Suddenly the furnace is not igniting, we can light it manually and the furnace kicks on, but once it reached the desired temp, it will not ignite itself when the temp goes back down. Where do we start troubleshooting?

Take a close look
The startup and run sequence should be
1. thermostat call for need to heat
2. the induction fan comes on
3. the igniter glows (can be seen pretty easy)
4. the gas valve opens and burners fire up
5. after short time the bigger blower will come on and blow heat into your home

I`m guessing the igniter isn`t glowing and porbably needs replacing

Check it out and let us know,, it also should be giving a blinking light code from the curcut board