If you can't hear clicks, then very likely it is the "microswitch" that is a rectangular white plastic box that can be seen at the bottom of the unit from underneath. It's purpose is this: when there is gas pressure, a pushrod turns the switch on, thereby allowing electrical current to flow from the "hydrogenerator" to the igniter unit. The hydrogenerator generators from about 1 to 2.5 volts AC (not DC) when water is flowing, powering the generator which powers the igniter unit....assuming the microswitch is allowing electrical current (the voltage) to pass through. It is a "self-adjusting" switch because the pushrod can come out at different pressures. So if your gas fluctuates, the microswitch will be fixed to a higher pressure level, but not be able to turn on if the pressure drops. This is because the pushrod is not pushing as far. It's simple to unscrew the plastic box and push the teethed switch setting back to the initial default position, and then carefully replace the plastic cover. Then do it again next time it stops working, a year or two it seems for many people. You can take out the switch and connect it's two wires directly, but if there is no gas pressure and you let water flow through, the igniter unit will constantly be sparking and thereby might fail early. If there is no gas pressure, then maybe it is because of a local leak and thereby bypassing the switch is a built-in explosion hazard, but that would require the sparking to no longer be internal to the ignition unit.
so, even when I manually click the microswitch when attempting to light the pilot, no attempt occurs and thus the pilot doesn't light. Why?
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Igniter can be ordered, but that may or may not be the issue.
Your tankless unit -and- plumbing system need total check-up by trained service technician.
Open following links for Bosch troubleshooting resources including contact Bosch, manuals, error codes, water flow etc.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://waterheatertimer.org/pdf/No-Error-Code.pdf
Check electrical outlet with circuit analyzer to make sure polarity is correct.
Typically, gas water heaters, and especially tankless water heaters require yearly maintenance by service technician who can check the gas pressure and supply, venting, electrical connections, PC board, sensors, burner orifices and igniter parts are clean and positioned correctly, vent is draining correctly and not dripping back into combustion chamber, correct temperature is achievable, incoming water temperature is not too cold for temp setting, proper incoming air supply, adequate air supply, no chemicals stored in air supply pathway, filters are clean, plumbing check valves, mixing valves, pressure valves etc are permitting one-way flow of hot water, water is softened to less than 11 grains hardness, water is filtered and not dirty, aerators and other faucet parts are clean and support water flow necessary to activate burner.
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