If your vehicle is showing car won't start in wet, rainy, or damp weather, here are the most likely causes and the parts you'll need to fix each one.
Over time, the rubber insulation on spark plug wires breaks down and becomes porous. In damp or rainy weather, moisture creates a conductive path that allows the high-voltage spark to jump to the engine block instead of reaching the spark plugs.
On older vehicles, hairline cracks in the distributor cap allow moisture to enter the housing. This moisture causes the ignition spark to scatter or ground out before it can be distributed to the cylinders.
Modern coil-on-plug systems can develop microscopic cracks in their plastic or epoxy housings due to engine heat. High humidity allows the electrical current to arc through these cracks to the engine ground.
If spark plug boots are not pushed down fully until they click, or if dielectric grease was omitted during a recent tune-up, moisture can easily penetrate the connection and short out the spark.
Moisture acts as a catalyst for corrosion. In damp weather, borderline battery connections experience increased electrical resistance, preventing enough current from reaching the starter motor.
If the weather seal on a vital engine sensor connector is broken or missing, splashing water from wet roads can short the signal, preventing the engine computer from knowing when to fire the spark or fuel.