Your Corner Wrench: Diagnosing and buying your own parts, part II

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Everybody loves a deal, and why not? Saving money is a tempting way to lessen the pain of dreaded auto repair bills. But when it comes to doing your own diagnosis, parts sourcing, and purchasing, there are a few caveats to be aware of to avoid wasting your time and money. Here are a few of the more common over-the-counter purchases that parts pros encounter.

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“It must be a defective computer”

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Why DIY shade-tree techs pick on this particular part can be a mystery. For most vehicles it can be the most expensive part on the vehicle, excepting an engine or transmission. Automotive computers, like home or business units, are basically a garbage-in-garbage-out affair. Yes, they can fail due to age and environment, and most older-model vehicles can’t benefit from software updates. But before you whip out the credit card for a replacement computer, you need to do some diagnosis.

First, they’re usually not returnable if they don’t solve your problem. Once they’re plugged in they leave telltale marks on their communication pins, meaning any decent parts person can tell you’ve tried them in a vehicle, thereby rendering them yours forever. Used units may be an option, but only if they’re not vehicle-specific. Some engine control computers can’t be swapped from one vehicle to another. Most salvage yard operators know which ones can and can’t be used, but if you’re shopping online, it’s a case of buyer beware. There are supposed computer refurbishing companies out there, but if they haven’t got access to the carmaker’s circuit diagrams (and most companies keep that info well guarded), they’re pretty much selling snake oil

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If you don’t have access to an accurate diagnostic manual or the proper electrical test tools, spend the money to get a pro to test the related circuits; you’ll likely save yourself a bundle. The same can be said for just about any sensor or actuator related to fuel injection or emissions (or any other electronic system). Unless it’s a particularly cheap part that’s easy to swap out, it’s always better to know for sure before you buy.

“Just a set of brake pads please”

This is a common enough purchase that most parts counterpersons have their reply memorized; “there’s no warranty on these pads against noise”. Popping a set of new brake pads (or linings) on an older rotor with rusted edges and glazed surfaces is almost guaranteed to give your ride a transit-bus sized squealing noise that will announce your arrival a city block away. As well, it can lead to overheating of the pads to the point that their lining material can crack. And by the time you realize you can’t live with the racket, those new pads will either require replacing again along with the new rotors, or will need to have a measurable amount of their life sanded off to reach a quiet, serviceable condition.