My Bosch AquaStar 240 NG tankless began throwing an error A2 and cuts off. I followed the trouble shooting guide, cleared the filter (which was pretty clogged), checked the fuse with a multmeter, fuse reads okay. When I reset it clears, and I see and hear it turn to ready mode... but then when I turn on the hot water in a few seconds, like 30 or so the copper part where the pilot window is unit makes a rumbling sound and vibration, it lights up big inside and shuts down and then reverts to the same error. I get about 2-3 seconds of warm water. I worked okay for the last 5 years....

Hi,

1. Check for flue gas leakage around the top and bottom seals of heat exchanger as well as the viewing window. Look for any noticeable burn marks or warping of the gaskets. If a leak is detected, seals will need replacing.
   
2. Verify that venting meets specifications outlined in chapter 3.6 of the installation manual. Long vent lengths, incorrect venting material, venting with more than three elbows, blocked vent or combination venting may cause this failure. Verify backdraft flapper inside flue gas collector is not stuck shut.
   
3. Check flue gas limiter wire connectors for tightness. Clean with pencil eraser if any corrosion is evident.
   
4. Trace wires from flue gas limiter to black connector. Verify the connection is tight.
   
5. If limiter is tripped due to overheating, it must cool down before error will clear on the control board.