takagi tk2 problem error code not in manual

i have owned this takagi for 4 years. i clean the filters out once a year. i have the remote panel. today during a bad storm, it suddenly fires up for no reason (no water usage) beeps and gives error code 21. this code is not in the manual. i was wondering if i'm getting water coming in from the exhaust valve (it's a side wall exhaust not straight up). it's only happened today during this really big storm when lots of rain is hitting the outside exhaust.

actually, my paper copy doesn't have it, but the pdf from takagi does. error code 21 is vent pipe. i took the front panel off and there is water dripping down onto the base part of the inside. i don't think it's condensation, i think water is coming through the vent pipe and stopping the burner from initiating.

But as you said the burner was firing up with out any water being used. I would be worried that some electronics might of gotten wet.