80% Gas Furnace Not Infighting - No voltage to Gas Valve - Soft Lockout every time

My furnace burner isn't producing flame. It was working when it last ran early this year. The only blink code I can get out of it is for "Soft Lockout." 80% Rheem / WeatherKing: 92-101654-01-01 IFC board: 1012-83-9257a Here's what happens at a high level when I turn on the furnace with thermostat requesting heat.

1. Purge for a couple of minutes
2. Induced Draft Motor runs
3. HSI glows for about 10 seconds
4. HSI stops glowing without infighting
5. IFC blinks code 1 (soft lockout)

Here's what I've already checked.

1. The thermostat is requesting heat.
2. The pressure switch closes and has about 27 volts across it after IDM starts. (Sorry, I was wrong about this. The 27 volts is there after the IDM stops, and briefly before it starts. It is 0 when the IDM is running.)
3. The gas valve does not gets any voltage to it.
4. The wires connecting the control board and and gas valve have continuity.
5. The pressure switch remains closed once initially closed during startup (actually, it opens again once the Soft Lockout occurs and IDM stops), but is open before IDM runs (i.e. does not appear to be stuck closed or open).

A troubleshooting guide was inside the door. I highlighted the path I took through it in green here. I am not sure what specifically to check for the "Ensure 24V between appropriate pins on connector of IFC." part. I don't know which pins are the appropriate pins in this context. I do get 24 volts between C and W, but I don't think that's what this is taking about. I have 2 of the same furnace (one in attic, one in basement). The working one in the attic gets 24 volts to the gas valve shortly after the HSI starts glowing. Everything before that seems the same as the non-working one. Any ideas on what else to check?