I'm pretty sure mine comes one only at start-up when it's cold but it has come on at idle too. I believe it is mostly when the exhaust temp is below a certain level. Now that I have the MIL eliminators, it will be interesting to see when it cycles.
It is loud when it comes on though. Not too bad but noticeable. I run my cooling fan just about all time too so it drowns it out.
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