New Master Temp - IGN ERR

This is not a silly question as I've seen it on too many installs. Are both the gas cock (outside valve) AND the main gas valve (inside the cabinet) in the "On" position? Sometimes a gas cock is installed at the meter as well (a second one). If this is propane (you don't say) is the valve on the tank open?

Both open. How about inside? Is that heater new or "new to you?" The exhaust side looks like this heater has been in service. If so, you need to do some troubleshooting with multimeter or have an experienced pool tech do it. Do you smell gas when the heater attempts to light? If not you are not getting gas to the combustion chamber. First place to start - gas supply.

On a side note, if you have a 400k btu heater, your main gas meter is too small, likely rated at 275 cfm max. A 400K heater wants 400cfm to function properly.