Wont stay running

100% sounds like an electrical problem, something is not allowing it to go into a duty cycle.

It sounds like it may not be getting enough fuel to the carb(s). Check to make sure that fuel is flowing from the tank. If not, it may be a problem with the petcock or the vents on the gas cap may be clogged.

Shuts down in 5 seconds? If so, it is supposed to. If the flame is not perfect. Read on..

The ignitor (glowbar) only glows for 5-7 seconds- just long enough to create combustion. Once the flame starts- the flame must keep itself going. So if at anytime the flame goes out- it stays out- there is nothing to restart it.

What most people don't realize is that the photocell is looking for a flame with perfect color, which means the heater is not emitting pollutants. If the flame runs too rich or too lean (polluting) the photocell shuts the unit down. No self re-start.

Start troubleshooting by setting the pump pressure. Pump pressure is directly related to fuel delivery. Poor fuel delivery equal poor flame equals shutdown.

Troubleshooting tips

If it runs for awhile and then smokes and then shuts off, the problem is usually in the fuel system. If you have never serviced your unit, it may be time to do it. Most fuel tanks start sweating during the hotter times of the year. This moisture builds up and settles on the bottom of your tank. Your suction fuel line is real close to the bottom and the water in the fuel ends up spraying on the ignition plug or glow plug causing it to put out the flame and the units circuit board shuts down the unit because of a flame was not sensed. Check the fuel if it is not fresh, change it. Usually there is a drain in the bottom of the tank. Good luck.

did you install the oil pressure switch origional to the tahoe? The tahoe uses a low oil pressure shutdown, instead of an inertia switch to shut off the fuel pump in the event of an accident. If the pressure circuit does not close within 3 seconds after the engine starts the fuel pump shuts off. This is in case the truck is on its side or upside down the pump will shut off.

It has a sensor that shuts the gas flow off if there is no heat. This is a saftety in case the flame goes out. Often the sensor is not placed close enough to the flame to get hot enough, or it's bad. I'd try finding the sensor and budging it a little closer to the flame, and it should stay on, otherwise, it will need replaced. 

at first i thought this was a bad flame rod but the fact that it burns on low and not on high tells me that it is not getting enough gas since it burns less gas on low. there will be a tap on the burner side of the regulator, take off the cap and turn the screw clockwise to increase the gas pressure. do this in small amounts until it corrects the problem. If you have or can get a manometer, the gas pressure should be set to @7 inches of water column

most likely cause is the CAD flame sensor.

John- the ignitor does not stay on for the duration of the running time. It only stays on for the first 5 seconds. This is long enough for the flame to ignite and perpetuate itself, assuming the fuel delivery system is up-to-par. The photocell monitors the flame color and if the flame becomes off-color then the photocell tells the control board to shut the unit down. Naturally, too rich or too lean affects flame color.

Now, I'm confused by your description "the heater cycles 3 times". Tell me more.

I'm glad to see that you checked the pump pressure. Have you used soapy water to check for air leaks all the way to the nozzle? A leak in the airline, fuel line, or a crack in the nozzle adapter will cause no fuel delivery, or reduced fuel delivery as well as reduced atomization of the fuel - all of which can cause a lack of combustion. A restricted fuel filter will cause starvation.

Eliminate all possibilities of a fuel delivery problem before you start blaming electronics because there is not much way to test the board or the photocell successfully without DESA's diagnostic tool. Unfortunately, replacement of the componants is cheaper than the tool.