Trane XR90 Struggles to Start

[senick]

How * Can you tell what the sequence of operations is, or which operation is making it *

Take the top cover off and set the thermostat to call for heat. Your sequence will be inducer motor for 10-20 sec. then through the sight glass you'll watch the ignitor glow then you'll hear the gas valve open and you should have flame. Wait 30 sec to 1 min for the blower to kick on. Figure out what it is and is not doing.

If it kicks on and shuts back off, you might have a dirty flame sensor.