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Your CEL was flashing because the computer picked up that you had a misfire.. Based on the fact that it was raining, this most likely means some water got splashed up onto the engine and got onto the ignition coils and possibly down beside them onto the spark plugs. You need to visit your local parts store and get the codes stored retrieved for free. Now if its time for a tuneup then go ahead and change all 6 plugs and apply di-electric grease to all the coil boots. Make sure and mount the one which was showing a misfire on cylinder 4 or 5 or 6, the ones closest to the radiator so in the event the one coil showing a misfire is indeed shorted out and bad it will be much easier to change if the misfire returns. If you just recently had a tuneup then just change the coil that is storing a fault code.