Temperature drops in Methane combustion

CPRL

Hi. I am trying to simulate methane combustion with air using Unpremixed/Diffusion and Chemical Kinetics scheme. There are separate air and methane inlets to the chamber and the flow is laminar. This is a single step global reaction. The ignition cells are set to 4000 K. In the beginning, when the ignition starts (from 400 to 500 iteration), I see high temperature rise and the combustion products (H2O and CO2) around the ignition cells. However, as the iteration number increases the temperature drops and reaches the normal room temperature and CO2 and H2O disappear. Anyone has idea why is it happening? Thanks for the help.