With Tom and Ray Magliozzi, hosts of NPR’s “Car Talk,” hanging up their coveralls and retiring from the radio this fall, there will be a serious void in car diagnostics for ordinary people. We will no longer be able to imitate (very poorly) the sounds that our cars make and get help over the air. So where can we turn now, short of visiting an actual mechanic? Our colleagues a few parking spaces away at Consumer Reports Cars discovered and tested a free Android and iPhone app called Car Trouble that supposedly helps diagnose, well, car trouble. Is it of any use? Yes and no.