What's wrong it a 99 Honda won't start when it rains, then won't start at all even though it cranks up?

What's wrong it a 99 Honda won't start when it rains, then won't start at all even though it cranks up?

The number one common cause on Honda vehicle in the same era for not starting is the Main relay. Of course, temperature, moisture or dampness will affect it also.

You still have to check into the other possibilities when a vehicle has failed to start and what systems/ components involve: Fuel and ignition systems. See this video:

Would it be the fuel pressure is weak or deplete due to a failed fuel pump and cause the fuel to return to the tank on the supply side? Have you done a fuel leak down test? Did the fuel pressure hold?

Other things like cross firing, ICM, sensors, even the PCM would be affected and temporarily out of business when dampness or temperature occur. Look at my answer to another member about 6 months ago:

Hope I've answered the question and if you have more questions or need more help send me an e mail.

Probably the spark plug wires or the coil, both can be affected by dampness.