The main burner stays lit for 10 seconds, followed by cutting out at exactly the same time the blower motor turns on. The normal startup sequence is for the main burner to stay lit for 20 seconds followed by the blower motor turning on.

There are no fault codes. "OK" LED stays green and flame LED goes from solid to off when the main burner shuts off after 10 seconds. I cleaned the flame sensor even though I had no fault code for that - didn't help.

Note: I just replaced the blower motor (confirmed bad via bench test). This problem began immediately, although the blower now works fine. Potentially, something was damaged when the broken motor continually failed to spin up after the main burner lit. I'm not sure if the system was short cycling before I realized the motor was broken. Also, I potentially didn't connect everything correctly when replacing the blower, although I took pictures and double checked it.

Any leads would be much appreciated!