Water Heaters - Tankless water heater ignition probem

I have a Bosch tankless LPG water heater with a capacity of about 3.3 gallons. Its about three years old and mounted outdoors. Recently I began to have problems getting it to ignite the gas when a faucet was opened. At first this was intermittent. I would go out and find error code 11 flashing in the control panel. This is a 'no ignition' code, and the prescribed remedy is to turn off the power and turn back on. Occasionally I would also get an error code 12, which indicates 'flame failure' and has a similar suggested remedy, which worked for a while.
Recently the problem became worse. Sometimes the gas would ingite for a few seconds and then fade out. Usually, though, once it started there was no problem and the water was hot as you like. Now, however, the unit will not ignite no matter how often I turn the unit on and off. I can hear the fan start and the igniter clicking rapidly inside the unit. I can also smell the gas venting from the exhaust at the top (via the fan - I dont think theres a leak). There is no problem with gas or water pressure. The drain valve is not clogged. There are no blockages I can see to the air inlet or exhaust conduits.
I'm trying to narrow down what the problem might be. It has seemed to coincide with a month of exceptionally heavy rain where I live. I dont think there is any leakage inside the unit (it is fairly well protected from the elements) but the air has been saturated with moisture. Unfortunately it hasnt recovered as the rain has gone away. I'm wondering if this could indicate a problem with the igniter?
Any suggestions?
Thx for any feedback.