Standard procedure for troubleshooting tankless

Hi fellas, I was curious what your procedures were for troubleshooting non functioning tankless heaters. We went out to a no hot water call and the 9 year old outdoor Rinnai had a leaking heat exchanger, and water dripping out of the fan cage. Remote controller was functioning, no codes, and no demand indicator light when running water.

I am still learning, and haven't had any formal training in them, so beyond pulling the filter and impeller to check for free movement, I didn't have any other ideas on what else to check, or how to check it.

We called tech support, and they had us check for source voltage (red wire) to the flow sensor, and we had 12v. So he is sending a flow sensor, heat exchanger and combustion fan, all under warranty. But he also was adamant that those parts might not fix the unit.

I have seen this before, you start throwing the parts bin at it and after all the parts you have to be the A-hole that walks to the front door and says that didnt fix it, after they paid the labor and diagnostic fees on the first one and have been out of hot water while the first round of parts comes in, then more time without HW while the second round comes in. I hate being in that position.

The unit would go through it's robot noises when powered up, but just wouldn't fire when you ran water. I would love to be able to prove out the flow sensor by sending a pulse to the computer and if the unit fired, I could without a doubt order the sensor. Or prove the sensor is good with an oscilloscope. I just hate to throw the parts bin at it blindly..........

Thanks for any help you can offer.
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