Tankless gas HWH weak flame, sometimes no flame

[ChicagoSuburban]

I have hard water. I have a small generic Chinese tank-less natural gas HWH which is no longer heating water properly. Water gets lukewarm. It seems like the size of the flame got smaller over the last year. Sometimes the flame would be so tiny that the water will stay cold. When that happens, if I turn the faucet off and then on again, the larger flame will come back on (sometimes). Is it a bad gas valve?

 

[GReynolds929]

If it has never been flushed you have mineral buildup reducing flow thru the unit and not triggering the flow sensor, or the flow sensor is faulty.

 

[ChicagoSuburban]

I know the spinning flow sensor impeller creates a low voltage, low current pulse (PWM) that drives a circuit to open and close the gas valve. That impeller is probably not spinning properly due to mineral buildup. Is there any way to override the flow sensor and feed it a PWM signal until I get a chance to replace the faulty/clogged flow sensor? I take full responsibility for my actions.

 

[GReynolds929]

You may not need a new flow sensor. Run a flush on the unit and see what happens. There are lots of videos out there on how to do this, and a lot of the time you can use straight white vinegar. Check your owners manual to see what it says.

 

[ChicagoSuburban]

I don't have a flush system. I have an aquarium pump and a windshield wiper pump. Is there a way to make a DIY flush system out of those?
The flow sensor is on the hot water outlet next to the temperature sensor and there are 2 screws holding the flow sensor in. Since I do not have a flushing system set up, do you think removing only the flow sensor and dipping it in vinegar may help?
BTW, there is no flame anymore as of this afternoon. I see the ignition sparks, and hear the gas valve solenoid thunk sound, but no flames at all now.