad or it is the inducer motor control circuit is bad. How do I troubleshoot to determine which is bad?

I am getting fault 42 Inducer outside valid speed range and fault 32 Low pressure switch fault. When I power cycle the furnace, sometimes the inducer spins up for 5-30 seconds, sometimes it doesn’t even spin. After the a half dozen power cycles sometimes it ends up coming on fully and it works and I get heat; but inevitably every morning, it won’t start.

The cold start problem seemed like a motor issue, but upon inspection the motor seems to be in great shape, the bearings are fine and the windings look really good, the shaft spins freely and easily and it never gets hot to the touch. That got me to think it may be the control relays that run the inducer (which is variable speed 3ph DC). But I do not know how to tell without talking the motor out and driving it to an electric motor house to put on a test bench. Also, if I determine the relays are the problem, can the relays be replaced or does the whole board need to be replaced?

Thanks for your help.

Yoyizit

Then very likely it is not the inducer motor.

Try a Google search on your symptoms using the "advanced search" option and
find out how much the parts place wants for a new controller.
If a new controller costs half the price of a service call you should only gamble on your replacing the controller if you think your odds are better than 50-50 and that there is no undiscovered root cause causing the failure of the controller.

Post a photo; a board that uses some electromechanical relays might be easier to fix than all surface mount components and solid state stuff.

Controllers can be viewed as "state machines", just like the TelCo, and it sounds like yours gets into an illegal state which is temporarily fixed by cycling the power on and off.