I have a bosch 250SX NG. I keep getting an A2

http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

no you do not, but it is a good idea to take the hydrogenerator apart and remove the waterwheel inside to protect it from damage(if you have anything that gets shot through the pump)
Connections are 1/2 inch.

Copy following link:
http://waterheatertimer.org/pdf/Bosch_A2_error_code_troubleshooting.pdf

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

The following link has general troubleshoot steps for Bosch A2 error
http://waterheatertimer.org/pdf/Bosch_A2_error_code_troubleshooting.pdf

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Before calling service technician, open following link for Bosch troubleshooting steps.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

If you are not getting error code, then problem could be somewhere in plumbing, as you suspect.
For example cross-over on single-handled faucet, or faulty check valve on re-circulation system, or sediment clog at elbow, or varied water-well pressure. Install in-line pressure tank.
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/pdf/No-Error-Code.pdf

Be sure to read manual for yearly maintenance protocol.
Tankless must be maintained, and cannot be exposed to hardness in water greater than 11 grains, and should be de-limed each year. Expect maintenance, service, and potential repairs and parts for tankless to far exceed expected $1-8 monthly savings. All costs are energy.

All gas tankless brands can be repaired by homeowner for $330.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

The 250 MUST have the condensate line connected. EC comes from gas supply problems and CO2 levels not being adjusted. LP units require 3/4" smooth gas pipe or 1" corrugated pipe minimum size. The LP regulator needs to be turned up to about 13 inches of water column and with all the gas appliances on, measure no less than 11 inches of water column. The CO2 levels must be measured by a qualified gas technician with a calibrated (with in 6 months) CO2 analyzer.

hi ill try to help the fault is in the overheat cutoff fuse its the white overheat wire fuse that wraps around the heat exchanger disconnect the plug connector between the fuse wire and the control unit ! using a multimeter check the resistance through the fuse to see if its bad !if its bad check to find the cause ie .a leakage of flue gasses on the heatexchanger also check the black rubber seals located at the top and bottom of the heat exchanger also inspect the seal around the viewing window on the heat exchanger then call tec support and report leakage areas also for fuse replacement the only number for tec support i can find is 08006423111 i do hope this helps

Model????? The 635Es, 250SX and 2400E are same. The 2400ES and 2700ES are World 2 heaters. Any error codes as I can't imagine this problem w/o codes. Flow initiated but you have spark. Is there any ignition? If so flame rod may be bad and not reporting ignition. Igniter sets on some early 2400/2700 have proved to be bad. Water pressure has zero to do with this. If you have an EC or EA code you have gas problems (meter, line size/length or gas valve). Also if 3005 or 3007 Software you can pull the last 5-10 error codes from Controller.
You need to call Bosch and get an RSA out there. This is a gas appliance that if you screw up can destroy your house. FYI any water in the gas valve condemns it. It will not dry out and work correctly. Is the fan running at ignition and/or purge? Any Controller errors require replacement. You have 2 years on parts warranty from installation date so don't screw around trying to DIY. Warranty Labor is not covered.
Flue obstruction? If you used the AQ3 installation Kit you have the wrong termination box to prevent freeze up and/or water penetration You need the Z-Vent drop nosed termination. Look for a fine tan dust in the fan intake (brown plastic) the intake throat or in passageway to the gas valve. Call Bosch unless you're a Gas Tech this is beyond you.

http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/250SXTechSupport/Troubleshoot/tabid/563/Default.aspx
click on this link and it has the troubleshooting in strucktions for the unit . hopt that it helps you out.

Replace the thermocouple.

Thats the flame sensor.

If the gas valve is not sensing flame it will not stay lit.

T-couple's are about 9 bucks.