I have a bosch Es 2700 water heater that keeps giving me an e2 error code .

SOURCE: omega tankless water heater giving error code E2 incomplete burn

it sounds like your flu pipe may be clogged,, they just snap together or have little screws holding them together,, take the sections apart and make sure nothing is is the line reinstall and try it out

SOURCE: I have a Bosch Tankless

Yes. Contact Bosch.
Error codes are considered start-point for troubleshoot.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Tankless are complex electronic machines.
If on-site plumbers cannot figure out problem, then you need trained service technician with a service manual for your specific product, who can ohm-test the electronics.
Bosch will have a list of trained service technicians. And maybe Bosch has another step for fixing problem yourself.

Fix most tankless problems yourself, same day, for $330.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

SOURCE: Bosch tankless watwer heater GWH

Low flow means only one of three things to me: 1. You don't have a water softener ahead of the heater and it is choked full of minerals. Solution: If it was correctly installed (with a softener) proceed to step 2: Step 2: Remove, clean and replace the water filter screen on the water inlet. The manual will show/tell you how to do this. If this does not get your flow going (it will ALWAYS be about 30% below your cold flow at best if your incoming water is quite cold and you are heating it toward maximum temp) go to step 3: Step 3: Order and replace the water valve. I think the manual recommends every three years that it be disassembled and rebuilt. I got just over five years out of my first one, and a customer got over six years before we had to replace their water control valve. I am not inclined toward rebuilding them. Replacement is much easier in my opinion. The price of the valve is between $250 and $300 (For the Bosch 117K btu B-Vent models) if memory serves. Don't even THINK about using a tankless heater without the protection of a WORKING WATER SOFTENER if you have hard water. I have seen minerals DESTROY AND RENDER USELESS a tankless water heater IN LESS THAN SEVEN MONTHS in hard water areas. Mike the Master Plumber