I am getting an error code ags

AGS = Auto Gas Shutoff...
What is happening is the heater is monitoring (or sorta, by reasons difficult to explain here) Water Flow and Water Temperature. In this case, the AGS is triggered by a low water flow, or high Incoming Water Temp.
High internal water temp = triggered AGS.
If you have and can confirm, PROPER water flow, water temp coming into heater is Lower than 104°, then it is possible you have a faulty AGS.
But in most cases, it's only doing its job. Locate the reason for the fault, or why the AGS Switch turned the heater off.

This error is Automatic Gas Shutdown. Here is what you can do:

1. Check for fluctuating pump pressure (air in strainer, pressure guage bounce)
2. Check all wiring connections
3. Replace ignitor/flame sensor assembly

Depending on your expertise, it could also be the gas valve or ignition control (aka the IID).

Since the burners do fire briefly, I would suspect the flame sense rod first. Attached is a link to the owners manual in PDF.

Lo Nox heaters can be a little tough so follow the directions exactly.

Automatic gas Shut Off . The system Locks out if it does not ignite. Shut the power off to the unit at the breaker then try to start it again.

AGS switch wiring may be bad. Jumper the switch at the molex connector on the board using a paper clip, with the plug still plugged on. Restore power.