I have a riello 40 burner it fires for 2 seconds then cuts out what could be the problem

Check thermostat since unit is working but is shutting off prematurely.

My first guess would be that it sounds like a clogged oil filter, air in your fuel line, is piping overhead, or leak in a fitting or connection causing it to loose it's prime when running.

I have also seen primary controls that are faulty do this, watching the complete cycle of the run time will give you more clues as to when the control actually locks out. Does it lock out during the run cycle as the boiler reaches temp.? If so,check for oil line and suction issues as stated above.

If it locks out on start up after reaching boiler set temperature, if hitting reset right away burner doesn't start and lock out occurs with no motor engaging, check motor (Note: Some motors when HOT will not kick in when powered, but after they cool down naturally they start, motor shaft may also be bound up tight, replacement is necessary) if burner motor spins but does not fire, check for fuel flow to the nozzle, be sure oil is pumping to the nozzle, if it is, clean cad cell eye, and check transformer for spark, if all is good.. replace primary control.

Disclaimer:
Of course all this information is technical troubleshooting information and should be performed by a Licensed Technician, not knowing how to properly test for flame failure can cause injury, fire or worse! Consult a technician in your area if you are not proficient at oil burner repair!

Tankless?... call service technician

Tank? clean arrestor screen, replace gas valve thermostat
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Ignoring the pressure drop which often is normal when the burner goes out, note that the pressure may also go up when burner is lit.
After 1 min. - suggest the flame detector needs cleaning. It's situate in the combustion chamber, usually at the rear, and looks like a right angled 1 inch pin above the gas jets. Clean with 240 grit emery cloth and also clean the nearby gas jets with a tooth brush

You need to measure the manifold pressures before and after the gas valve and adjust accordingly to fuel used, 3.5" NG or 11" Propane.

Make sure you pull connector off and clean it and contacts on board. Be sure igniter is in flame good too. Does it have the thermal cut off in-line? Its the little resistor looking piece inside the plastic cover. If it does, and still running a few seconds and going out, remove it from the circuit, and connect wire end to where you removed cut out from, thus eliminating it from the circuit, and test it. If that cures it, replace that cut-out, as it is a safety device in case of flame up, it cuts gas off to prevent fire. Just leave it out long enough to test it.

is the filter clean u might of picked up sludge in bottom tank also blow out the supply lines and re fitt try again

1) Product manual has burner info page 11, maintenance 12, and troubleshoot flow charts starting page 17
Otherwise, Rheem does not post service manuals.
http://bostonheatingsupply.com/Rheem/Rheem%20Commerical%20Gas/Rheem_CommGas_Extreme_Use%20and%20Care%20Manual.pdf

2) Post question on pro water heater forum.
http://www.thetankatwaterheaterrescue.com/forums/forum3/

3) Is air intake clear? Is the burner sooted? Is the flame large and clear blue? Is the vent obstructed? Does air draft when blower activates?

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Remove the burner access cover under the control valve. Press the pilot light button (gas control) and hold it down. At the same time, light the pilot. Continue to hold the lighting button for a few seconds until the thermocouple heats up. Release the button. Twist the gas control knob to the ON position, and the burner should fire up after a few seconds. Replace the access cover. Good luck.