[LG Gas Dryer] gas ignites only for a second

Hi,

I have been trying to fix this dryer for last two months. I tried replacing every single component in the unit with n success. Wife wants to buy a new dryer but i want to give it a last try.

Symptom: Gas comes on for a second or two before going off, instead of burning until high temperature is reached. I was able confirm this by running the dryer with front panel removed.

What i have tried so far: replaced main control board, gas valve, gas ignitor, flame sensor, and hi thermostats (unit has two)

I want to star from scratch, setting aside my assumptions.

Where do I begin? Thanks for your help.