Marey tankless hot water heater 16ng

Most on demand or tank-less units require a minimum flow rate before the unit will fire up, they designed them that way so small water leaks (drips etc.,) keeps the unit from going on needless;y. Try opening the shower hot water fully and see if it comes on, If it does, try closing it slowly to see where it shuts off. Make sure your shower head is clean and not blocked with calcium, which will restrict the water flow. You could also try changing the shower head to one that allows more gallons per hour flow rate. Also make sure the cold inlet and hot outlet valves on the unit are fully open and the diverter on the shower is fully open as well.

Sometime the batteries in the drawer fade (yes even new ones). I am asking, is that after the flow initially stops, the Automatic Start will not fire up when there is a demand for hot water (Is this what you meant when you say "The water is turned off" and "Then on again". Check the batteries for being the right way around (I Have installed them 1/2 forward and 1/2 back ward as a set, the one cancelled the other one out....my Bad....Ooops) The battery should be only needed if the power fails, but not sure about this model. Does the Owners manual say anything about no hot water? IF that was tried, you may need to call the heating / cooling techs that are familiar with this unit.

Before calling service technician, open following link for Rheem troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

List of causes MIGHT include one of following:
Bad household circuit breaker that powers electronics, or loose wire inside outlet where tankless plugs in.
Tankless fuses and control board.
Water flow rate too low. Minimum .8 gallon required (depending on model)
Tankless inlet filter clogged.
Household aerators clogged at faucets.
Crossover at single-handled faucet
If installed, there can be crossover at mixing valve.
Faulty household check valve, including check valve on re-circulation system if that system is installed.
Restriction in household water line, such as clog at elbow.
Unequal pressure in household hot and cold lines, caused by restriction in hot line. When single-handled faucet turns on, the cold water pressure is higher so hot does not flow and tankless will not turn on.
Loose wire connection inside tankless.
Unit temperature set too high.
Unit temperature set too low.
Water temperature is very hot coming out of tap, so more cold is added, but that reduces amount of hot water flow, so tankless turns off because flow rate is too low.
On some tankless, if water is preheated by solar or other indirect source, the burner will not activate.
High mineral content in water > delime tankless yearly
Misaligned temperature sensor
Low gas pressure. Wrong type of gas. Undersized gas line.
Too many faucets on at same time.
Faulty tankless water valve. Water valve stuck.
Fluctuating household water pressure, for example MUD and PUD district water pressure problems or water well pressure.
Faulty household pressure reducing/regulating valve if installed.
Faulty household pressure tank, if installed -or- pressure tank needs to be installed.

Ok i intall tankless water heaters and sell them and repair them . The trouble your are hving is more than likely ,the mixing valve on your shower has a ajustment on it to prevent you from burning your self . that is the number one reason in showers when water never get to temp . What it is doing is addng cold water to your shower for you bummer . Take handle off and adjust water temp to your hottest setting . Some times you have to remove the low flow washer in shower head to get the right flow . also every time you start up your hot water it shouldbe set all the way ot other wise withe the battery 2 D models the unit will not sence flow and shut off the unit ,with some units it is a fine balancing act to get them adjusted right. If your buying a tankless water heater always buy a blower type unit that has a computer that runs it . You will be happy . If the Battery operated unit is for a cottage or a smallcabin then your just going to have to fix the pumbing to make it work right many of the safety features of the newer showers are not made to work well with tankless water heaters .

http://www.greatergoods.com/store/energy-savers/tankless-water-heaters/rheem-rtg-74-dvn-natural-gas-tankless-water-heater/prod_386.html
manual above.
I professionally recommend that you get a Plumber/Gasfitter, to check this over, as this isn't usual, and I wonder if there are not "Other" problems too... with anything that uses Gas one has a responsibility to the entire family to have it professionally checked over, in my humble view..

That is the temperature pressure relive valve (TPV) . It is either foiled with some thing or going off because of a pressure or temperature problem. If it is leaking continuously it is foiled and needs cleaning. If I where there I would replace it with new.

if your on a private well, it sounds as though your loosing enough water pressure to shut the heater off,.(which is right before the water pump comes on),....tankless heaters turn on by sensing water flow, and of course off when flo stops,......water pressure tanks often need to be re-charged with air for pressure to stay even,....does your water pressure fluctuate?

lpg overgassing best leave to the experts

Ok the way this system works is from a pressure and flow switch. The pressure switch picks up the slight drop in pressure when the hot side water is turned on then turns on the burner. The flow switch measures the amount of water flowing and adjusts the size of the burner to the amount of water flow. So a couple different things might be happening. The flow switch might be sticking causing the water heater to throttle down the burner but not off when the water flow is off. Or the gas control valve might be by-passing gas when off causing the burner to stay on even when the demand is off. Run the hot water on high demand then turn the water off. If the burner is still on try banging on the water heater with your hand and see if the burner shuts off completely. If it does the gas control valve might be by-passing. You would also need to check and see if the pressure/flow switch is giving any voltage signal changes when the water flow is off. These tankless water heaters are more comlplicated to work on than a standard water heater and you may need a professional's help...good luck

You are doing everything you can. Your dishwasher probably isn't drawing enough hot water to activate the unit so opening the faucet and flowing more water will activate the unit. Just try to adjust the flow from the faucet to a point it, plus the draw from the dishwasher, will activate the unit. This will come with trial and error as I am sure you have already experienced. Also, open the dishwasher when it does get filled and check the water temperature. You might have the unit set at a low discharge temperature. Do you have a touchpad to control the water temperature adjustment?