My 1999 Corolla's check engine light comes on only when it's wet outside and will go off after a couple days of dry weather or sometimes will turn off when I turn on the rear window defogger. What could be the problem?
by 
Dan Busser
ANSWERS: 2
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It's hard to tell without checking the on board diagnostic computer, but this often happens when your serpentine belt or alternator belt is slightly loose or worn. The condensation that accumulates on the pulleys causes the belt to slip just enough to trigger a timing fault code in the computer. As the pulleys dry out the problem resolves but the trouble code stays in the computer often for several days. Turning on the rear defogger increases the load on the alternator and belt, which probably sends a timing adjustment code to the computer that clears it. Solution? Replace the belt and make sure the tension is correct and the tensioner (for a serpentine) is tightened and functioning properly. Don't use spray-on belt dressing, it will make the problem worse.
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You might want to check the plug wires. My '98 wouldn't start if it was raining out (check engine light came on) and the car was cold. The wires were arching when it was wet out.
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