"Press SW*" on the display is normal when the pump isn't running, and no action is needed. It's also normal if there's a safety device connected to the "Fireman Switch" and it has opened the heater's safety circuit.
If the pump is running, this code can mean that it has low pressure due to a dirty filter or clogged valves in the plumbing. Clean the filter and look for blockages.
"Press SW*" can also indicate a defective pressure switch, in which case you should contact an authorized service person.
High Limit Switch
"Hi Lim Sw" can mean that the water temperature has climbed beyond the acceptable limit, or the switch is defective. Both of these situations require attention from an authorized service person, who will either find the reason for the overheating and repair it, adjust the switch or install a new one.
Extra Switch 1
"Ext Sw 1" on the display can indicate that the vent switch for the flue was triggered and needs to be manually reset. Before resetting, be sure that the fan is operating correctly and that the flue isn't blocked.
This code can also mean that the vent switch or the fusible link is defective. Contact an authorized service person, who will find and repair any loosened connections or install new switches.
Air Flow Switch
"Air Flo Sw" means the fan isn't running or is running slowly, or that there's a bockage at the air intake or the air discharge. Inspect the tubing to the fan and the air flow switch for cracks, breaks, splits, kinks or a disconnection.
Install new tubing if needed. If the fan is defective or you suspect faulty wiring, contact an authorized service person.
If the air flow is blocked in any way at the flue or the discharge, the clearance around the heater is not sufficient or an enclosed room isn't ventilated correctly, contact an authorized service person.
Automatic Gas Shutdown
"AGS" can signal that the pressure in the pump is fluctuating. Clean the filter.
If necessary, find the reason for the fluctuation and repair it. "AGS" can also mean low gas pressure This issue requires a call to an authorized service person to determine whether the correct size of supply pipe was used, as outlined on Page 5 of the owner manual.
If the pipe size is incorrect, the service person should replace it. If the pipe size is correct, the service person should check for blockages and remove them.
"AGS" also indicates a lack of flame at the burners. An authorized service person should inspect the unit for loose wires or issues with the igniter, sensor, gas valve or ignition controller.
"Temp Sensor*" can indicate defective or loose wiring. Check the wiring on the sensor and make sure it's connected to the rear of the control panel.
This message can also mean a defective or worn sensor, which will need to be replaced by an authorized service person.