| To the untrained eye, a bad main relay will look just like a good one. Upon closer inspection, however, one will see one or more "cold" solder joints on the printed circuit board (PCB). Cold solder joints are cracks in the solder around pins in the PCB. They come about from poor soldering during manufacture and from constant heating and contraction during use. In most situations, a bad relay will bench test okay--it's when it gets hot that it fails. My advice is to replace them every 120,000 miles or 10 years, whichever comes first. | 
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