Hayward gas heater is Flashing 111 and says service. Have no idea what to do.

SOURCE: Hayward H400 heater problem

LO usually indicates low water pressure. Check the pressure in your filter. You may need to backwash. If the filter is to clogged then the water pressure in the heater is so low that it won't activate the pressure switch. While checking the pressure turn the pump off and then on to make sure the pressure gauge is actually working. Also check the pressure coming out of the inlets into the pool. You should be able to tell if you have sufficient water pressure.

SOURCE: Hayward H300 Spa Heater will not fire up

try to replace the thermo sensor it is located on the left side of tghe unit and has 3 wires running to it.

SOURCE: CE error code on hayward pool heater

CE= Communication Error between Control Module
and Display Interface Assembly
1. Disconnect and then reconnect power to heater.
2. Check for faulty wiring or connection.
Inspect Display Interface Wiring. Ensure Display Interface Plug is securely attached to Control Module. .
3. Control Module and/or Display Interface Assembly
are defective.Replace Control Module and/or Display Interface Assembly.
Here is a linkt oH aywardmanuals:
http://www.haywardnet.com/inground/products/manuals.cfm