Opened the pool for the season but can't get heater to start

AGS - Auto Gas Shutoff = Because of a problem sensed, the gas valve is closing, stopping gas supply and hence heater shutoff.
Improper water flow through heater! Check filter, pump, water supply lines, anything that will reduce water flow into heater.
A faulty Pressure Switch may cause this problem as well.
or...
In coming water to heater is over 104°
If all is good with the above, replace the AGS Switch.

Could be that the plumbing got clogged while the system was idle

go into set up menu to calibration

Replace the Thermal Reg and the temp sensor. Thermal Reg will cause the noise and the Temp sensor will cause the 118.

You might have a much bigger issue than the temp sensor. When the heater is running hold the "pool" or the "spa" button for 12 seconds. This will show the exhaust temp, if the temp is rising above 375 degrees with in 1-2 mins then your not getting the heat out of the heat exchanger.
One other thing to check would be an external by-pass. If you have an external by-pass make sure the by-pass is set correctly. Opened or closed to much can cause the problem your having.

This could be a number of things. Most often it will be the internal by-pass. You find the assembly inside the water header, it controls water flow through the heater. When it goes bad it can open at a low temp and keep the water from going through the heater.
Also you will want to check the temp probe on the side of the header. If this is bad it will usually show up on the temp display though.

check gas line to ignitor for cobwebs

themal bypass.. looks like an automotive thermostat ,locater in the header.
temp should roughly 20 degrees difference from in and out

The 126 is not an error code, but it maybe the temp of the water that the heater is reading. If the heater thinks the water is 126 degrees, it will not start at all.

If the heater is reading the wrong water temp, the problem is the Thermister, which is the sensor that is suppose to read the water temp. You will need to replace this sensor in order to correct the problem with the heater. It is really easy to do!

If you have a manual for the heater, if you go to the Replacement Parts Listing for the Electrical System, page 53 in the online version, the Thermistor is Item #9. It goes into the left side of the manifold in the to place, between the first two manifold bolts.

Anytime the heater is reading the water temp to be above what it actually is, especially 126, the problem is the Thermistor, or Temp Sensor, for the heater. This is a very easy part to replace. The Thermistor is located on the manifold, on the left side, between the top two bolts.

Sorry this is a late response! Hope that it helps!