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Engine fails to start after the vehicle has been left out in heavy rain or after washing the engine bay. The cause is moisture tracking across the ignition components causing the high voltage to earth to the engine block rather than the spark plugs. This is most common on older vehicles with conventional distributor ignition systems but can also affect modern coil-on-plug systems with damaged coil boots.
Based on 351 driver confirmations
Modern car, no distributor. Wouldn't start when wet but fine when dry. Code P0335 crankshaft position sensor. The sensor itself was fine but the connector had corroded and was letting water in. Cleaned connector, applied dielectric grease, sealed with self-amalgamating tape. No issues since.
Parts replaced: Dielectric grease, self-amalgamating tape
Only happened after rain or car washing. Turned out two of the HT leads had small cracks in the insulation — fine when dry but tracked to earth when wet. New leads and the problem never returned. Could see faint blue sparks jumping in the dark which confirmed it.
Parts replaced: HT lead set
Car refused to start after heavy overnight rain. Tried everything then my dad suggested spraying WD40 inside the distributor cap. Started first turn. Moisture had got in and was tracking across the cap. New cap and rotor sorted it permanently for £18.
Parts replaced: Distributor cap and rotor arm
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