Stein and Mana stressed verifying the proper operation of the electrical and mechanical parts of the system in question because, in many instances, the electronics are working properly. "Most of the driveability problems I see are caused by vacuum leaks, which can trigger lean fuel misfire DTCs," Stein tells us. Mana related a similar story: "I've worked on cars with plugged fuel filters that triggered a random misfire code. On the scanner, the pump was delivering the proper fuel pressure, but the filter was so plugged, the pump couldn't deliver the proper volume of fuel, especially under heavy load." He also tells us about an Escalade a customer brought in with random misfire and faulty MAF sensor codes. Rather than automatically replacing the MAF, he checked the air filter and ducting and found a giant rat's nest clogging the inlet to the airbox. "You've got to treat the scanner as another tool to use while diagnosing problems, not as your only source of information," he says.
More often than not, it comes down to experience. The more you know about how a component or system is supposed to work, the easier it will be for you to figure out how to fix any problems that arise. The more you work at it, the better you will be. END
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