Rheem model p-m266a-en-c digital gas pool heater runs, shuts down

Agree, start with filters. Also check for clogged pump impeller.

On Rheem gas fired pool heaters SNS is a temp sensor failure.
1. Water temperature is below 36F or above 110F
2. If the two thermistors are more than 2 degrees apart, the SNS code will appear.
3. Loose or corroded connection at P1.
4. If the sensor is not mounted all the way into the JACO fitting on the header, it may give a false reading.

GVC means Gas Valve Closed. This fault can be caused by many different problems, or even a faulty gas valve. Here is a link to a PDF file that give a step by step troubleshooting chart. This may help put you on top of the actual problem. http://shop.solardirect.com/pdf/pool-heaters/gas-pool-heaters/raypak-r-series-oim.pdf

The Fan code would indicate a problem with the combustion air blower. The is a pressure switch to indicate there is sufficent combustion air, and if not then unit shuts down. Either the fan isn't running, or there is an obsturction in the fan or exhaust piping, preventing enough air flow.
cfh would be call for heat, which would be a normal display when water temp is below setpoint
See the link below for a manual. Once you open the page look on lower right side for documents, then install manual
http://www.rheem.com/product.aspx?id=2388EEB7-3DEC-4601-AE76-EC7A07F17630

1) common problem for this heater. Likely the pilot is igniting but the gas valve is not getting the signal back saying pilot is lit so open the main valve to burners. Easy fix with new pilot assembly. Good technician can diagnose this and change part in under 20 minutes.
To diagnose yourself go out after dark (to make pilot easier to see), turn on heater to make it spark. After it sparks 15-20 seconds pilot should be lit. With an inspection/cosmetic mirror you will either see the pilot sparking or lit. It will be in bottom right if burner cabinet, just follow the small silver tubing.
2) Second scenario, Pilot is not getting fuel. Could be clogged pilot assembly, stuck/faulty gas valve, supply line shut off valve not open.
In the event heater is in older, rusty/corroded condition there is the remote possibly of a hole in pilot tubing so gas isn't getting to the right location, just dumping into heater. If this is the case do not loose your eyebrows looking for pilot in scenario one, call a pro.