I have a double tank heater. Neither one will currently stay lit. They will light as long as long as I hold the button in. They shut down as soon as I let it go. I have the tank level.

Sounds like you need a new thermo coupler that is the outfit that heats up to a certain temperature so the pilot can stay on.

Some heaters have to be "warmed up" by holding the button down for approx. 30 seconds after it lights. These are ones with whats called a "thermocouple". The thermocouple has to remain a certain temp. or the unit will shut off. If the thermocouple goes bad, it can be replaced

The pilot generator (aka thermocouple) has gone out. This is the little peg that is mounted close to the front of the burner. The flame heats the thermocouple and it puts out a millivolt current that holds the gas valve open after you release the button. The TC is no longer putting out enough current to hold the valve open.
I beleive the part # is TT15c-11, (about $15)

Sounds like the button is bad. Try replacing it

Either the temperature probe (at the pilot light) that signals the gas valve to open is defective or the gas valve is.

The power pile (pilot generator) is defective. It produces a milivolt signal to ensure to keep the gas valve open. If it goes bad, the valve thinks the pilot is out, even when you are holding the button down, so when you release the button, it shuts the gas off to prevent excess gas buildup and a possible explosion.

It sounds like a thermocouple has gone bad. this is a very common problem with gas powered things ie; water heaters, furnaces, hot tubs. The telltale sign is when you release the pilot light button it won't stay lit. This is a safety feature and is relatively inexpensive and easy to repair.

Change The Thermostat Cause If Your Getting Gas But The Water Heater Doesn't Detect The Heat It Will Close The Valve. When Lighting You Hold The Button Down For About 30 seconds so the thermostat will detect flame and stay lit. Check To Make Sure The Thermostat Is Where It Should Be. Heres A Picture Of What It Will Look Like.


You Can Replace Them If Need Be Will Be Available At Sears Or Home Depot. Also They May Sell Them At other Local Hard Ware Stores.

Happy Fixing And Thanks For Using Fixya.com
Sincerely,
Michael Scott

I'm not sure about that model, but I think you have to hold that button down for a minute or more to "prime" it.

I have a different model coleman propane heater that sits on the 20# tank. Mine is probably getting rusty, last time I used it I had to hold that button down for a long time.

I'm looking for a new heater now myself, as mine won't stay lit outdoors in much wind.