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A car engine refusing to start can be any number of issues. A car that wont start in the rain can still be any number of issues, but it gives you a place to start looking.
Spark Plug
The most common reason for a car not starting in the rain is found in the spark plugs. Wire insulation can wear thin and this causes the voltage to arc to the metal of the car engine rather then going into the spark plug. You can check for this visually by opening the hood when its wet outside and trying to start the car. You might be able to see an arc from the spark plug wires.
Distributor Cap
The second most common issue is the distributor cap. Take a good physical examination of your distributor cap for any signs of a cracks. Also remove the distributor cap and check the inside of it for moisture. If you find any, wipe it clean; it wont work while wet.
Other Issues
If a check of the spark plugs and the distribution cap has not solved your problem, then you have a bigger issue. There are dozens of other reasons a car wont start, from battery corrosion to a fuel filter issue keeping gas from reaching the engine. Though few of these other issues will only affect your vehicle during the rain.
