Look on the front of the heater for the "Press SW" code. This indicates that the pressure switch on the heater is not working properly. Contact a technician to replace your pressure switch.
Look for the "HI LIM SW" code on the front of the display. This indicates that there is a problem with your Hi Limit Switch. You are likely to feel the water getting extremely hot when this switch fails. Contact a technician to replace the Hi Limit Switch in the heater.
Look for the "AIR FLOW SW" code on the heater's display. This indicates that there is a problem with the tubing connected to the airflow switch. Go to the back of the unit and locate the airflow tubing on the bottom of the heater. Make sure that the tubes are not kinked. Straighten the tubing out to clear this code. If the code does not clear, call a qualified technician to replace your internal fan.
Look for the "AGS" code on the front of the heater. This means that there is a problem with the automatic gas shutdown switch. A certified technician needs to check the gas flow to the heater. Your supply pipes might not be the correct size for the heater.
Look for the "TEMP SENSOR" code on the display. This indicates that the temperature sensor has come unplugged from the unit. Go to the back of the unit and look for the part labeled "Temperature Switch." Plug it back into the "Temperature Switch Port" labeled on the back of the unit.