Depending on how old the unit is it may have a thermopile "generator". the thermopile will have 2 wire leads that usualy attach to the main gas valve by 2 screws. set your multimeter to mvdc and tke a reading usualy anything over 350 mvdc will be able to energize the gas valve. any bad connections or open limits or safty switches will not alow the main gas valve to energize. hope this helps.
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Looks like your pilot generator may have some corrosion on it. Follow the stainless steel tubing on the gas valve as it goes down to the burner area. At the end of this is the pilot assembly. If you can remove this so your able to work on it you will se a short metal rod that the pilot flame touches when lit. This generates current that keeps the gas flowing to the pilot to keep it lit. Take some sand paper and give it a light sand to remove any corrosion on it. Reassemble at put it together. You may also need to clean the pilot tube(stainless tube from valve). Small spiderwebs can reduce the flow to the pilot and cause this problem also
Your thermopile that tells the heater how hot the water is has failed. It should kill the heat at a temperature of 104f. This also indicates that the limits are not doing their job, as they should be killing the heat at 135f (low limit) or 150f (high limit). Schedule 40 pvc is rated at about 140f.
This many failures at once is rare, and typically indicates that the heater has been rigged. Get a tech out to fix it correctly. You are literally playing with fire.
It sounds like you need to replace the pilot generator. This is the metal cylinder that sets in the flame of the pilot and supplies voltage to the system. When it gets weak, it will support the pilot valve to remain open, but when the main valve tries to open the pilot generator cannot provide enough electricity, and everything shuts down. It's a prime suspect in your situation.
if it is direct vent chances are it has electronic pilot.. it does not have a standing pilo and match size upt. if in fact it did have a buning pilot there is a part called thermocoupling. it will look like a 1/8 in. copper wire coming out of the bottom of the gas valve and goes right to the pilot. anywhere from 10 in. to 20in.. unscreew nut at gas valve pull out from pilot area [ it just clips in at pilot] take it to any home center
You probably have a defective thermocouple. The thermocouple sits in the flame of the pilot, and the heat generates a small electrical current conducted back to the gas valve, keeping the pilot gas line open after you switch it to the "on" position. That's why you have to manually hold the gas valve depressed (on) during lighting in the "pilot" position, allowing the thermocouple time to heat up enough to generate its current. A defective thermocouple will not generate the holding current.
If your pilot assembly is corroded, you will have trouble lighting the pilot. I would highly recommend replacing the pilot assembly. By doing this, you will have a new pilot generator that will be producing a larger amount of voltage to run the gas valve. Also, you will have a clean pilot tube and orifice so gas can run unobstructed.
There should be instructions on the inside of the heater access door. Make sure you are comfortable working with gas heaters. Make sure the shutoff valve supplying gas to the heater is on. Turn the gas valve to "pilot", press and hold it while attempting to light the pilot. Once the pilot is lit, continue holding the gas valve knob down for about 30 seconds. The pilot should stay lit once you release the knob. If the pilot stay lit, turn the gas valve to "on". Replace the heater access door and make sure you have proper water flow and attempt to light the heater. If it will not light, or the pilot will not stay on, you will probably have to replace the pilot generator. It provides voltage to run the heater and after time it wears out and produces less and less voltage. A millivolt heater runs on less than one volt so any power loss can be a problem. If you are able, test the pilot generator with a multimeter to see if you are getting at least 500mv when the pilot is on. If not, you should replace the pilot generator or even the entire pilot assembly. Hope this helps...