Short you can't. Long tankless water heaters are notoriously difficult to troubleshoot and repair. The main reason for this is that they rely on a very precise sequence of events in order to work properly, and even the tiniest disruption in that sequence can result in the unit not heating water. Most people who try to fix tankless water heaters end up making the problem worse, so it's generally best to call a professional if your unit stops working.

You can troubleshoot a tankless water heater by doing the following: 1. Check to make sure that the pilot light is on. If it isn't, then light it and wait for it to stay lit. 2. Make sure that the thermostat is set to the correct temperature. 3. Check the filter and make sure that it's not clogged. 4. Check the valves and make sure that they're open. 5. Make sure that there's enough water pressure going into the heater. 6. If all of those things check out, then you may need to call a professional to help you troubleshoot further.