Starts then shuts off

REAL FACT is: If all individual components of the heater are at 100% but the air pump pressure is not on it's mark, then the heater WILL NOT continue to run!
Air Pump Pressure is directly related to fuel delivery. Read that statement again! It is the most important line of text on this website.
If the pump pressure is incorrect, then, the fuel delivery will be incorrect. As a result, the heater will not run to specs.
Instructions for setting the air pump pressure:
Air pump pressure is the pulse of the heater. Pump pressures range from 3 psi to 6 psi depending on heater BTU size (see chart below). A loss of ONLY 1 psi translates to a 30% to 17% loss of FUEL FLOW (starvation). You must know the specified required pressure and set the pressure accordingly.
Heaters with Hot Surface Ignitor technology are extremely critical to pump pressure. If the pressure is off by 1/2 psi, the heater may run for 5 seconds, then shut down because of IGNITION FAILURE. The only way to correct this problem is to set the pump pressure to DESA's recommended pressure specification.

1. The FIRST STEP in diagnosing any heater problem should be checking the output of the air pump. Determine the air pressure required for the heater model that is being tuned. The air pump, in turn, is responsible for fuel delivery.
2. If you do not know the required pressure look for a decal on the side (usually) of your heater. This decal will state the model number, serial number, fuel capacity, pump pressure, etc.
3. If your plastic end cover looks like this......

M29609 has 2 threaded holes. The hole on the left has a square plug inserted. Remove the plug. Screw in the nipple end of the gauge into the threaded hole. Check for air leaks with soapy water.
or
If your plastic end cover looks like this......
M16545 has 2 threaded holes. The shorter projection with a hole on the left has a square plug inserted. Remove the plug. Screw in the nipple end of the gauge into the threaded hole. Check for air leaks with soapy water.
4. The other hole (longer projection) has a screw with a hole which adjusts a ball and spring relief valve assembly. Carefully adjust the screw until the gauge is reading the pressure level that is specified on the heater's label. If operating properly, the pump should produce more pressure than is required to operate the heater. The relief valve allows the excess air to escape before being sent to the nozzle.
5. Replace plug. Test for leak.
6. If you are unable to reach the desired pressure spray soapy water around the seal of the end cover- at the same time check the plastic cover for a crack. A cracked end cover will not allow the heater to perform properly and will not allow for an accurate reading. The pump must be set at the specified pressure in order to have adequate fuel delivery an adequate fuel atomization. Replace if needed.
7. If you set the pressure to specs and the heater does not perform properly or the cone does not glow a soft red, then check for other fuel delivery problems such as a clogged fuel filter, cracked rubber hose, clogged or worn nozzle, or a cracked nozzle adapter. Soapy water lightly sprayed in these areas will quickly identify the problem most of the time. Any air leak will create problems.