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Unit will provide hot water most of the time so long as we leave our single handle faucets at about 1/2 water flow. When I try to use full hot water flow, the fan comes on and I have a flame for about a second and then code 12 comes on a few seconds later. When I use a single handle faucet at about half available water flow the unit works most of the time but if I then increase to full flow the flame stops and I get code 12. There is the odd time that with a single handle faucet being used at half water flow it will show a good amount of flame for 10 seconds or so and then the flame drops down considerably and within 5 to 10 more seconds the flame goes out and we end up with a code 12 again. When using our only dual water faucet, I can turn the hot water tap to full and I get hot water on a consistent basis.
So far I have purged the unit with 4 gal of vinegar twice. I cleaned the inlet water filter. I have replaced the electrode, the flame rod , the gasket as well as the igniter. I have cleaned the heat exchanger. I am using a new combustion fan. I have checked the venting to make sure all connections are secure. I have checked the water flow turbine to make sure that it is turning freely and that there are no restrictions. I don't believe that there is any gas pressure problems involved with this situation and there is no water seen in the tube below the gas valve. Also checked the immersion sensor on the hot water exit tubing and it was clean.
At this point, I can only surmise that the problem could be related to a malfunctioning water flow servo-sensor.
The unit being 11 years old I am not sure if it would be wise to spend more money at this stage.
Hopefully someone can help me out with some suggestions as what the problem might be.