Many cars either come standard with dual exhaust or can have it added on, but exhaust will not always come out of both tailpipes. There are times when that is normal, but it can also mean there is a problem.
Function
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Dual tailpipes are used to expel exhaust after it has gone through the muffler and the catalytic converter. A butterfly valve at the base of the dual exhaust pipes directs the exhaust through the two pipes.
Warming up
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In some cars, the exhaust is directed only through one exhaust pipe for the first 30 seconds to a minute. It will look as if the exhaust is only coming out of one pipe. Once the car has warmed up, the butterfly valve opens and directs the exhaust through both pipes.
Blockage
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The other reason why exhaust would only come out of one pipe would be if the other pipe was blocked. Occasionally, dirt, snow and other material can become impacted into the pipe or the pipe can be bent from an accident and prevent the flow of exhaust through the pipe.