Before calling service technician, open follow links for troubleshooting resources.
Bosch site does not show TF250, but shows 250SX and 250SXO.Gas Products Technical Support: 800-642-3111http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.htmlhttp://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/tabid/370/Default.aspx
At no time when visiting tankless website have I read suggestion that people fix tankless as a do-it-yourself project. In fact each site says repeatedly to call service technician for installation and repairs.
When reading service bulletins, the repair protocols are generally long and very sequential, even if a thermocouple seems straightforward there are many interrelated parts and sensors that must communicate exactly with each other.
Example: following pdf outlines operation sequence for non-standing pilot modelhttp://waterheatertimer.org/pdf/Tankless-gas-operation-overview.pdf
The dirty water also suggest that tankless has not gotten regular maintenance that is required for continued operation. Tankless are not install-it-and-run-flawless-for-years type of machine.
Read manual for specific maintenance required for your model. For example yearly flushing the heat exchanger with vinegar to avoid lime build-up is suggest most models. Tankless are succeptible to lime build-up.
All costs are energy. Maintenance alone can easily eat up the promised $1-8 monthly savings from tankless vrs tank-type water heater. One service call can wipe away years of expected savings, plus replacement part can sometimes cost as much as whole new tank-type water heater, installed.