Is this new installation? Or old installation that stopped working?
Here's the 100% solution. And it applies to all brands, models, types, of tankless water heater, no matter which error code:http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
People should avoid complicated water heaters that claim 'energy star' and '98% efficiency' while proclaiming 'endless hot water' as if the laws of physics, economics and common sense can suddenly be suspended if only the customer would buy expensive, complicated water heater that cannot be maintained or repaired by customer, and there are no qualified technicians who live in your area.
Otherwise you need a service manual, or contact eternal manufacturer. 1-866-946-1096http://eternalwaterheater.com/pdf/downloads/GU100.pdf
Error E02 does not show in manual.
E2 is ignition failure, which on other brands can be failed PC board... and inadequate incoming air supply, or venting, or failed sensor, dirty combustion parts, air or water in gas line, low gas pressure, wrong gas, etc, or multiples from list above.