we need toi do one thing and then retry....do you mind a hands on project...?
First turn power off. You will need to be able to reach to the front of the burner. In front of the burner is a metal rod with a white ceramic base. We need to wipe that rod down with steel wool before we proceed.
do you see any wires or tubes that enter the burner area and head toward burner front?
Any wire will be leading to either ignitor or sensor...this should enable you to follow wire to sensor
Excellent....clean with steel wool....turn unit back on...then tell me if we retrieve code 7 again
Now....exhaust fan starts....ignitor glows then gas valve clicks and opens....
do you see ignitor glow or hear gas valve click?
Well this is the issue then...your ignitor is most likely cracked which is very common.
It has 2 white wires that go to it. Odds are you see a fine white line across the black section of ignitor.
You will need to replace ignitor which is available at any heating/cooling shop.