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Rinnai gas heater burner goes out when ignition button is released?

I just bought a 2nd hand Rinnai Econoheat 850 gas heater.
When I press the control button down, the ignition works fine and burners come on (gas supply present) and start turning bright. But when I release it after holding for 10 seconds, the burner goes off. I have tried ventilating the room, replacing the battery and also holding the button down longer (30 sec, 1 mn, even 3 mins) but these don't help. Can someone please help?

Your problem could be something simple, such as dirt or some other debris interfering with the establishment of a proper pilot flame. The pilot flame must impinge properly on the thermocouple tip in order for it to function correctly. With the unit turned off you might try blowing compressed air around the pilot assembly area. If you are handy you might try taking the pilot assembly apart, clean it, and reinstall. You might also try, but do it gently, a light, rapid tapping in the area of the pilot flame (sometimes this will dislodge any debris). If air, cleaning, or tapping do not work, you should try a new thermocouple. Gas valves are expensive, so you want to leave that until last. Good luck.

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