I cleaned out the inducer housing. Checked the vacuum, with a manometer. It holds about 1.5 in H2O constant. The pressure switch activates and completes the circuit.

When it is operating I have 24V at the limit switch, so it is closed, as it should be. It maintains 24V at the yellow lead to the gas valve.
Also have 24V at the rollout switch.

The flame sensor has 1.3 M ohms resistance, and provides 0.000,007 Amps vs a specification of 0.001 Amp. If I disconnect the sensor, the unit runs through the pre start sequence, fires up, then shuts off after 5 seconds; which seems about right. So it seems like it is reading the flame presence, but maybe low on ampereage?

This morning it was blowing cold again. I cycled the power and it lit up after the second attempt.
Is it possible that the flame sensor amperage is low, causing the furnace to shut off? If so, where should I buy a new sensor?

Clean flame sensor with steel wool or sand paper. This is the normal trick for this problem.

Tried that already, sorry I didn't put it in the message.